Humans in a Changing Climate — Science and Solutions

Andy Revkin
69 min readJan 10, 2018


A user-friendly, spin-free, bite-size guide to global warming drawing on three decades of reports from the National Academy of Sciences and other peer-reviewed science.

Scientists gauging the hardness of tundra on Alaska’s North Slope, 2004 (A. Revkin)

By Andrew Revkin

This is a summary of research pointing to a rising, long-lasting and consequential human influence on the climate system and resulting impacts on communities, resources and ecosystems, as well as science and scholarship on ways to respond.

This climate change guide is implicitly a snapshot of the state of science in relevant fields, drawing mainly on reports, through 2015, from the National Research Council of the National Academies, the leading scientific advisory body in the United States. (The National Academies produced a similar guide to energy, called What You Need to Know About Energy, which can be found online here:

There’s a section on behavioral and social science research illuminating how humans react, or don’t react, to risks on long time scales or embedded with persistent uncertainty.

This work was done a couple of years ago. I’ll be trying to do an update at some point, should new research offer fresh clarity on key points.

Probe it. Dispute it. Help build on it. Post constructive comments, ideally including links to supporting information. Then find the facet that suits your skills and passions and dive in. The one good thing about complicated problems is there is something for everyone to do. You can find lots more from me on climate change science and policy on Dot Earth.

1 Climate Basics

1.1 What Is Climate?

1.2 Climate Cycles and Variability

1.3 The Human Factor

1.4 Climate Dynamics

1.4.1 Climate Forcing

1.4.2 Feedback Processes

1.4.3 Inertia and Lags in the System

1.4.4 Tipping Points

2 Climate Change Research

2.1 Studying Climate

2.2 Probing the Past

2.3 Observing the System

2.4 Climate Modeling

2.5 Climate and People

3 Impacts of Climate Change

3.1 Our Warming World

3.2 Ecosystems and Natural Resources

3.2.1 Freshwater Resources

3.2.2 Ecosystems and Biodiversity

3.2.3 Agriculture, Forests, and Fisheries

3.3 Communities and Infrastructure

3.3.1 Coastlines

3.3.2 Cities and the Built Environment

3.3.3 Public Health

3.3.4 Transportation

3.3.5 Energy Supply and Use

3.4 Climate and Security

4 How We Respond

4.1 Limiting Climate Change

4.1.1 Reducing Energy Demand

4.1.2 Increasing Energy Efficiency

4.1.3 Low-Carbon Energy Sources

4.1.4 Carbon Trapping and Removal

4.1.5 Strategies to Reflect Sunlight

4.2 Adapting to Climate Change

4.2.1 Reducing Our Vulnerability

4.2.2 Ecosystems

4.2.3 Agriculture and Forestry

4.2.4 Water

4.2.5 Health

4.2.6 Transportation

4.2.7 Energy

4.2.8 Coasts and Rivers

4.3 Taking Action

4.3.1 Climate and the Mind

4.3.2 Government Policy

4.3.3 The Climate Divide

4.3.4 Education and Innovation

Humanity’s planet, photographed March 28, 2012, by the International Space Station Expedition 30 Crew, NASA

1 Climate Basics

Earth’s climate is a complex, dynamic system shaped by a host of factors, including cycles, forcings, and feedbacks. Learn about these concepts and other facets of climate science, including the most basic question of all: What is climate?

1.1 What Is Climate?

You don’t need to consult a weather report to know whether you should pack a down parka for a July trip to Phoenix, Arizona. (You shouldn’t.) But you may regret that you neglected to bring your umbrella when you discover that Phoenix turns out to be uncharacteristically rainy during your weekend visit. This, in a nutshell, illustrates the difference between weather and climate. Climate is what you expect. Weather is what you get.

To put it more scientifically, weather is the state of temperature, moisture, winds, and atmospheric pressure in one place and at one time. Climate is the average weather over a long period of time — generally a few decades; it tells us what’s “normal” for a particular region season by season.

The general conditions of climate are determined by factors as basic as the flow of energy from the Sun; the changing orientation of the Earth’s surface to the sun, both in its annual orbit, which results in seasons, and in variations over longer time periods that contribute to the coming and going of ice ages; the chemical composition and behavior of the Earth’s atmosphere; the capacity of different surfaces — from oceans to ice floes, from fields to forests — to reflect, absorb, and retain heat; and the heat-storing properties of the Earth’s oceans.

”Greenhouse gases” in the atmosphere — chiefly water vapor, carbon dioxide, and methane — are particularly influential. They allow sunlight to reach the Earth’s surface but they block much of the heat radiating back toward space from surfaces warmed by the Sun. Trapped energy is retained in the atmosphere and returned to the land and oceans (much as heat is retained in a greenhouse). This effect is crucial to life on Earth. Without the warming that comes from this natural blanket of greenhouse gases, Earth’s atmosphere would be frigid, its surface water frozen, and the planet a very inhospitable place.

The characteristics of a landscape, whether cloaked in dark, moist forests or sheathed in asphalt and concrete, can have a big impact on climate conditions. And up in the atmosphere, clouds and aerosols (tiny particles and droplets) also affect climate, either causing heating or cooling depending on their makeup and where they reside.

Oceans, which cover two thirds of the planet’s surface, are a critical influence on climate because they absorb, hold, and move vast amounts of heat. The top 10 feet (3 meters) of ocean water store as much heat as is held in the entire atmosphere. As a result, the oceans are a storage bank for heat and a buffer against sudden temperature changes in the atmosphere. That largely explains why coastal areas generally do not warm or cool as rapidly as inland areas.

Shifts in oceanic conditions over months, years, and even decades can dramatically influence atmospheric patterns — and thus weather and climate — above both oceans and continents. The best-known example is the “El Niño-Southern Oscillation,” or ENSO, a seesawing climate pattern that typically shifts states every several years, involving changes in sea surface temperatures in the tropical Pacific Ocean and resulting in widespread shifts in atmospheric pressures, winds, and rainfall. In the spring of 2011, one half of that pattern, La Niña, likely contributed to massive flooding in the Midwest and intense drought in parts of the South and Southwest.

1.2 Climate Cycles and Variability

One of the most valuable sources of insight into how the climate system works is the study of past climate changes, a field known as paleoclimatology. Research reveals that Earth’s history is characterized by climate shifts occurring on many different time scales.

Studies of layered seabed sediments and other evidence have shown that over very long time spans, the climate has had warm periods lasting tens of millions of years, and shorter, much colder glacial periods. Scientists have shown that some of these dramatic shifts have been driven by large-scale processes such as the slow drift of continents and the rise of mighty mountain ranges like the Himalayas.

Big changes have occurred on shorter time scales, as well, as revealed by studies of ancient ice, tree rings, and other indicators of past conditions. During the last 1.8 million years, for example, glaciers and ice sheets have repeatedly advanced across large swaths of the continents and lingered for tens of thousands of years, retreating during shorter warm intervals. We live in such an “interglacial period” right now. Scientists have linked this pattern to recurring variations in Earth’s orbit and the tilt of the planet’s axis. By affecting the amount of sunlight reaching certain parts of the Earth, both of these factors are seen as playing a significant role in causing the cooling and warming that trigger ice sheets to expand and contract. Resulting changes in the atmospheric concentration of greenhouse gases have been shown to amplify the upward and downward changes in temperature as well as to spread the impact from pole to pole.

On occasion, unusually large bursts of volcanic activity, collisions between Earth and an asteroid or comet, and other rare events have jolted Earth’s climate system, producing substantial cooling or warming by temporarily altering the composition of the atmosphere in ways that reflect sunlight or trap it in the form of heat.

On even shorter time scales varying from centuries down to decades, years, and months, the climate also exhibits substantial variability. Some cool spells in past centuries appear related to periods of reduced solar activity, such as sunspots and solar flares. Other variations, some lasting a century or many decades, appear related to combinations of slow changes in ocean currents and patterns of atmospheric pressure. For example, NASA scientists provided evidence[1] that a rare combination of unusually cold Pacific Ocean temperatures and Atlantic Ocean warmth triggered the disastrous Dust Bowl drought in the American Great Plains from 1931 to 1939.

On the scale of individual seasons, the turbulent and chaotic nature of the swirling atmosphere, in concert with variations in ocean conditions, can lead to unusual episodes of severe cold, heat, snow, rain, and storminess. They can also induce periods of relative calm.

Simple chance also drives climatic variability. For example, unusually warm sea surface temperatures, which are highly favorable for spawning and sustaining a powerful hurricane, may be in place. But other factors present at the same time may reduce the ferocity of any storm that gets going. Wind shear, a marked difference in wind speeds moving up through the atmosphere, is one such factor. The phenomenon tends to prevent hurricanes from gaining power.

Amid all these natural influences on climate, a new one has arrived: the rapid buildup of greenhouse gases[2] added to the atmosphere by people.

1.3 The Human Factor

Through most of human history, the relationship between climate and people was a one-way street. Rainfall or temperatures changed; ice sheets, coastlines, and deserts advanced or retreated; and communities thrived, suffered, or adapted by changing how or where they lived.

Now the relationship goes in two directions. Over the past few decades, a robust body of scientific evidence has shown that humans are having a significant and consequential impact on the climate, with more to come.

A 2010 report, Advancing the Science of Climate Change,[3] states the basics clearly and succinctly: “A strong, credible body of scientific evidence shows that climate change is occurring, is caused largely by human activities, and poses significant risks for a broad range of human and natural systems.”

Many lines of evidence support the conclusion that most of the observed warming since the start of the 20th century, and especially over the last several decades, can be attributed to human activities and not to natural factors.

Among these, as described in the 2010 report, are:

● Both the basic physics of the greenhouse effect and more detailed calculations dictate that increases in atmospheric greenhouse gases should lead to warming of Earth’s surface and lower atmosphere.

● The vertical pattern of observed warming — with warming in the bottom-most layer of the atmosphere and cooling immediately above — is consistent with warming caused by greenhouse gas increases and inconsistent with other possible causes.

● Satellite measurements conclusively show that solar output has not increased over the past 30 years, so an increase in energy from the Sun cannot explain the extent of recent warming.

● Direct measurements likewise show that the number of cosmic rays, which some scientists have posited might influence cloud formation and thus the climate, have neither declined nor increased during the last 30 years.

While much has been learned, important questions persist about human-driven climate change and its impacts: What quantity of greenhouse gases will humans produce in coming decades? Just how much will the planet be warmed by a given rise in concentrations of these gases? How fast will seas rise as ice sheets on land melt?

And there’s another challenge: The variability inherent in the climate system means there could be plenty of temperature jogs, both cooler and warmer,[4] even as the human warming influence builds. No one storm, drought, or exceptionally snowy winter can be attributed to human-driven climate change. But as researchers learn more over time, we analysis of patterns and dynamics in such events can help determine which are most affected by the building greenhouse effect. UPDATE extreme events attribution report

The complicated nature of the climate system fuels persistent debates about both the level of danger posed by human-driven global warming and possible responses. But there is no serious debate about the basic science. Researchers with substantially different personal interpretations of the level of danger agree on several key conclusions:

● Greenhouse gases make the planet warmer than it would otherwise be.

● A sustained buildup of these gases will substantially heat the planet.

● Warmer conditions through Earth’s history have come with significantly higher sea levels and big changes to ecosystems.

1.4 Climate Dynamics

The Earth’s climate is complex and turbulent. But science has gone a long way toward revealing factors, both within the system and affecting it from without, that can influence conditions over the short term and have long-lasting impacts. Among the important elements shaping climate are forcings, feedbacks, lags, and tipping points.

1.4.1 Climate Forcing

The term “forcing” applies to those natural or human-caused influences, such as a rise or drop in solar activity or an increase in CO2 emissions, that upset the energy balance in the climate system. Until balance is restored, the system will remain in a state of flux. Positive forcings, such as added greenhouse gases, warm the system. Negative forcings, including many of the kinds of particles (or aerosols) lofted by volcanic eruptions or the combustion of fuels, reduce incoming energy from the Sun and thus cause cooling. Generally, scientists understand how greenhouse gases warm the planet better than they understand how various aerosols cool it[w1] .[5]

Particularly in the last half century, a growing push on the system has come from the buildup of human-generated greenhouse gases, emitted mainly by burning fossil fuels and cutting forests. Carbon dioxide (CO2) is the most important human-generated greenhouse gas, in large part because of the amounts released — measured in the billions of tons annually in recent decades — and CO2’s long lifetime in the atmosphere. A range of studies estimate that between 20 percent and 60 percent of carbon dioxide emitted by fuel burning[6] will stay airborne for a millennium or more.

A clear signal of the human influence on the atmosphere is the sharp rise in the concentration of carbon dioxide that started with the advent of the Industrial Revolution. For at least 800,000 years, the concentration varied between 180 and 280 parts per million (ppm). The concentration rose through the 19th and 20th centuries, reaching 350 ppm around 1988, just when human-caused global warming first became a topic of wide interest. In 2013, the concentration passed 400 ppm.

The next most significant human-generated greenhouse gas is methane (CH4), produced by everything from livestock herds and rice paddies to emissions from oil and gas wells, along with a host of natural sources. (Natural gas used for heating, cooking, generating electricity, and other purposes consists almost entirely of methane.) Once in the atmosphere, most methane is broken down in less than a decade by chemical reactions.

But while aloft, methane’s potent heat-trapping ability makes it the second most important contributor to warming. The concentration of methane in the atmosphere as of the end of 2010 was in excess of 1,750 parts per billion (ppb)[w2] , up from just 700 ppb before the Industrial Revolution.[7] If parts per billion seems like a trivial amount, consider that each methane molecule is about 24 times more effective than a carbon dioxide molecule at trapping heat when their greenhouse impacts are measured over a 100-year span.

Humans also contribute other greenhouse gases to the air, including nitrous oxide and various synthetic compounds containing chlorine and fluorine. Although some of these are extraordinarily rare, they are thousands of times more effective at trapping heat than carbon dioxide. Production of many of these synthetic chemicals is being phased out because they damage the veil of protective ozone in the stratosphere, contributing to what has been called an “ozone hole” over Antarctica in winter. (Although this reduction in ozone high in the atmosphere does not measurably increase global warming, the reverse may be true — warming may make the ozone losses somewhat worse.)

1.4.2 Feedback Processes

In Earth’s climate, an initial warming can cause other changes that reinforce the warming, resulting in a cycle that can lead to considerably higher temperatures than the initial rise. This is what scientists call a positive feedback.[8]

The reverse can occur as well. An initial change in the climate may result in negative feedback, meaning a response that tends to counteract the change.

The most important positive feedback mechanism in Earth’s climate system involves water vapor — the most abundant greenhouse gas in the atmosphere. A small warming from, say, a buildup of carbon dioxide (CO2), is amplified substantially as more water evaporates from the seas, increasing the level of water vapor in the air, which contributes to greater warming. Positive feedbacks like this are thought to lead to more global warming from our carbon dioxide emissions than would otherwise be the case. And not by just a small amount.

As the Industrial Revolution began, carbon dioxide concentrations in the atmosphere stood at about 280 parts per million (ppm). Should atmospheric CO2 concentrations double to 560 ppm, that alone would warm the world by about 1°C. But the water vapor feedback from the added heat, by recent estimates, would roughly double that effect. Clouds, another consequence of adding water vapor to the air, add some uncertainty because they can produce a complicated mix of both cooling and warming (see below[w3] ).[9]

A rise in temperatures produces other feedbacks. Some are related to the reflectivity of Earth’s surface, known as its albedo. In the Arctic, for example, floating sea ice ordinarily helps reinforce cold temperatures. That’s because the bright ice reflects sunlight back toward space. But warming temperatures and changes in patterns of winds and atmospheric pressure in recent decades have greatly increased the amount of open water, especially in summer. This has allowed more of the Sun’s energy to be absorbed by the relatively dark ocean, thereby amplifying the warming in the region.

A negative feedback resulting from a warming-induced increase in water vapor could be the creation of more low-lying clouds, especially over ocean areas. As in the case of sea ice, such clouds would have a cooling effect on the Earth by reflecting solar radiation back to space. This negative feedback might offset some of the initial warming associated with rising amounts of water vapor. Still, under a push from accumulating greenhouse gases, the additional warming from changes to Earth’s albedo, along with other positive feedbacks, is projected to roughly add another 1°C of warming.

Biological feedbacks can also kick in under sustained warming. One such feedback involves permafrost — deeply frozen soils in places like Alaska, northern Canada, and Siberia. Warming already is causing permafrost to thaw. When this happens, microbes begin breaking down organic matter in the now thawed soil, releasing carbon dioxide into the atmosphere in drier locations, and methane in wetter ones. Recent research suggests there is far more carbon stored in Arctic lands than previously estimated.

1.4.3 Inertia and Lags in the System

Earth’s atmosphere and oceans have properties that can both delay and prolong climate change. This is particularly true when the shift is driven by fast-rising levels of a long-lived greenhouse gas like carbon dioxide.

The enormous capacity of the oceans to absorb heat has meant that about 80 percent of the energy added to the climate system so far through the greenhouse buildup has gone into the seas instead of the air.

It can take decades for this heat to fully mix in relatively shallow layers of seawater. Then it can take centuries more for the heat to move into the depths, circulate in currents around the world and eventually resurface.

The heat absorbed by the seas doesn’t go away; it commits the planet to extra warming later on. And this is no small issue. Even if emissions were cut sufficiently right now to stop the buildup of greenhouse gases, heat banked in the oceans will continue to warm the atmosphere for several centuries.

The “commitment”[10] to warming from this process could raise the global average temperature another 1°C over the long run.[11] In considering climate policies, this also means there really can be no quick fix. Just as important, the commitment issue points to the importance of making societies resilient to a changing climate — since we will continue to experience changes even if we were to stop all greenhouse gas emissions immediately.

The climate impact of the slow release of heat banked in the oceans is compounded by another factor: the long lifetime of carbon dioxide in the atmosphere. As long as it is present, this gas traps heat moment by moment. And as with the ocean heat-bank effect, carbon dioxide’s longevity in the atmosphere guarantees a long period of warming — many centuries, in fact — even if we were to stop emitting the gas tomorrow.

This effect also makes stabilizing the concentration of CO2 in the atmosphere at a manageable level particularly challenging. To understand why, it is helpful to imagine the flow of water into a bathtub with a partially open drain.[12] As water pours in, some of it goes down the drain, slowing the inexorable rise of water in the tub. Similarly, as we add carbon dioxide to the atmosphere by burning fossil fuels, some is being absorbed by plants and the oceans.

Even so, like the bathtub that continues to fill, the climate continues to warm — just at a slower rate than would otherwise be the case. Unfortunately, we can’t depend on this effect forever because the ability of the oceans and ecosystems to keep absorbing carbon dioxide may be weakening. In essence, the drain seems to be starting to clog. And if that’s true, just as a bathtub with a clogging drain would overflow sooner than it would otherwise, at some point when the oceans cannot absorb more carbon dioxide the climate may begin to heat up more rapidly, bringing on disruptions more quickly.

The factors that delay and prolong warming add up to a sobering reality: The warming of our planet is building in ways that could be difficult to reverse.

1.4.4 Tipping Points

A persistent concern in assessing risks from greenhouse-driven heating is whether and when resulting changes in the climate could trigger disruptive, largely unstoppable changes in systems or conditions important to people or ecosystems. .

Research published in 2008[13] and 2009[14] in the Proceedings of the National Academy of Sciences tried to narrow the definition of various “tipping elements,” as well as the probability that they were poised to tip. A number of thresholds could conceivably be crossed in this century, making them relevant to climate policy debates. For example, ice loss from Greenland and West Antarctica could quicken, resulting in an accelerating and prolonged rise in sea levels. The research also identified biological thresholds that could be crossed by the year 2100 resulting in the loss of the great northern “boreal” forests and transformation of the Amazon rain forest into a drier ecosystem.

The reduction in floating sea ice in the Arctic region in summers is another threshold issue. Some researchers have proposed that the expansion of open water in summers would allow more and more solar heating of the exposed, dark ocean surface, thereby delaying winter re-growth of ice and locking the system on a path to a largely ice-free Arctic in summer. If this were to happen, the change might be a boon to energy development and transport of goods in the far north. But it could also imperil ice-dependent wildlife, alter weather patterns around the Arctic, and contribute further to global warming.

How likely is it that the world will cross such thresholds? Research has not yet provided a definitive answer. In 2011, for example, new modeling studies[w4] [15] cut against the prospect of a tipping point in Arctic sea ice. What scientists can say with confidence, however, is that with rising temperatures come rising odds of unpleasant surprises.

2 Climate Change Research

Our understanding of how Earth’s climate works — and how it is changing — has evolved over decades of scientific research, some of it dating back more than 100 years. Everything from ice cores to tree rings, satellites to supercomputers, and weather balloons to submarines has helped fill out the details of the story. Although many basic facts and processes are understood, there is still a great deal to learn.

2.1 Studying Climate

Broadly speaking, scientists have four basic ways of understanding how humans are affecting the planet’s climate:

Physics: The basic physics of the greenhouse effect has been understood for more than a century. This is the starting point.

Paleoclimatology: Data mined from ancient ice core samples, tree rings, lake-bed and seafloor sediments, and other sources help reveal how climate change occurs naturally, including the role of greenhouse gases.

Observations: Real-time monitoring of rising greenhouse gas concentrations, and how it affects the climate and different environments, provides a current picture of our climate system at work.

Computer modeling of the climate: This technology helps give scientists confidence in their understanding of what’s going on and allows them to make sophisticated projections of anticipated future climate change under different scenarios.

Given the current public debate over whether humans are substantially influencing the climate by emitting greenhouse gases into the atmosphere, you might think this idea sprang only recently from controversial science. In reality, it rests on physics first identified early in the Industrial Revolution and developed in the late 1800s.[16]

The physics really are quite simple: The physical properties of atmospheric greenhouse gases make them transparent to sunlight. Incoming solar energy passes through the atmosphere mostly unimpeded, is absorbed at the Earth’s surface, and then is re-radiated upward as infrared radiation. If you’ve ever felt the heat emanating in the evening from an asphalt parking lot that has baked in sunlight all day, you’ve experienced this effect.

But those same atmospheric gases have another key physical property: They absorb the infrared energy coming up from the Earth’s surface, and then re-radiate some of it back down. This causes more heat to be retained in the atmosphere than would otherwise be the case, thereby warming the Earth.

This basic finding pointed toward the potential for global warming[17] from human-generated greenhouse gases, particularly carbon dioxide from burning fossil fuels. But would those greenhouse gases actually build up in the atmosphere?

In 1957 Roger Revelle[w5] [18] and Hans E. Suess[19] of the Scripps Institution of Oceanography helped answer that question. They were interested in the capacity of the world’s oceans to soak up some of the carbon dioxide humans were adding to the atmosphere. By analyzing different forms, or “isotopes,” of carbon, and employing other means, they determined that the oceans actually had a limited capacity to store carbon dioxide.[20] Just before their paper was sent for publication, Revelle added a line that has been heavily cited ever since: “Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future.”

The “experiment” Revelle referred to consists of a substantial buildup of carbon dioxide in the atmosphere, and the experimental subject is the climate. By 1960, the results of the experiment were becoming clearer. That year, Charles David Keeling of Scripps published a scientific paper[21] documenting a stark fact: carbon dioxide was, indeed, building up in the atmosphere. Keeling’s systematic measurement of carbon dioxide concentrations[22] over the ensuing years produced an iconic graph now dubbed the “Keeling Curve.”[23] This graph shows in objective detail just how much of an impact humans have been having on carbon dioxide concentrations.

For a long while the question of what all that extra atmospheric carbon would do to the climate was confined mainly to scientific circles. But by the late 1980s, that had changed. In 1988, as public attention to greenhouse-driven warming[24] was growing, the United Nations formed the Intergovernmental Panel on Climate Change (IPCC),[25] the first scientific body established to offer periodic assessments of an emerging field of science to policymakers.

The word “assessment” is important. It means that instead of conducting its own scientific research, the IPCC reviews, analyzes, and evaluates existing scientific, technical, and socioeconomic information on climate change. It does not make policy recommendations. Rather, it attempts to provide a credible baseline of information that can help governments weigh options for addressing climate change and it helps them understand the possible outcomes of those various options.

Since it was formed, the IPCC has produced five detailed sets of assessment reports — roughly every six to seven years. Through two decades, the panel’s core conclusions about a rising human role in driving climate change have moved from tentative to nearly conclusive. In its fifth assessment, released in 2013 and 2014,[26] there was virtually no doubt remaining about a core conclusion: By adding greenhouse gases to the atmosphere, humans are causing the average temperature of the Earth to rise, and further emissions will cause temperatures to rise further, increasing the risk of disruptive impacts to ecosystems and human societies.

The IPCC is by no means alone in reaching this bedrock conclusion. The National Academy of Sciences has produced more than two dozen reports on humans and climate in recent decades, with a similar trajectory toward high confidence in the basics of the problem. In a 2009 report, “Global Climate Change Impacts in the United States,” the federal Global Change Research Program echoed the IPCC findings:

“Impacts are expected to become increasingly severe for more people and places as the amount of warming increases. Rapid rates of warming would lead to particularly large impacts on natural ecosystems and the benefits they provide to humanity. Some of the impacts of climate change will be irreversible, such as species extinctions and coastal land lost to rising seas[JV6] .”

But just what are those impacts and how much should we be worried about them? From the possible pace of sea-level rise to the specific impacts on particular regions and even the precise extent of warming itself, many aspects of climate change are not fully understood. Scientists continue to probe the past for clues, observe current conditions for telltale patterns, and simulate possible outcomes using basic theories and powerful computers.

2.2 Probing the Past

Earth’s history holds important lessons about factors that can influence our planet’s climatic life support system. No one was around with a thermometer or other instruments many thousands of years ago to determine just how much, and why, temperatures and weather patterns varied. So scientists seeking to understand the extent of past climate changes and the forces behind them have to use indirect means. Among these so-called “proxy records”[27] are the patterns of growth seen in ancient tree rings. The width of these rings, which reflect the quality of each growing season, can reveal information about past changes in both temperature and precipitation. Similarly, stalagmites — those formations that grow from the floor of caves like upside-down icicles — also contain clues to past shifts in precipitation, revealing periods of ancient drought as well as times of plenty.

Ice cores drilled from mountain glaciers and the vast ice sheets of Greenland and Antarctica contain valuable samples of the ancient atmosphere, as well as the chemical fingerprints of temperature changes. Using these records, scientists can reconstruct the concentration of greenhouse gases in the atmosphere, along with patterns of temperature change, going back as far as 800,000 years. And sediments extracted from the beds of lakes[28] and seas can extend the climate picture back millions of years.[29]

Surprises continue to emerge from this work. In 2006, for example, the first deep cores extracted from the Arctic Ocean seabed near the North Pole revealed that 55 million years ago, surface waters there in summer were a balmy 74 °F (23.3 °C).[30] This was a hot, climatically turbulent period called the Paleocene-Eocene Thermal Maximum.[31] Research suggests that a burst of greenhouse gases (from sources still undetermined) warmed the planet some 4 to 8°C in as little as 5,000 years — a very brief moment in geological time. This rapid warming triggered enormous environmental changes[32] around the world that persisted for several million years. In fact, about 49 million years ago the Arctic Ocean surface appears to have been matted with an ancient precursor to the Azolla duckweed that sometimes cloaks contemporary ponds.

Research like this has made enormous contributions to our understanding of climate change. But given that greenhouse gas concentrations could within 100 years reach levels unequaled on the planet for 30 million years, a more concerted exploration of climate on the scale of millions of years is warranted, a panel convened by the National Academy of Sciences concluded in 2011. The panel called for a comprehensive program[33] to examine deep-time climate change. One goal would be to focus on early periods when concentrations of greenhouse gases in the atmosphere were much higher than at present. Such work could help clarify just how sensitive the climate is to rising levels of greenhouse gases now. Another outcome could be improved understanding of the potential for current emissions trends to push the climate across a “tipping point,” triggering changes so abrupt that society would have difficulty adapting.

Research on far more recent climate history, just within the last 5,000 to 10,000 years or so, holds important implications for contemporary civilization. Sobering findings from the American Southwest and sub-Saharan Africa[34] suggest that communities in these regions should prepare themselves for protracted drought, which could occur as a result of natural variability or human-caused climate change. Proxy records, including tree rings and sediment cores extracted from lakebeds, show that both regions have seen extraordinary periods of intense dryness, some lasting more than a century, in recent millennia. In other places, including the northeastern United States, studies of layers of storm-washed gravel in lakebeds reveal past periods in which episodes of extreme rainfall have been far more severe and more common than at present.[35]

These changes in the distant past occurred because of natural climatic variation. But that does not necessarily let us off the hook regarding humans’ role in causing harmful climate change. It is possible that our activities could equal or augment a previous era’s natural climate influences and trigger the same kinds of dramatic changes that occurred naturally in the past. For example, in the American Southwest, researchers employing a mix of evidence from the past and from computer simulations, have argued that greenhouse-gas-driven warming is now tipping conditions[36] into a new period of sustained dryness.

2.3 Observing the System

Studies of past atmospheric and climate conditions provide an important foundation for setting the human factor in context and charting possible futures. But another vital need is for careful and sustained observation of conditions right now — from the composition and dynamics of the atmosphere to the behavior of ice sheets, from the temperature of permafrost to the chemistry of the seas.

Since the mid-twentieth century, when Charles David Keeling began his pioneering measurements of CO2, monitoring efforts have expanded dramatically. Today there are more than 140 sites worldwide[37] — from the South Pole to the Arctic — where atmospheric monitoring is done. In addition to this, airborne sampling is carried out at more than 30 locations.

What about temperature? Through the 19th and early 20th centuries, reliable readings of the temperature on land and on the ocean surface were sparse and the measurement methods fairly unsophisticated. To monitor the temperature of the ocean surface, for example, mariners would simply drop a bucket overboard and haul up a water sample for a thermometer reading. This may have provided a reasonable measurement, but vast portions of the oceans went unmonitored.

Since then, independent research groups in the United States, Britain, Japan, and elsewhere have compiled consolidated archives of temperature data spanning large portions of the globe. And with this information, they’ve produced a clear view of a warming planet: These data show that during the first decade of the 21st century, the Earth’s average surface temperature was 1.4ºF (0.8ºC) warmer than it was during the first decade of the 20th century, with most of the warming occurring in the past thirty years.

But scientists would like to be able to track more rapid changes occurring on smaller geographic scales. They’d also like to be able to make data-sharing easier in order to facilitate more detailed and transparent analyses. To achieve these goals, an effort is under way to create a single data archive[38] where all instrumental temperature records could be maintained and made freely accessible.

Dozens of satellite missions have also yielded valuable findings about the climate and how it is changing. Some satellite sensors measure tropical rainfall[39], which could change as the world warms. Others focus on water vapor,[40] a potent greenhouse gas. A Japanese satellite, Gosat,[41] is measuring concentrations of carbon dioxide and methane from orbit, helping researchers determine how these greenhouse gases are distributed around the globe. And NASA’s Grace mission[42] measures variations in Earth’s gravitational field that can reveal changes in ice sheets, sea level, and other conditions. This project is helping scientists get a more accurate picture of how much ice is being lost in Greenland and Antarctica, and thus is essential to projections of sea-level rise.

Meanwhile, microwave sensors on satellites, called sounding units, have tracked the temperature of broad slices of the atmosphere. After competing groups worked out some differences[w7] ,[43] the data from these sensors helped confirm that the troposphere, the layer closest to the Earth’s surface, was warming while the higher layer, the stratosphere, was cooling. This pattern was precisely the fingerprint scientists expected to find if their theories about warming from a buildup of heat-trapping carbon dioxide and other gases were correct.

The vast oceans, which play such a vital role in absorbing and transporting heat, had been only sparsely studied until the last couple of decades. But the Argo program[44] — deploying thousands of instrument-laden buoys that descend and rise in the ocean waters, collecting data at various depths — is starting to fill the gaps. Even at the North Pole, where a sheath of drifting sea ice presents a daunting obstacle to research most of the year, scientists have been deploying an anchored two-mile-long strand of instruments[45] each year to measure changes in the Arctic Ocean.

Monitoring is vital not only to improve understanding of the climate but also to bolster human capacity to limit harmful impacts as conditions change. Unfortunately, some parts of the world with the most human exposure to both flooding and drought risks, particularly sub-Saharan Africa, have barely any capacity to collect and disseminate climate information. And even in industrialized countries, tight budgets and competing priorities have eroded many climate-monitoring programs.

In its 2010 report, Advancing the Science of Climate Change, the National Research Council stressed the importance of reviving such systems and filling data gaps, saying that observations are “critical for developing, initializing, and testing models of future human and environmental changes, and for monitoring and improving the effectiveness of actions taken to respond to climate change.”

2.4 Climate Modeling

A traditional scientific approach to assessing the consequences of rapidly rising concentrations of greenhouse gases would be to conduct Roger Revelle’s “geophysical experiment” against a controlled case with no such changes — on a parallel planet. But there is only one Earth — with no parallel planet available — and the experiment is already well under way here.

The next best option is computer simulation of the climate system, examples of which are incorporating more and more relevant features of the oceans and ecosystems. Over the last half-century, these mathematical marvels — some now consisting of several hundred thousand lines of computer code — have grown from rudimentary representations of the flows of energy and mass through the atmosphere and ocean surface to increasingly complicated replicas of the dynamic Earth. Some now include the flows of carbon to and from forests, soils, and seas, the coming and going of floating sheaths of polar sea ice, and other features.

So far, models have been used mainly to project possible future conditions under various scenarios for greenhouse gas emissions. They’ve also been used in so-called “attribution” studies intended to clarify the role of greenhouse gases amid all the other influences on climate — from variability in the Sun’s energy output to shifts in ocean conditions. In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) concluded with high confidence[w8] [46] that human-generated greenhouse gases have been the dominant influence driving global warming since 1950. And computer simulations played a significant role in the research that led to the IPCC’s conclusion.

Many seasoned model designers and researchers stress that this technology is part of what the climatologist Ronald Stouffer has called the “triangle” of observation, models, and theory. Advances in each arena help move overall understanding of climate change forward, but progress is hard to achieve without all three.

Researchers have not yet been able to narrow significantly the range of possible warming scenarios from a given buildup of greenhouse gases — a response called “climate sensitivity.” Part of the problem is that climate models are not yet good enough to simulate the impact of clouds and particles in the air. As a result, the 2007 [w9] report from the IPCC projected essentially the same range of warmer futures[w10] [47] from a doubling of atmospheric carbon dioxide over the pre-industrial concentration as was sketched out in a 1979 report from the National Academy of Sciences.[48]

New challenges confront climate modelers as they try to move from using simulations to analyze changes and project a range of futures toward using them for actual climate forecasts on scales of decades or even a century. The quality of a forecast depends not only on the ability of a model to behave like the real planet, but also on how well the initial conditions set up at the start of a simulation match conditions around the planet. Small variations at the outset can lead to big changes, and errors, as a simulation unfolds.

Nonetheless, as the climate changes, pressure is building for scientists to move from analysis to prediction, a vital need for city planners and farmers, for instance. In such circumstances, managing expectations about the value of climate simulations may end up being almost as important as continuing the work and investment necessary to keep improving them.

2.5 Climate and People

Understanding the human impact on future climate change, on the one hand, and the impact of climate on people, on the other, is complicated by a simple fact: Modeling or predicting the behavior of humans [w11] [49] is a big challenge.

Future trends in populations, economics, energy choices, and land use will exert powerful influences on the sources and amount of greenhouse gas emissions in coming decades. Predicting how those trends play out is more difficult than simply extrapolating into the future. That’s because we don’t really know for sure how people will respond to stresses, and opportunities, as environmental changes and technological innovations unfold in a warming world.

Limiting the magnitude of human-caused warming will require research into how people make decisions when faced with risks where the worst case — think rapid sea-level rise or the high end of warming projections — is the reason to act but also the least certain outcome. Moreover, there are many possible responses, the benefits of which may not be apparent for quite some time, further complicating decision making.

To help with these challenges, scientists are moving to develop more layers in modeling and analysis. The result is what’s known as an “integrated assessment,” which can identify opportunities both for limiting our contribution to rising concentrations of greenhouse gases in the atmosphere and fostering resilient responses in communities that are potentially in harm’s way. One recent analysis of this sort[50] found that a lower rate of population growth through mid-century could provide 16 to 29 percent of the emissions reductions that some studies have concluded would be needed by 2050 to avoid dangerous climate change.

There is ample potential for human ingenuity and market forces to influence energy choices, and thus greenhouse-gas emissions paths, in surprising ways. For instance, in the wake of disruptions to the supply of oil to the United States in the 1970s, an intensive effort was made to analyze energy use through the rest of the 20th century. Almost all of the resulting forecasts underestimated the ability of American citizens and businesses to trim energy use.[51] More recently, the enormous, and unexpected, increase in natural gas production from shale rock is predicted to cut into the demand for coal for electricity generation, according to the Energy Information Administration. But abundant, cheap gas could also limit the prospects of renewable energy option, other studies have found.

All of this illustrates the importance of improving assessments of the human factor in climate analysis, as described this way in a 2010 report by the National Research Council:

“Human and social systems play a key role in both causing and responding to climate change. Therefore, in the context of climate change, a better understanding of human behavior and of the role of institutions and organizations is as fundamental to effective decision making as a better understanding of the climate system.”

3 Impacts of Climate Change

From the Arctic to the tropics, climate is changing in ways that could profoundly reshape ecosystems and human affairs. Ice is retreating. Rain forests and coral reefs are showing signs of stress. Growing seasons are expanding in temperate zones, yet the odds of scouring floods and severe heat are also rising in many populous places. Learn about some of the impacts that have experts most concerned.

3.1 Our Warming World

The climate has varied enormously through Earth’s history. But relatively stable climate conditions over the last two centuries helped sustain adequate food and water supplies even as the human population soared from 1 billion to nearly 7 billion. Now, as human numbers and resource appetites continue to rise, accumulating greenhouse gases are poised to push the climate system beyond that comfort zone.

Earth is warming. On this issue, climate science is unequivocal, thanks to well-documented increases in air and sea temperatures, along with observations of melting snow and ice. In terms of the global average temperature, it is likely that the second half of the 20thpast three decades were the warmestcentury was the warmest 305-year period in at least the pastof the last 1400 years13 centuries.[52] [JV12] Most of the warming of the climate since 1950 has almost certainly been driven by our emissions of carbon dioxide, methane, and other greenhouse gases, which have been steadily accumulating in the atmosphere since the dawn of the industrial era.

Of course humans do not really experience the average temperature of the entire globe. We experience climate right where we live — in the form of patterns of heat waves and cold snaps, droughts and floods, severe storms and quiet spells, not to mention the rhythm of the seasons. At this local level, change is increasingly evident as well.

In regions of the world where climate conditions have been carefully tracked over the last century, there is strong evidence of shifts that match what climatologists would expect to see from increased greenhouse warming. Heat waves have become longer and hotter while cold snaps have become shorter and less severe. More precipitation is coming in heavy bursts. The area covered by snow in the Northern Hemisphere has been declining. Rivers and lakes are generally freezing later and thawing earlier. Communities of animals and plants, as well as their ranges, are changing in ways that appear linked to warming and related shifts in precipitation. Sea levels are rising in most places as warmed seawater expands and melt water from eroding ice sheets and glaciers adds to the volume of the oceans.

It’s always possible that nature could throw us a climatic curve ball — some unexpected shift that could temporarily slow or reverse these changes. But given current understanding of how the climate system works, the long-term prognosis is for rising average temperatures. The prime reason? The concentration of carbon dioxide in the atmosphere today already appears to be at its highest level in 2.1 million years. And recent research suggests that without reductions in CO2 emissions, it could be headed toward its highest level in 20 million years.

Additional warming will occur even if deep cuts in emissions were somehow accomplished quickly. The long lifetime of important greenhouse gases and the slow release of heat banked in the oceans essentially guarantee that decades, and probably centuries, of human-driven warming lie ahead — although with plenty of ups and downs along the way.

3.2 Ecosystems and Natural Resources

Earth’s sheath of life is showing signs of being influenced by climate shifts attributed to the buildup of greenhouse gases. At the same time, important sources of water for agriculture and drinking supplies are being affected by warming, with a mix of harms and benefits.

3.2.1 Freshwater Resources

In temperate regions, including much of North America, climate change already appears to be influencing the hydrologic system — the source of the fresh water that humanity depends on. More precipitation is now coming in heavier downpours,[53] as was long predicted given that warmer air holds more water vapor. Lakes have been affected too, with many getting warmer. And runoff in glacier- and snow-fed rivers has increased, with peak flows occurring earlier in the spring. In turn, these changes are affecting the plants and animals that live in fresh water, prompting, for example, some fish populations in rivers to head to sea on their natural migrations earlier than before.

Looking toward the future, increasing greenhouse-driven warming is projected to trigger further shifts in precipitation and, over time, to shrink mountain snowpack and glaciers, which are frozen storehouses of fresh water. In general, wetter areas are expected to grow wetter while drier areas such as the American Southwest will dry more by mid-century, according to the latest federal report on “Global Climate Change Impacts in the United States”[JV13] and the 20137 report[54] of the Intergovernmental Panel on Climate Change. [JV14]

The IPCC also found that river runoff and water availability are expected to rise 10 to 40 percent at high latitudes and in some parts of the tropics. Meanwhile, a 10 to 30 percent drop in river flows is expected in some dry regions of the mid latitudes and the tropics. [JV15] For millions of people around the world, that could present big challenges.

A prime example in the United States is the Colorado River, a source of water for 35 million people living in seven states. (The Colorado also supplies water to Mexico.) Computer modeling suggests climate change could bring a reduction in flow in the Colorado Basin of 10 percent over the next 50 years. At the same time, both the frequency and duration of droughts are also likely to increase in coming decades. But the problem is not just one to be confronted decades down the road. Since 2002, in fact, total use of water in the Colorado Basin has actually exceeded the quantity of water flowing through it.[55][JV16] Disaster has been averted thanks to the hydrologic equivalent of savings banks: reservoirs. But as with a savings account, those hydrologic reserves could run very low if demand continues to grow while the naturally limited supply of water is put under stress by climate change.

Warming also has other impacts on fresh water supplies. It increases evaporation from soil and water surfaces. Moreover, the response of plants to both climate changes and additional carbon dioxide, which is used in photosynthesis, can potentially draw down groundwater. At the same time, changes in land use, such as paving over urban areas or replacing forests with fields, affects the local climate, flooding patterns, and water availability.

Challenges from climate change are expected to be particularly acute in the world’s poorest regions, particularly sub-Saharan Africa, where there is little capacity to cope with either too much or too little rain and population growth rates remain high, putting more people at risk from today’s climate variability, let alone any changes that are coming.

Granted, general projections of precipitation trends are not much help to particular communities, and substantial uncertainty about impacts on water resources in many rich and poor regions of the world is likely to persist. This reinforces the importance of building capacity on a regional or even local level to conserve and manage water in a warming world.[56]

3.2.2 Ecosystems and Biodiversity

Around the world, biologists have measured substantial changes in the ranges and habits of species, as well as the composition and condition of ecosystems, which appear to be related to altered patterns of temperature and precipitation stemming from the building greenhouse effect. In some cases ecosystems are being disrupted as the timing of interrelated and synchronized events — like the flowering of a certain plant and the emergence of a specific pollinating insect — is thrown off by shifting climate conditions, resulting in damage, in this case, to both plant and insect communities. Some of these changes could have significant impacts on the services humans derive from nature. More research is needed to clarify such impacts.[57]

In many instances, discriminating the contribution of climate change amid other influences on species remains a challenge. In Central America, for example, human-driven warming was at first blamed for the demise of some frog species. But a fast-spreading fungus is now thought to be the culprit.[58] The connection to climate change, if any exists, is unclear. The uncertainty comes in part from the large natural swings in conditions in the region during El Niño events and a lack of long-term weather data.

In other instances the fingerprint of human-driven warming on ecosystems and biodiversity is emerging. A decades-long trend toward reduced summertime sea ice floating on the Arctic Ocean, linked in large measure to the building greenhouse influence, has caused problems for polar bears and walruses.[59] At the same time, this change has also prompted some whales and salmon to expand into Arctic waters.

In mountainous regions, the ranges of some insects, plants, mammals, and other species are substantially shifting in ways linked to temperature or precipitation changes. Wolverines, while still abundant in Canada, are progressively vanishing from the lower 48 states. The U.S. Fish and Wildlife Service cited climate change as a justification for listing this species as threatened under the Endangered Species Act. (The agency has deferred formal listing while it focuses on other species that are considered a higher priority.)

In the oceans, substantial changes in the distribution of some fish species are evident, though a lack of long-term records complicates efforts to attribute the shifts to recent climate change. While a wide array of coral species have endured tens of millions of years of change, iconic reefs are subject to increasing stress through a combination of warming waters, fishing, and coastal pollution. In addition, the alkalinity of ocean waters is dropping as carbon dioxide absorbed in seawater forms carbonic acid. Laboratory and field studies point to rising risks to corals, some plankton, and other marine life, as this change in the pH of seawater impedes the ability of organisms to form reefs and shells.

3.2.3 Agriculture, Forests, and Fisheries

In agriculture, forests, and fisheries, different parts of the world are expected to see profoundly different outcomes. Farms in some of the most troubled regions, where populations are rising fastest and poverty is persistent, will likely experience more climate stress. In some parts of Africa adjacent to the Sahara Desert, warmer and drier conditions have already reduced growing seasons and contributed to flat or reduced crop yields. Central and South Asia are projected to experience up to a 30 percent decline in yields by 2050.[JV17] But a 20 percent rise in agricultural productivity is foreseen in eastern and Southeast Asia.[60] [JV18]

The 2014 federal report on impacts of climate change in the United States found that while a rise in carbon dioxide levels, fewer frosts, and rising temperatures could benefit some crops in some regions, many threats to food production will likely intensify with warming.[61] [JV19] A faster-growing cereal crop, for instance, puts less energy into production of the grain itself, reducing yields. And while elevated carbon dioxide levels cause some plants to use water more efficiently, increased frequency and intensity of droughts and hot spells, along with downpours, can disrupt planting and pollination.

A mix of influences will likely shape the fate of temperate and boreal forests. The substantial recent warming in large stretches of the Arctic has already spurred the expansion of forests into areas that were long covered in brushy tundra, with much more northward expansion foreseen.

In the American West, a variety of factors have led to a four-fold increase in large and long-burning forest fires over the past 30 years.[62] There, earlier snowmelt, increased drought, and rising temperatures have fostered combustible conditions. And warming has increased the length of the fire season by more than two months in some locations.

Warming has also benefitted forest pests in the American West — most especially pine beetles that kill trees. As trees there have become stressed by warmer temperatures and drought, widespread infestations of pine beetles since 2002 have killed tens of thousands of square miles of conifer forests.

In the Amazon River basin, there is evidence that drought patterns are being exacerbated by global warming, with some studies projecting an ecological shift away from the rain forests that have cloaked the region for millennia toward a drier forest type. If the fragmentation of forests in the tropics is reduced, however, some biologists see regions like the Amazon maintaining their vigor for decades to come, even with some drying.[63]

Fisheries in fresh and salt water are expected to change with changing conditions. Already, communities along Alaska’s Arctic shores have noted salmon spawning runs moving into Arctic rivers that had no history of such activity. Elsewhere, important fish stocks, such as Peruvian anchovies, are clearly sensitive to natural climate variations like the warm and cool Pacific Ocean cycles characteristic of El Niño and La Niña. But overall, there is substantial uncertainty about how global fisheries will respond to a warming climate.

3.3 Communities and Infrastructure

Human lifestyles and habitats — from schoolyards to skyscrapers — are largely shaped around the climate conditions that have prevailed through the last couple of centuries. The world is entering a period in which, if projections hold up, the new climate norm has changed. What are the impacts already happening and what’s likely in the future?

3.3.1 Coastlines

One of the most basic responses of Earth to a warming climate is a rise in sea levels, both from expansion of warming seawater and melting water and ice from eroding glaciers and ice sheets.

In developed countries, experts foresee substantial costs as cities gird against the increasing odds of severe storm flooding that come with each inch of rise in sea level. Areas vulnerable to tropical cyclones, such as river deltas, could face an added risk given the likelihood, derived from computer models and recent observations, that climate change will cause more hurricanes and typhoons to reach the most powerful categories and produce greater rainfall totals.[64]

More than 600 million people live 10 meters (33 feet) or less above sea level. Hundreds of billions of dollars worth of infrastructure lies exposed to a rising risk of inundation. Depending on the pace and extent of sea level rise, developed countries may be able to adapt by building coastal defenses, and over the long run, moving some developed areas inland. But less well-off nations may not have the resources to be so ambitious in the face of climate change.

In developing countries the main threat from sea level rise is to poor communities living in highly vulnerable areas along the coasts. Meanwhile, some nations consisting solely of low-lying atolls face an existential peril: If sea level were to rise by up to 2 meters in this century, which is at the high end of projections, hundreds of small islands would disappear beneath the waves.

Wedged between rising seas and human communities, coastal ecosystems such as marshes and wetlands are likely to face a space crunch. Along parts of the East Coast, the saltmarsh sharp-tailed sparrow, for example, could see its habitat vanish entirely within a few decades under the twin threats of sea-level rise and coastal development.[65] Shrinking wetlands can also result in the loss of valuable ecosystem services such as water purification, flood control, and shoreline stability.

3.3.2 Cities and the Built Environment

More than half of humanity now resides in cities or towns instead of rural areas. With much more urbanization coming in the next several decades, there is a greater need than ever for urban buildings and systems to be able to cope with changing climate conditions, including more frequent extreme heat or rainfall.

In developing countries, where less than 2 percent of the cost of catastrophes[66] is typically covered by insurance, urban flooding can slow economic growth. The rapid growth of many cities has made it difficult to detect evidence that climate change is contributing to flood-related losses. But there is no question that the kinds of weather-related disasters that are projected to worsen in a warming world can devastate poorly prepared communities.

The rapid growth of cities in the tropics poses a double-edged challenge: There is growing demand for air conditioning, amplified in part by warming; but this is also adding to greenhouse-gas emissions, thereby contributing to warming. One recent study[67] of trends in air conditioning use around the world found that just one city in India, Mumbai, is heading toward a demand for air conditioning equaling about one-fourth of all the air conditioning usage in the United States.

Some big cities have already begun investing to build resilience to climate extremes. For instance, Chicago[68] is implementing a climate action plan[69] that includes installing pavement that is permeable to water, planting heat-tolerant tree species, and mapping hot spots. Programs are under way in many California municipalities to transform as many roofs as possible from dark to white surfaces to reflect sunlight and thus keep both buildings and city air cooler.[70] On the other side of the globe, Kuala Lumpur, Malaysia, has a system of privately built highway tunnels that double as emergency storm drains when dangerous downpours occur.

3.3.3 Public Health

The impacts of climate change on health are likely to be felt most in communities with poor public health services and regions where people, through poverty or geography, are most exposed to the elements.

While deaths from exposure to cold are expected to drop, mortality from heat waves, particularly among vulnerable populations like the elderly and the poor, could rise. In the United States, heat waves are the leading causes of weather-related illness and death. The number of hot days and nights and protracted hot spells has risen in recent decades, with the frequency of such conditions projected to rise in a warming climate. Climate change could worsen air quality by promoting the formation of ground-level ozone, a contributor to smog. With warmer temperatures and more carbon dioxide, plants are expected to produce more pollen. For people with allergies or asthma (or both), this might make their conditions worse.

More frequent extreme weather conditions could lead to increased infiltration of water into indoor spaces. Dampness and water intrusion create conditions that encourage the growth of fungi and bacteria and may cause building materials to decay or corrode, leading in turn to harmful chemical emissions released into the indoor environment.

Another result of extreme weather events can be an increase in psychiatric disorders, such as anxiety and depression. This is likely due to disruption in the home environment and other economic losses.

The ranges of some diseases carried by ticks, mosquitoes, and other insects or rodents could expand as conditions favorable to those pests expand. Microbes that cause food- and water-borne illness, such as Salmonella bacteria and the organisms that cause cholera, could experience more favorable conditions in coming decades, through warmer temperatures and dispersal in more frequent heavy rains and flooding.[71][JV20]

In vulnerable developing countries, increases in malnutrition are likely where agriculture is disrupted by more frequent drought or floods. Almost everywhere, the negative health effects of climate change will either be counteracted or made worse depending on the extent of economic development and access to medical care.

3.3.4 Transportation

Human mobility, from shipping to aviation and highway traffic, can be greatly hampered or helped by shifts in climate conditions. In the Arctic, where warming has been more pronounced than in most other parts of the planet, both kinds of impacts can be seen. Thawing permafrost in Alaska has caused ground to subside, buckling roads and railways. But Arctic warming has also reduced the extent and thickness of sea ice, allowing some companies to start shipping goods from Asia to Europe along Russia’s northern sea route.

At lower latitudes, warming temperatures are expected to have less dramatic effects on transportation systems. In northern regions of the United States, warming winter temperatures will bring about reductions in snow and ice removal costs, lessen environmental damage from the use of salt and chemicals on roads and bridges, extend the construction season, and improve the mobility and safety of passenger and freight travel through reduced winter hazards.

Along with these potential benefits come problems, however, including increased stress in summer months on bridge joints, highway surfaces, and train tracks not designed for high heat. In many regions highway traffic is expected to be increasingly disrupted by flooding as the frequency of heavy rain rises. Changes in river flows, up or down, could limit barge and shipping traffic on the Mississippi River and other important waterways. Rising sea levels, according to several studies, will threaten billions of dollars of existing infrastructure at ports and coastal airports along the Gulf of Mexico and the Atlantic seaboard.

In the long run, climate-driven shifts in settlement patterns and the distribution of agricultural production and other economic activities could increase stress or congestion in some transportation corridors.

3.3.5 Energy Supply and Use[72][JV21]

Our current energy system, with its dependence on greenhouse-gas-emitting fossil fuels, is not only a contributor to climate change but is also likely to be affected by it. Research on the possible impacts on energy systems suggests a number of potential issues. A warming climate is likely to raise the demand for electricity for summertime cooling, creating the need for more generation capacity to meet peaks in use. Conversely, the demand for fossil fuels for winter heating will probably decline.

Reduced river flows in some regions, along with rising water temperatures, could hamper electricity generation at both fossil-fueled and nuclear power plants reliant on water for cooling. Water flows in rivers harnessed with hydroelectric dams may increase[73][JV22] or drop[74] [JV23] in different regions, impacting the output of electricity. Large-scale deployment of bioenergy may cause new stresses on water supplies for growing biofuel crops and processing them into usable fuels.

Changes in the severity and frequency of some disruptive weather events, such as hurricanes and ice storms, could affect the operation of a variety of energy installations, from electrical transmission and distribution lines to oil and gas drilling platforms. General changes in circulation and weather patterns may affect the efficiency of electricity generation by solar and wind farms. For example, increased cloudiness could reduce solar energy production, and wind energy production could be reduced if wind speeds increase above or fall below the acceptable operating range of the technology.

Within and among both industrialized and fast-growing developing countries, political and diplomatic disputes are intensifying as demand for fossil fuels clashes with concerns about the resulting buildup of greenhouse gases in the atmosphere. Policy changes could require significant shifts in the main sources of energy and in how energy systems operate.

3.4 Climate and Security

Shifting climate conditions are projected to increase the risks of conflict in several ways. In regions already struggling with a mix of poverty and vulnerability to drought or other climate extremes, intensification of weather conditions could tip the balance toward turmoil. In the Middle East and Asia, declines in river flows or aquifers could add to existing tensions among countries sharing water resources. Rising sea levels are a concern to countries adhering to the United Nations Convention on the Law of the Sea, under which marine economic zones are demarcated in relation to shorelines.

Rising seas and changing climate patterns could also increase migration within and between countries in poor vulnerable regions, increasing the potential for unrest or conflict.

The Defense Department has identified a variety of risks posed by substantial climate change to its installations and missions. Some are straightforward, including the impact of rising sea levels on coastal military bases.[75] Some are indirect. For example, the military is an enormous consumer of fossil fuels. Any factors that hinder access could undermine the ability of the military to carry out its missions. In addition, some potential impacts could strain Pentagon resources and require shifting priorities — one being potentially rising pressure to respond to climate-related disasters.[76]

Warming and the reduction of summer sea ice in the Arctic is opening up resources and shipping routes, increasing pressure on nations to add patrols and raising the odds of disputes over the control of waterways and economic boundaries. The United States lacks the vessels it would need to secure such waters.[77]

4 How We Respond

For decades, evidence for humans’ influence on the climate has been mounting. But societies, rich and poor, have been slow to shift to energy[JV24] habits and technologies that could cut heat-trapping emissions. At the same time, growth in human populations and economies is creating more exposure of people and property to climate and coastal hazards. Many steps — some difficult, others easy — could limit warming and vulnerability.

4.1 Limiting Climate Change

Human-driven warming is a building force that will almost certainly influence temperatures, precipitation, and other conditions for decades, and centuries, to come. Nonetheless, risks could be reduced through a sustained worldwide effort to cut energy waste and boost the affordability and adoption of energy technologies that produce few or no emissions. At the same time, communities could work to build resilience to climate change.

4.1.1 Reducing Energy Demand

Nearly every activity in modern life — from searching the Web to cooking a meal, from driving a car to manufacturing a solar panel — requires energy. But there are many affordable ways to reduce waste and make the most of the energy used.

Energy conservation comes through adopting technologies, practices, or policies that result in less energy used. Energy efficiency, discussed in detail elsewhere, is achieved by using less energy to accomplish a certain task. As one example, conservation is driving less; efficiency is driving a car that gets more miles to the gallon. The potential savings from both approaches is enormous. A variety of actions, using existing technologies and techniques to consume less energy overall and to use energy more efficiently, could cut United States emissions of greenhouse gases from household activities[78] [JV25] by up to 20 percent within 10 years with little or no perceived reduction in quality of life.

One large reduction, with the least need for behavior changes, would come from improving insulation and reducing air leakage in homes, cutting energy use and thus emissions by some 25 million metric tons a year. Other energy savings would require more conscious and sustained changes in behavior. For example, a similar reduction in emissions to that from home weatherization could result from shifts in driving behavior that reduce fuel use, or coordinating shopping and other excursions to get the most out of each trip. Even greater reductions, some 36 million tons a year, could come through carpooling. Any chance to do work from home or conduct a business meeting or class via Web video cuts greenhouse gas emissions related to travel.[79][JV26]

In parts of the world where both populations and economies are growing the fastest there is great potential to avoid ballooning energy waste and emissions of greenhouse gases. That’s because wasteful patterns of development and energy use are not as deeply rooted there as in the developed world. If urban expansion in China and India involves planning and investments focused on efficient building designs and access to public transportation, those countries can avoid the kind of sprawl and energy waste that is common in older cities elsewhere.[80]

Taking the broadest view of energy trends, there’s one other factor that can matter enormously — the number of people on the planet. Research[81] has found that expanded access to family planning and education for women, along with other policies known to slow population growth (while improving health and general welfare), could avert as much as 1 billion tons of global greenhouse gas emissions annually by the middle of the century. The population of the United States is likely to approach or surpass 400 million by mid-century,[82] mainly through immigration. That is a substantial driver of emissions growth, but here — as in many countries — there’s no simple path to policies that could counter such a trend.[83] Of course, the issue of population growth is highly contentious and societies are unlikely to try to control it simply out of a desire to do something about climate change. But should population growth be tempered for other reasons, emissions of greenhouse gases would be reduced.

4.1.2 Increasing Energy Efficiency

As with cutting energy use altogether, using energy more efficiently can be a valuable path toward limiting emissions of greenhouse gases. This is particularly true if such shifts are coupled with policies that limit the human habit of using something more if conservation makes it cheaper. In designing and maintaining buildings, in manufacturing, in farming, and in the transportation of goods and people, there are abundant opportunities to get more results with less energy.

In industry and transportation there are plenty of ways to cut energy use and emissions while making a profit, such as moving more goods that have to go long distances by train instead of truck. The fuel-saving benefit can be enormous[84] (from two to five times the amount of freight moved a particular distance per unit of fuel), and comes with reductions in smog-forming pollution and highway congestion.

Some oil and gas companies have profitably cut energy waste at the source — for example by stanching leaks of natural gas,[85] which is a valuable fuel in a pipeline but a potent heat-trapping substance in the atmosphere (because it is mostly methane).

The potential for improving energy efficiency in the United States is large. Already, dramatic improvements in the efficiency of everything from refrigerators to automobiles have come through tightened standards. Steady improvements are being made in lighting and air conditioning, as well. By replacing older technologies with newer ones that save both energy and money, energy use could be reduced by up to 30 percent below current 2030 forecasts, according to the National Research Council.[86] This would yield substantial cuts in fossil fuel use, as well as reductions in air pollution and emissions of greenhouse gases.

Achieving such results will require some combination of regulations, tightening energy or emission standards for appliances and vehicles, incentives, investment in research and development, and improved communication of information that can inspire behavior change. One barrier to a substantial shift toward more efficient technologies, even when they save money in the long run, is higher initial cost. Consumer education and financing options that spread the cost could smooth the transition to a more energy-efficient and low-emission future.

4.1.3 Low-Carbon Energy Sources

From the sunlight bathing the planet to the heat generated in the depths of the Earth to the power locked in atoms, there is vastly more non-polluting energy available than humans could ever need. But it would take new policies and sustained public and private investments to shift to such energy resources from the fossil fuels that have underpinned human progress for two centuries.

The main reason is that fossil fuels remain relatively cheap and abundant and provide a concentrated, portable source of energy while the costs attending an unabated buildup of carbon dioxide from burning fossil fuels lie mainly in the future. It should be no surprise, then, that fossil fuels provide about 85 percent of the world’s energy, with renewable sources — hydropower, wind, solar, biofuels, and geothermal — accounting for just 10 percent.[87]

Though there may be no “silver bullet” solution to the problem of satisfying rising global energy demand with affordable, low-carbon sources, progress could come from a combination of approaches that moves society in the direction of a more sustainable energy future.

Natural gas is an abundant fossil fuel that is substantially less polluting than oil or coal and produces fewer greenhouse-gas emissions per unit of energy. But extraction from vast deposits in shale presents other environmental risks that industry, states, and the federal government are just beginning to address.

Nuclear power, while generating no greenhouse gases once a plant is built, remains extremely costly to harness. In the wake of the Japanese nuclear crisis in 2011, this energy source faces substantial public unease. China is building several new nuclear power plants with a reactor design that is thought to be nearly impossible to destabilize. But even if the world moved aggressively toward deploying a new generation of nuclear plants, there is scant evidence this would curtail the use of cheaper coal.

The Sun is being harnessed in several ways, most notably through photovoltaic panels that convert sunlight directly into electricity and concentrating arrays of mirrors and other devices that heat fluids to drive generators. But solar power is hampered by the initial expense of the technology and a lack of cheap, large-scale systems for storing power until it is needed. For now, it is also starting from a tiny baseline, constituting about 0.1 percent of the energy used worldwide in 2008.[88][JV27]

Another way to exploit the Sun is to harness photosynthesis in crops or algae and generate biofuels. Substantial advances have been made in this area. But in many instances the amount of energy used to grow such crops and refine the resulting fuel greatly cuts into any climate benefit and also raises the cost of these alternatives. Around the world, the dominant biofuels are still firewood and dung. Research is under way to produce fuels more efficiently from the fibrous, or “cellulosic,” parts of plants or, in the long run, develop what amounts to artificial photosynthesis.

Turbines powered by the wind can play a role where conditions are right, and are cheap enough to compete with fossil fuels in some situations. But wind technologies face challenges as well. These include the dispersed nature of the energy resource, which would require hundreds of towers, along with related transmission lines, to equal the capacity of even one large coal-burning plant. As with solar power, wind power is intermittent, requiring a resilient electrical grid to handle shifting flows of electricity and large-scale storage systems for banking the power until it is needed. Research is in progress on both fronts to surmount substantial hurdles related to cost and efficiency.

Hydroelectric dams are the most established renewable-energy technology and the only greenhouse gas emissions come during the construction of facilities. The potential for expanding conventional hydropower lies mainly in developing countries, and even there the ability to harness more rivers is constrained by political instability and resistance from communities in regions targeted for dam construction. As of 2008, dams generated 16 percent of the world’s electricity and 2.3 percent of the total energy supply. Efforts are also under way to expand the harnessing of the power in tides, waves and even engineered waterways like irrigation canals.

Geothermal heat[89] could, by 2050, theoretically supply as much energy in the United States as now comes from nuclear power plants. An investment of about $1 billion in large-scale demonstration projects over 10 or 15 years could make the process profitable.

4.1.4 Carbon Trapping and Removal

Given the enduring dominance of coal, oil, and gas, along with the projected growth in fuel burning as developing countries move out of poverty, scientists and engineers have focused increasing attention on ways to capture carbon dioxide and sequester it. This could be done directly from power plants, or from the air itself. Once captured, the CO2 would be stashed within geologic formations, or even in the deep sea.

Most of the methods are already in use. Carbon dioxide has long been pumped into oil wells to increase production. And various studies have identified ways of capturing the gas from power plants and the atmosphere. Some small-scale prototypes are running, handling up to 1 million tons of carbon dioxide a year, and as of 2011 a few larger-scale tests were getting under way. But energy analysts say that far more testing at industrial scale will be required to determine whether such gas-trapping methods could someday handle billions of tons of CO2 a year and sequester the gas reliably and permanently.

Changes in the way farmers till the land, along with other agricultural practices and large-scale forest planting have the potential to remove substantial amounts of carbon from the atmosphere. In fact, these kinds of activities already result in a net removal of CO2 from the atmosphere, offsetting some 15 percent of all U.S. emissions of the gas.[90] But there may be some limitations. For example, it is not clear if reforestation is a true long-term solution, since holding the carbon depends on ensuring that the trees are not cut or burned in the future (which releases the carbon back into the atmosphere). Also, soils can continue sequestering carbon drawn out of the atmosphere only to a point, after which they become saturated.

Just as plants on land draw carbon dioxide out of the atmosphere through photosynthesis, so can phytoplankton, microscopic organisms that live in watery environments, including the sea. The problem is that their growth is limited in large swaths of the world’s oceans by a lack of a simple nutrient: iron. Small tests have been done to see if fertilizing ocean waters with iron causes blooms of phytoplankton that would, in turn, pull substantial amounts of carbon dioxide out of the air and deposit it on the seabed. To date, the results have been equivocal at best. There are also questions about the ecological impacts if ocean fertilization were done at large scale.

Scientists are also exploring ways of capturing CO2 directly from the air. Among the possibilities are what might be called artificial trees. As air passes through their tree-like structure, a CO2-absorbing chemical (such as sodium hydroxide) would remove significant amounts of the gas, which could then be piped elsewhere to be stored, probably in geologic formations. Though not yet economically or technically feasible on a large scale, the concept of “air capture” is an important area for research and testing. Unlike carbon-capture systems at power plants burning coal or gas, technologies such as artificial trees could capture carbon dioxide added to the atmosphere from untold distributed sources like vehicles and small factories, which are responsible for nearly half of all greenhouse gas emissions globally.[91]

Suggestion: include nature-based strategies as mitigation — and adaptation — solutions. [JV28]

4.1.5 Strategies to Reflect Sunlight

For many years, some scientists have proposed ways to counter the building greenhouse effect by artificially blocking or reflecting sunlight. Some techniques are quite simple and local — painting roofs and other surfaces white, for example. Others are monumental and global, such as lofting millions of tons of reflective sulfur-containing compounds into the stratosphere, or generating bright clouds over remote parts of the world’s oceans.

Such proposals, while important to explore, raise daunting technical, environmental, and diplomatic questions. They also are largely considered a “backstop” — a last technical option when all else fails — that could never substitute for reductions in emissions of greenhouse gases.[92]

Although past volcanic eruptions demonstrate the cooling potential of sun-blocking aerosols, the fast disappearance of particle veils after an eruption stops demonstrates that any effort to artificially replicate such cooling would have to be continual. Any disruption in the process, given the ongoing buildup of greenhouse gases, could lead to disruptively turbulent climate shifts, according to some research. Reduced sunlight would also have impacts on agriculture and the output of solar panels.

But the geopolitical questions are perhaps toughest. Who, for instance, gets to set the planet’s thermostat? Different countries would have different views. Who pays for the ongoing cooling effort? Who pays if some country claims it is harmed, for instance by shifting rainfall or crop patterns?

4.2 Adapting to Climate Change

Much can be done to limit the vulnerability of populations ,economies, and nature to predicted changes in water supplies, weather patterns and coastlines. Arguments for prompt action are built around the reality that the human-amplified greenhouse effect has already guaranteed significant climate change and sea-level rise for generations to come. Moreover, the slow rate at which energy systems transform essentially guarantees more emissions and heating before any stabilization occurs.

4.2.1 Reducing Our Vulnerability

From America’s Mississippi River floodplains to sub-Saharan Africa, from Mumbai to New Orleans, human societies are already vulnerable to weather extremes — particularly too much or too little rain, or powerful storms. Research shows that greenhouse-driven climate change can tip the odds toward trouble in many regions, while also raising coastal risks as seas rise.

Opportunities to limit impacts exist in “hot zones” for climate risk around the world. In sub-Saharan Africa, high fertility rates and enduring poverty create growing vulnerability in a region where frequent extremes, particularly drought, are already the norm. Studies of lakebed sediment show past patterns of protracted and profoundly severe drought. Vulnerability is also high in the American Southwest, which has a similar history of long dry spells, and in America’s hurricane zones because of substantial growth in populations and economies in recent decades.

Efforts to boost the resilience of farming, water supplies, transportation systems, and urban centers in the face of extreme conditions can pay off in a host of ways. Given persistent uncertainties about how global warming will play out in particular regions, any policies for adapting to change will have to be resilient, themselves, evolving as new information about trends and impacts emerges.

One route to limiting risks is increased investment in systems for monitoring and forecasting environmental conditions — from satellites to stream gauges. Such investments have been slowed by competing priorities and pressure to cut federal spending in the face of persistent deficits. This trend prompted the U.S. Geological Survey to create a map of threatened stream gauges, in the same way wildlife agencies map endangered species.[93][JV29]

4.2.2 Ecosystems

In many places around the world, important ecosystems — from reefs to rain forests — face growing pressures as human numbers and appetites surge. More stress is coming as temperature and precipitation patterns shift, with many species of animals and plants already measurably changing ranges and habitats. Species with ranges restricted by elevation on mountainsides or other geographic limits (one example is polar bears, which depend on Arctic Ocean sea ice as a seal-hunting platform) are particularly vulnerable in a warming world.

The challenge is not restricted to weighing risks to biological diversity for its own sake, but also to the services that ecosystems provide — from tourism revenue, coastal protection, and fish stocks for human consumption to the moderating influence on local climate and flooding provided by forests and wetlands.

In almost every instance, managing ecosystems in a changing climate is complicated by the many kinds of other pressures they face, including changes in land use, pollution, and invasive species. Climate change adds a new source of stress on top of these existing problems, and it adds uncertainty to current management practices.

One strategy that may become more prevalent is to move from trying to maintain an existing mix of species within a certain area to focusing more on maintaining the vigor and value of a system even as the composition changes over time due to shifting climate patterns.

In many instances, there are ways to boost the resilience of ecosystems to climate stress. In the Amazon River basin, which appears to be experiencing more drought with warming, biologists have concluded that limiting fragmentation of forests by road building can enhance their capacity to withstand drying.

Scientists have identified several parts of the Arctic Ocean, especially regions near northern Greenland and the Canadian islands to the west, that are likely to be sheathed with summer sea ice through this century. If these areas are set aside as bear refuges, the predator’s long-term prospects could improve.

In more developed parts of the planet where the ability of species to shift ranges is hindered by human settlements, establishing or maintaining “wildlife corridors” could maintain ecological vigor.

4.2.3 Agriculture and Forestry

Farmers have adapted to varying climate conditions and weather extremes as long as there has been agriculture. But challenges will be amplified in coming decades. In essence there will be a three-pronged race between rising impacts from extreme flooding, drought, or heat; rising demand for food; and the development and diffusion of improved crop varieties and farming methods.

But the current rate of investment in agricultural research, particularly aimed at advances that could make a difference in the world’s poorer regions, is seen as lagging well behind what is needed.[94] This is especially true because other threats to yields — including diseases like wheat rust — are spreading even as climate change unfolds. Persistent opposition to genetic technology in some countries and constituencies could also slow the development and diffusion of genetically engineered salt-, pest-, or drought-tolerant crops.[95]

Improving the availability of insurance to farmers in poor regions and the capacity of many African nations to monitor and forecast weather conditions and communicate such information to farmers could build capacity to endure more climatic hard knocks. A shift in such regions away from rain-fed agriculture toward efficient use of irrigated water could help in the short run.

In the long run, however, insurance and irrigation can lead to overdevelopment of agriculture in regions that can support it only marginally. As further climate change occurs, this could lead to big setbacks.

One approach to limiting climate risk to agriculture would be to pursue policies that increase farmers’ natural capacity to shift to alternative crops and cultivation methods that can suit changing environmental conditions.[96]

4.2.4 Water

From the great rivers of Asia to the baking deserts of Africa and the rivers and reservoirs of North America, there are likely to be ever more instances of either too much water or not enough. Combined with population growth, the threat posed by this pattern can be reduced by greatly improving the capacity to manage this vital resource.

Severe flooding can paralyze cities, ruin farmers’ crops, and overwhelm wastewater treatment plants. Deep droughts can threaten agriculture and the cooling capacity of power plants.

But climate change could also affect water quality, with big ramifications. Extreme rainfall, like the downpours that struck the U.S. Northeast with the remains of Hurricane Irene in 2011, can stir sediment in reservoirs, threatening to increase dramatically the cost of purification.[97]

In dealing with water scarcity, wealth and technology can make a big difference, as displayed in Perth, Australia. The capital of West Australia, in a region prone to severe drought, has greatly expanded systems for turning seawater into drinking water to bolster its supply while generating much of the electricity for desalination systems with wind turbines.[98]

In the United States since the 1980s, regulations and policies, along with rising awareness, have cut water use for many purposes — from industry to irrigation — even as the economy and populations have grown.[99]

Some strategies can work at both ends, harvesting water when it is plentiful so it is available during dry periods. In rainy seasons in central Florida, water authorities are starting to pump excess flows[JV30] [100] that would otherwise threaten to overtop the banks of Lake Okeechobee into natural subterranean reservoirs where the water will be available during droughts.

4.2.5 Health

Many communities have already demonstrated ways to cut vulnerability to some of the anticipated health impacts of climate change. Cities that have developed methods for getting information and help to the most vulnerable populations have already seen reduced rates of illness and deaths in heat waves. Philadelphia’s Hot Weather-Health Watch/Warning System is a notable example. When federal forecasts predict severe hot spells, announcements in the media encourage volunteers or neighbors and relatives of elderly or infirm residents to make daily visits to ensure that the most susceptible individuals have water, ventilation, and other means of coping with the heat.

Making progress on related threats to public health can limit climate-related risks. For example, heat can work in concert with pollutants such as volatile organic compounds to elevate levels of ground-level ozone, a lung irritant. Work to reduce emissions of such chemicals can potentially limit this harmful side effect of warming.

There is evidence that climate change could boost the prevalence of some water- and insect-borne diseases, but it is also true that improved sanitation and public health efforts can increase the capacity of communities to limit such risks.

4.2.6 Transportation

Developed countries and older cities in poorer countries will face the challenge of adapting existing airports, subway systems, and roads to threats such as more extreme heat, rising seas, and more frequent storm surges or flooding rains. There is great potential in the world’s fast-growing developing countries to take into account the anticipated impacts of climate change when developing transportation systems.

In both situations, progress would most likely come through integrated planning involving experts in climate, engineering, and other relevant disciplines at the time long-term investments are considered, to make sure a bridge or wharf or runway is built to withstand the conditions likely to be encountered as the climate changes. Bolstering research aimed at creating roads and other structures able to resist extreme heat, cold, and precipitation will benefit efforts to build, or rebuild, robust transportation systems in the face of sustained environmental change.

One reality complicating such efforts is the enduring uncertainty related to how global warming will affect regional and seasonal conditions. For example, while warmer winters could reduce transportation disruption related to extreme cold, shifts in the frequency or intensity of snowfall and ice storms could exacerbate hazards. Nevertheless in some places, as with warming permafrost in Alaska, the risk to roadways, rails, and runways is easier to assess.

4.2.7 Energy

Climate change will pose some technical and economic challenges to maintaining adequate energy supplies as human numbers and needs grow.

Developing distributed sources of electricity generation could make transmission and distribution systems more resilient and reliable in the face of more frequent or severe extremes of heat or storminess.

Shifting over time to power plant designs that rely less on water for cooling can help in regions that might face reduced river flows or warmer water temperatures.

In fast-growing regions like the American Southwest or South Asia, ensuring that buildings are designed to be cooled as efficiently as possible could moderate electricity demand.

Electricity demand at times of peak heat can be reduced through fairly simple changes. Programs are under way in many California municipalities to transform as many roofs as possible from dark to white surfaces to reflect sunlight and thus keep both buildings and city air cooler. New York City is in the midst of a project to plant one million trees, which shade buildings and can potentially help lower energy demand from air conditioning.

Urban redevelopment can be shaped to reduce both energy demand and temperatures. A congested corridor in downtown Seoul, South Korea, was rebuilt in the mid 2000’s to remove an inefficient, aging elevated highway and restore a long-hidden stream and adjacent vegetated banks. As a result temperatures, traffic, and air pollution all dropped, creating a soothing haven amid concrete canyons.

4.2.8 Coasts and Rivers

Humans, rich and poor, have a tendency to congregate along coasts and rivers, creating different challenges in different places. As warming leads to an era in which sea levels could rise for centuries to come, the world’s established coastal megacities will face substantial challenges. But history has shown an enduring ability[101] of important cities to adjust to changing environmental conditions. Metropolises like New York City and London are already planning for a future with rising seas and other disruptive change that could attend global warming.[102] [JV31]

Policies that could be helpful in addressing the risks of climate change include reductions in subsidies that encourage development in hazardous regions along coasts, estuaries, and floodplains; incentives to design or retrofit coastal facilities in ways that can accommodate a sustained rise in sea levels and increased exposure to storm surges; and, where possible, staging an orderly retreat by letting coastal regions revert to dunes or shrubbery.

In poorer places around the world, there is less infrastructure at risk but there are more people in harm’s way, creating different challenges. Poorer communities on vulnerable coasts or river deltas, notably in Bangladesh, have shown a strong capacity to limit losses in flooding storms by establishing storm warning systems and building platforms or other refuges. Bangladesh is also studying ways to exploit the massive amounts of sediment washing down from the Himalayas in trying to stay ahead of sea-level rise at least for a good part of this century.

From South Asia to the Gulf of Mexico, efforts are under way to protect and expand wetlands and other coastal ecosystems that can offer a buffer in the face of severe storms. In Europe and the American Midwest, communities and federal agencies are starting to shift practices and policies in ways that will give rivers, long hemmed in by development, more room to flood.

4.3 Taking Action

Global warming has many characteristics — including persistent uncertainty and dispersed and delayed impacts — that have kept the issue low on lists of public concerns for decades. As a result, some analysts have dubbed it a “super wicked” problem.[103] Nonetheless, a suite of societal choices have been identified that experts say could limit harm and also bring multiple benefits.

4.3.1 Climate and the Mind

The human mind has been an extraordinarily adaptive tool, allowing our species to thrive from the Arctic to the Equator, visit the dark side of the Moon and the deepest ocean trench, exploit the gene and the atom. But that same mind, which evolved mainly to avoid real-time risks and capitalize on real-time opportunities, appears to have a limited ability to deal with long-term, cumulative threats. This trait has been noted not only in dealing with climate change, but also with financial boom-bust cycles, zones where earthquakes or floods recur and other such threats.

There is often a deeply held bias toward immediate concerns, a tendency, over time, to become inured to slowly building vulnerabilities and to perceive some single action — say, changing light bulbs — as solving a large problem.

People also appear to have a wide range of reactions to risk and approaches to problem solving. This has likely contributed to persistent divisions over the evidence pointing to a rising human influence on climate and what, if anything, to do about it.

Despite the challenges imposed by the nature of the human mind, several studies show durable trust in scientists, support for scientific inquiry, and enthusiasm for expanding the menu of energy options that come with limited environmental costs.

Lessons learned through decades of research on how individuals and communities respond to, or ignore, risks[104] of various kinds are beginning to be applied by sociologists and other researchers to assessing how humans might respond to greenhouse-driven warming.[105]

4.3.2 Government Policy

For 20 years, many countries, environmental groups, and economists have judged that transforming the world’s energy systems would require that the full long-term costs of fossil fuels, including the anticipated costs from the impacts of climate change, be included in their price.

One mechanism for achieving this would be a rising tax on carbon dioxide emissions or the carbon content of fuels. Another would consist of setting lower national or worldwide caps on greenhouse-gas emissions, along with a system that would enable companies and others who achieved deep cuts to trade emissions credits on the open market. This so-called “cap-and-trade” system would also cause the price of carbon-based energy to rise.

New approaches are focusing on an array of policies to accelerate the deployment of climate-friendly energy technologies and practices that can blunt emissions in the short term, while also building the scientific and engineering capacity to produce the breakthroughs required if emissions rates are to decline worldwide in decades to come.

Some near-term options include tightening energy standards for appliances and vehicles; using the buying power of the federal government, including the military, to build a market for extra-efficient materials, lighting, and other goods and services; reexamining the merits and drawbacks of subsidies and tax breaks for all energy choices; and restricting emissions of greenhouse gases under clean-air laws in the same way more traditional pollutants have been regulated.

Even as near-term efforts are pursued, transitioning to a low-carbon energy system in the midst of a growing population and rising energy demand would also require fundamental advances in technologies for generating electricity without pollution, storing it cheaply and efficiently and providing — or substituting for — liquid fuels that will be at the heart of modern transportation systems for decades to come.

Achieving progress, by many estimates, would require increased and sustained investment in basic research, development, and demonstration on the frontiers of sciences related to energy.

4.3.3 Global Equity and Responsibility

One factor greatly complicates efforts to forge a global response to curbing human-driven climate change: deep divisions among the world’s 200-plus nations over which countries should assume the most responsibility both for curbing emissions of greenhouse gases and helping vulnerable populations cope with the projected impacts of rising seas and shifting climate patterns.

The divisions arise for two reasons. The first is related to the persistent, and thus cumulative, nature of both greenhouse gases and the additional heat banked in the Earth system. The world’s established industrialized countries, led by the United States and the nations of Western Europe, but also including Russia and Japan, are responsible for two thirds of the buildup of these gases so far.

But fast-growing developing countries, led by China, will be responsible for the majority of emissions going forward. So who should take the first step toward limiting their emissions?

Under the 1992 United Nations Framework Convention on Climate Change and subsequent agreements, all countries pledged to avoid dangerous human-driven warming on the basis of “common but differentiated responsibilities.” But negotiations over apportioning both emissions obligations, and the costs related to adapting to climate change, have been largely deadlocked for years.

The second source of division is that countries face different levels of vulnerability to the impacts of climate change, determined both by geography and economics. Countries in latitudes near the Equator appear to have the greatest vulnerability to heat and drought. And, with the exception of Australia, they have little capacity to use wealth to withstand such risks.

Industrialized countries that have acknowledged the greatest historic responsibility for emissions, such as the United States,[106] [JV32] are mostly in middle and higher latitudes where, at least through mid-century, impacts may be offset somewhat by benefits including increased agricultural productivity through longer growing seasons and expanded arable regions. The industrialized countries also have the financial assets to boost resilience using such tools as crop insurance and desalination plants for drinking water.

A variety of scenarios have been proposed to narrow these differences. Several focus less on comprehensive new accords and more on forging research partnerships. These relationships are intended to help in things like developing farming methods that can hold up in changing climates while fostering global competition for energy technologies that come with the fewest environmental impacts.

4.3.4 Education and Innovation

On a crowded, complicated planet with a changing climate, efforts to foster scientific literacy and research and maximize the human capacity for invention and collaboration could go a long way toward limiting costs and regrets.

The United States, while a leader in innovation and science for generations, has seen substantial erosion of student performance in science, math, and other vital skills and in both public and private investment in research and development in physical sciences.

In two reports on innovation and competitiveness since 2005, the National Research Council proposed an array of initiatives that could change this picture — from expanding teacher training in science and math to upgrading deteriorating research facilities and establishing an advanced research agency focused on energy.[107]

One response to such advice was the 2007 passage and 2010 reauthorization of the America Competes Act, which has funded a variety of initiatives aimed at fostering innovation and advancing scientific research and science education. Congress has created the Advanced Research Projects Agency, Energy, or Arpa-E, along with several “innovation hubs” focused on new nuclear reactor designs, energy-efficient building design, and processes for making liquid fuels from sunlight. But these and related initiatives face inconsistent levels of funding.[108][JV33]

Improved comprehension of the causes and consequences of climate change and the environmental and economic implications of energy choices could come through a better understanding of the scientific process and the framework it provides for evaluating environmental risks and human responses. Just as important is an understanding that uncertainty delineated by science is not the same as ignorance — but a form of knowledge that can shape responses to risk.

To achieve an energy transformation sufficient to curb greenhouse gas emissions, even as human numbers and appetites crest, near-term actions like cutting energy waste would need to be coupled with a long-term commitment to pursuing fundamental advances in science and technology.

One route to boosting the odds of revolutionary advances is to exploit the explosively growing capacity, through telecommunications and the Web, for ideas to be shared and shaped globally. Another is to cultivate a new generation of scientists, engineers, entrepreneurs, teachers, and leaders focused on pursuing progress in ways that do not simply satisfy the needs of the moment, but also build a durable, bountiful world for generations to come.



Andy Revkin

Pursuing progress on a finite planet at @columbia @earthinstitute. The rest? Family, friends, books, songs