Aug 23, 2019 | Global Documents |
answers to common questions about the science of climate change EvidEncE, impacts, and choicEs Climate Change How do we know that Earth has warmed? How do we know that humans are causing greenhouse gas concentrations to increase? How do we know the current warming trend isn’t caused by the Sun? How do we know that the warming trend is not caused by natural cycles? How much more warming can be expected? How is precipitation expected to change? How will sea ice and snow be affected? How will coastlines be affected? How will ecosystems be affected? How will agriculture and food production be affected? How does science inform the response to climate change? RefeRences National Research Council, 2010a, Advancing the Science of Climate Change National Research Council, 2010b, Limiting the Magnitude of Climate Change National Research Council, 2010c, Adapting to the Impacts of Climate Change National Research Council, 2011d, Informing an Effective Response to Climate Change National Research Council, 2010e, Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean National Research Council 2011a, Climate Stabilization Targets: Emissions, Concentrations, and Impacts for Decades to Millennia National Research Council, 2011c, America’s Climate Choices For more information, contact the Board on Atmospheric Sciences and Climate at 202-334-3512 or visit http://dels.nas.edu/basc. A video based on Part I of this booklet is available at http://americasclimatechoices.org. This booklet was prepared by the National Research Council with support from the National Oceanic and Atmospheric Administration (NOAA). It was developed by Nancy Huddleston and designed by Francesca Moghari. Special thanks to Ian Kraucunas, Antonio J. Busalacchi, Jr., Edward J. Dunlea, Robert W. Fri, Laurie Geller, Pamela A. Matson, Damon Matthews, Gerald A. Meehl, Claudia Mengelt, Raymond T. Pierrehumbert, Kimberly A. Prather, John P. Reisman, and Benjamin D. Santer for their helpful contributions. Photo Credits Main cover photo by Michael D. Dudzik; cover thumbnail (bottom) by Fuse; p. 1 by John P. Kelley (Image Bank); p. 17 by Joe McDonald; p. 19 courtesy United States Geological Survey; p. 20 by Randy Well (Stone); p. 23, p. 27 by David Haines; p. 30, Jupiter Images (Comstock); p. 36, kali9. Photo on p. 2 by Mike Waszkiewicz, courtesy National Science Foundation. Nicole Spaulding and Kristin Schild, students from the University of Maine Climate Change Institute, chip out near-surface ice samples as part of research into new methods for sampling the record of polar climate change. © 2012 National Academy of Sciences About the national Research council The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. The Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering and is administered jointly by both Academies and the Institute of Medicine. The National Research Council enlists the nation’s foremost scientists, engineers, health professionals, and other experts to serve on committees to address scientific and technical aspects of some of the nation’s most pressing problems. These experts serve pro bono and are screened for conflicts of interest to ensure that the committee is able to provide impartial and objective advice. Through these committees, the Academies produce about 200 peer-reviewed reports each year that provide thoughtful analysis and helpful direction to policymakers and stakeholders. Contents Part I. Evidence for Human-Caused Climate Change 2 How do we know that Earth has warmed? 3 How do we know that greenhouse gases lead to warming? 4 How do we know that humans are causing greenhouse gases to increase? 6 How much are human activities heating Earth? 9 How do we know the current warming trend isn’t caused by the Sun? 11 How do we know the current warming trend isn’t caused by natural cycles? 12 What other climate changes and impacts have been observed? 15 The Ice Ages 18 Part II. Warming, Climate Changes, and Impacts in the 21st Century and Beyond 20 How do scientists project future climate change? 21 How will temperatures be affected? 22 How is precipitation expected to change? 23 How will sea ice and snow be affected? 26 How will coastlines be affected? 26 How will ecosystems be affected? 28 How will agriculture and food production be affected? 29 Part III. Making Climate Choices 30 How does science inform emissions choices? 31 What are the choices for reducing greenhouse gas emissions? 32 What are the choices for preparing for the impacts of climate change? 34 Why take action if there are still uncertainties about the risks of climate change? 35 Conclusion 36 Just what is climate? Climate is commonly thought of as the expected weather conditions at a given location over time. People know when they go to New York City in winter, they should take a coat. When they visit the Pacific Northwest, they take an umbrella. Climate can be measured at many geographic scales—for example, cities, countries, or the entire globe—by such statistics as average temperatures, average number of rainy days, and the frequency of droughts. Climate change refers to changes in these statistics over years, decades, or even centuries. Enormous progress has been made in increasing our understanding of climate change and its causes, and a clearer picture of current and future impacts is emerging. Research is also shedding light on actions that might be taken to limit the magnitude of climate change and adapt to its impacts. This booklet is intended to help people understand what is known about climate change. First, it lays out the evidence that human activities, especially the burning of fossil fuels, are responsible for much of the warming and related changes being observed around the world. Second, it summarizes projections of future climate changes and impacts expected in this century and beyond. Finally, the booklet examines how science can help inform choices about managing and reducing the risks posed by climate change. The information is based on a number of National Research Council reports (see inside back cover), each of which represents the consensus of experts who have reviewed hundreds of studies describing many years of accumulating evidence. Climate Change EvidEncE, impacts, and choicEs 2 But how has this conclusion been reached? Climate science, like all science, is a process of collective learning that relies on the careful gathering and analyses of data, the formulation of hypotheses, the development of models to study key processes and make testable predictions, and the combined use of observations and models to test scientific understanding. Scientific knowledge builds over time as new observations and data become available. Confidence in our understanding grows if multiple lines of evidence lead to the same conclusions, or if other explanations can be ruled out. In the case of climate change, scientists have understood for more than a century that emissions from the burning of fossil fuels could lead to increases in the Earth’s average surface temperature. Decades of research have confirmed and extended this understanding. The overwhelming majority of climate scientists agree that human activities, especially the burning of fossil fuels (coal, oil, and gas), are responsible for most of the climate change currently being observed. EvidEncE for Human-Caused Climate Change Part I 3 Scientists have been taking widespread measure- ments of Earth’s surface temperature since around 1880. These data have steadily improved and, today, temperatures are recorded by ther- mometers at many thousands of locations, both on the land and over the oceans. Different research groups, including the NASA Goddard Institute for Space Studies, Britain’s Hadley Centre for Climate Change, the Japan Meteorological Agency, and NOAA’s National Climate Data Center have used these raw measurements to produce records of long-term global surface temperature change (Figure 1). These groups work carefully to make sure the data aren’t skewed by such things as changes in the instruments taking the measure- ments or by other factors that affect local tempera- ture, such as additional heat that has come from the gradual growth of cities. These analyses all show that Earth’s average surface temperature has increased by more than 1.4°F (0.8°C) over the past 100 years, with much of this increase taking place over the past 35 years. A temperature change of 1.4°F may not seem like much if you’re thinking about a daily or seasonal fluctuation, but it is a significant change when you think about a permanent increase averaged across the entire planet. Consider, for example, that 1.4°F is greater than the average annual FIGURE 2 FIGURE 1 NASA’s Global Surface Temperature Record Esti- mates of global surface temperature change, relative to the average global surface temperature for the period from 1951 to 1980, which is about 14°C (57°F) from NASA Goddard Institute for Space Studies show a warming trend over the 20th century. The esti- mates are based on surface air temperature measure- ments at meteorological stations and on sea surface temperature measurements from ships and satellites. The black curve shows average annual temperatures, and the red curve is a 5-year running average. The green bars indicate the margin of error, which has been reduced over time. Source: National Research Council 2010a (bottom left) Climate monitoring stations on land and sea, such as the moored buoys of NOAA’s Tropical Atmosphere Ocean (TAO) project, provide real-time data on tempera- ture, humidity, winds, and other atmospheric properties. Image courtesy of TAO Project Office, NOAA Pacific Marine Environmental Laboratory. (right) Weather balloons, which carry instruments known as radiosondes, provide verti- cal profiles of some of these same properties throughout the lower atmosphere. Image © University Corporation for Atmospheric Research. (top left) The NOAA-N spacecraft, launched in 2005, is the fifteenth in a series of polar- orbiting satellites dating back to 1978. The satellites carry instruments that measure global surface temperature and other climate variables. Image courtesy NASA EvidEncE for Human-Caused Climate CHangeHow do we know that Earth has warmed? 4 temperature difference between Washington, D.C., and Charleston, South Carolina, which is more than 450 miles farther south. Consider, too, that a decrease of only 9°F (5°C) in global average temperatures is the estimated difference between today’s climate and an ice age. In addition to surface temperature, other parts of the climate system are also being monitored carefully (Figure 2). For example, a variety of instruments are used to measure temperature, salinity, and currents beneath the ocean surface. Weather balloons are used to probe the temperature, humidity, and winds in the atmosphere. A key breakthrough in the ability to track global environmental changes began in the 1970s with the dawn of the era of satellite remote sensing. Many different types of sensors, carried on dozens of satellites, have allowed us to build a truly global picture of changes in the temperature of the atmosphere and of the ocean and land surfaces. Satellite data are also used to study shifts in precipitation and changes in land cover. Even though satellites measure temperature very differently than instruments on Earth’s surface, and any errors would be of a completely different nature, the two records agree. A number of other indicators of global warming have also been observed (see pp.15-17). For example, heat waves are becoming more frequent, cold snaps are now shorter and milder, snow and ice cover are decreasing in the Northern Hemisphere, glaciers and ice caps around the world are melting, and many plant and animal species are moving to cooler latitudes or higher altitudes because it is too warm to stay where they are. The picture that emerges from all of these data sets is clear and consistent: Earth is warming. How do we know that greenhouse gases lead to warming? As early as the 1820s, scientists began to ap- preciate the importance of certain gases in regulating the temperature of the Earth (see Box 1). Greenhouse gases—which include carbon dioxide (CO2), methane, nitrous oxide, and water vapor— act like a blanket in the atmosphere, keep- ing heat in the lower atmosphere. Although greenhouse gases comprise only a tiny fraction of Earth’s atmosphere, they are critical for keeping the planet warm enough to support life as we know it (Figure 3). Here’s how the “greenhouse effect” works: as the Sun’s energy hits Earth, some of it is reflected back to space, but most of it is absorbed by the land and oceans. This absorbed energy is then radiated upward from Earth’s surface in the form of heat. In the absence of greenhouse gases, this heat would simply escape to space, and the planet’s average surface temperature would be well below freezing. But greenhouse gases absorb and redirect some of this energy downward, keeping heat near Earth’s surface. As concentrations of heat- trapping greenhouse gases increase in the atmosphere, Earth’s natural greenhouse effect is enhanced (like a thicker blanket), causing surface temperatures to rise (Figure 3). Reducing the levels of greenhouse gases in the atmosphere would cause a decrease in surface temperatures. 5 Amplification of the Greenhouse Effect The greenhouse effect is a natural phenomenon that is essential to keeping the Earth’s surface warm. Like a greenhouse window, greenhouse gases allow sunlight to enter and then prevent heat from leaving the atmosphere. These gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor. Human activities—especially burning fossil fuels—are increasing the concentrations of many of these gases, amplifying the natural greenhouse effect. Image courtesy of the Marian Koshland Science Museum of the National Academy of Sciences FIGURE 3 EvidEncE for Human-Caused Climate CHangeBox 1 Early Understanding of Greenhouse Gases In 1824, French physicist Joseph Fourier (top) was the first to suggest that the Earth’s atmosphere might act as an insulator of some kind—the first proposal of what was later called the greenhouse effect. In the 1850s, Irish- born physicist John Tyndall (middle) was the first to demonstrate the greenhouse effect by showing that water vapor and other atmospheric gases absorbed Earth’s radiant heat. In 1896, Swedish scientist Svante Arrhenius (bottom) was the first to calculate the warming power of excess carbon dioxide (CO2). From his calculations, Arrhenius predicted that if human activities increased CO2 levels in the atmosphere, a warming trend would result. 6 Discerning the human influence on greenhouse gas concentrations is challenging because many greenhouse gases occur naturally in Earth’s atmo- sphere. Carbon dioxide (CO2) is produced and con- sumed in many natural processes that are part of the carbon cycle (see Figure 4). However, once humans began digging up long-buried forms of carbon such as coal and oil and burning them for energy, addi- tional CO2 began to be released into the atmosphere much more rapidly than in the natural carbon cycle. Other human activities, such as cement production and cutting down and burning of forests (deforesta- tion), also add CO2 to the atmosphere. Until the 1950s, many scientists thought the oceans would absorb most of the excess CO2 released by human activities. Then a series of FIGURE 4 The Carbon Cycle Carbon is continually exchanged between the atmosphere, ocean, biosphere, and land on a variety of timescales. In the short term, CO2 is exchanged continuously among plants, trees, animals, and the air through respiration and photosynthesis, and between the ocean and the atmosphere through gas exchange. Other parts of the carbon cycle, such as the weathering of rocks and the formation of fossil fuels, are much slower pro- cesses occurring over many centuries. For example, most of the world’s oil reserves were formed when the remains of plants and animals were buried in sediment at the bottom of shallow seas hundreds of millions of years ago, and then exposed to heat and pressure over many millions of years. A small amount of this carbon is released naturally back into the atmosphere each year by volcanoes, completing the long-term carbon cycle. Human activities, espe- cially the digging up and burning of coal, oil, and natural gas for energy, are disrupting the natural carbon cycle by releasing large amounts of “fossil” carbon over a relatively short time period. Source: National Research Council How do we know that humans are causing greenhouse gas concentrations to increase? 7 scientific papers were published that examined the dynamics of carbon dioxide exchange between the ocean and atmosphere, including a paper by oceanographer Roger Revelle and Hans Seuss in 1957 and another by Bert Bolin and Erik Eriksson in 1959. This work led scientists to the hypothesis that the oceans could not absorb all of the CO2 being emitted. To test this hypothesis, Revelle’s colleague Charles David Keeling began collecting air samples at the Mauna Loa Observatory in Hawaii to track changes in CO2 concentrations. Today, such measurements are made at many sites around the world. The data reveal a steady increase in atmospheric CO2 (Figure 5). To determine how CO2 concentrations varied prior to such modern measurements, scientists have studied the composition of air bubbles trapped in ice cores extracted from Greenland and Antarctica. These data show that, for at least 2,000 years before the Industrial Revolution, atmospheric CO2 concentrations were steady and then began to rise sharply beginning in the late 1800s (Figure 6). Today, atmospheric CO2 concentrations exceed 390 parts per million—nearly 40% higher than preindustrial levels, and, according to ice core data, higher than at any point in the past 800,000 years (see Figure 14, p.18). Human activities have increased the atmospheric concentrations of other important greenhouse gases as well. Methane, which is produced by the burning of fossil fuels, the raising of livestock, the decay of landfill wastes, the production and transport of natural gas, and other activities, increased sharply through the 1980s before starting to level off at about two-and-a-half times its preindustrial level (Figure 6). Nitrous oxide has increased by roughly 15% since 1750 (Figure 6), mainly as a result of agricultural EvidEncE for Human-Caused Climate CHangeFIGURE 5 FIGURE 6 Measurements of Atmospheric Carbon Dioxide The “Keeling Curve” is a set of careful measurements of atmospheric CO2 that Charles David Keeling began collecting in 1958. The data show a steady annual increase in CO2 plus a small up-and-down sawtooth pattern each year that reflects seasonal changes in plant activity (plants take up CO2 during spring and summer in the Northern Hemisphere, where most of the planet’s land mass and land ecosystems reside, and release it in fall and winter). Source: National Research Council, 2010a Greenhouse Gas Concentrations for 2,000 Years Analysis of air bubbles trapped in Antarctic ice cores show that, along with carbon dioxide, atmospheric concentrations of methane (CH4) and nitrous oxide (N2O) were relatively constant until they started to rise in the Industrial era. Atmospheric concentration units indicate the number of molecules of the greenhouse gas per million molecules of air for carbon dioxide and nitrous oxide, and per billion molecules of air for methane. Image courtesy: U.S. Global Climate Research Program 8 fertilizer use, but also from fossil fuel burning and certain industrial processes. Certain industrial chemicals, such as chlorofluorocarbons (CFCs), act as potent greenhouse gases and are long-lived in the atmosphere. Because CFCs do not have natural sources, their increases can be attributed unambiguously to human activities. In addition to direct measurements of CO2 concentrations in the atmosphere, scientists have amassed detailed records of how much coal, oil, and natural gas is burned each year. They also estimate how much CO2 is being absorbed, on average, by the oceans and the land surface. These analyses show that about 45% of the CO2 emitted by human activities remains in the atmosphere. Just as a sink will fill up if water is entering it faster than it can drain, human production of CO2 is outstripping Earth’s natural ability to remove it from the air. As a result, atmospheric CO2 levels are increasing (see Figure 7) and will remain elevated for many centuries. Furthermore, a forensic-style analysis of the CO2 in the atmosphere reveals the chemical “fingerprint” of carbon from fossil fuels (see Box 2). Together, these lines of evidence prove conclusively that the elevated CO2 concentration in the atmosphere is the result of human activities. – + = Emissions Atmospheric Concentration Growth Land Sink Ocean Sink FIGURE 7 Emissions Exceed Nature’s CO2 Drain Emissions of CO2 due to fossil fuel burning and cement manufacture are increasing, while the capacity of “sinks” that take up CO2—for example, plants on land and in the ocean—are decreasing. Atmospheric CO2 is increasing as a result. Source: National Research Council, 2011a Clues from the “fingerprint” of carbon dioxide. In a process that takes place over millions of years, carbon from the decay of plants and animals is stored deep in the Earth’s crust in the form of coal, oil, and natural gas (see Figure 4). Because this “fossil” carbon is so old, it contains very little of the radioisotope carbon-14—a form of the carbon that decays naturally over long time periods. When scientists measure carbon-14 levels in the atmosphere, they find that it is much lower than the levels in living ecosystems, indicating that there is an abundance of “old” carbon. While a small fraction of this old carbon can be attributed to volcanic eruptions, the overwhelming majority comes from the burning of fossil fuels. Average CO2 emissions from volcanoes are about 200 million tons per year, while humans are emitting an estimated 36 billion tons of CO2 each year, 80-85% of which are from fossil fuels. Box 2 9 Greenhouse gases are referred to as “forcing agents” because of their ability to change the planet’s energy balance. A forcing agent can “push” Earth’s temperature up or down. Greenhouse gases differ in their forcing power. For example, a single methane molecule has about 25 times the warming power of a single CO2 molecule. However, CO2 has a much larger overall warming effect than methane because it is much more abundant and stays in the atmosphere for much longer periods of time. Scientists can calculate the forcing power of greenhouse gases based on the changes in their concentrations over time and on physically based calculations of how they transfer energy through the atmosphere. Some forcing agents push Earth’s energy balance toward cooling, offsetting some of the heating associated with greenhouse gases. For example, some aerosols—which are tiny liquid or solid particles suspended in the atmosphere, such as those that make up most of the visible air pollution—have a cooling effect because they scatter a portion of incoming sunlight back into space (see Box 3). Human activities, especially the burning of fossil fuels, have increased the number of aerosol particles in the atmosphere, especially over and around major urban and industrial areas. Changes in land use and land cover are another way that human activities are influencing Earth’s climate. Deforestation is responsible for 10% to 20% of the excess CO2 emitted to the atmosphere each year, and, as has already been discussed, agriculture contributes nitrous oxide and methane. Changes in land use and land cover also modify the reflectivity of Earth’s surface; the more reflective a surface, the more sunlight is sent back into space. Cropland is generally more reflective than an undisturbed forest, while urban areas often reflect less energy than undisturbed land. Globally, human land use changes are estimated to have a slight cooling effect. When all human and natural forcing agents are considered together, scientists have calculated that the net climate forcing between 1750 and 2005 is EvidEncE for Human-Caused Climate CHangeHow much are human activities heating Earth? Warming and Cooling Effects of Aerosols Aerosols are tiny liquid or solid particles suspended in the atmosphere that come from a number of human activities, such as fossil fuel combustion, as well as natural processes, such as dust storms, volcanic eruptions, and sea spray emissions from the ocean. Most of our visible air pollution is made up of aerosols. Most aerosols have a cooling effect, because they scatter a portion of incoming sunlight back into space, although some particles, such as dust and soot, actually absorb some solar energy and thus act as warming agents. Many aerosols also enhance the reflection of sunlight back to space by making clouds brighter, which results in additional cooling. Many nations, states, and communities have taken action to reduce the concentrations of certain air pollutants such as the sulfate aerosols responsible for acid rain. Unlike most of the greenhouse gases released by human activities, aerosols only remain in the atmosphere for a short time—typically a few weeks. Box 3 10 pushing Earth toward warming (Figure 8). The extra energy is about 1.6 Watts per square meter of Earth’s surface. When multiplied by the surface area of Earth, this energy represents more than 800 trillion Watts (Terawatts)—on a per year basis, that’s about 50 times the amount of power produced by all the power plants of the world combined! This extra energy is being added to Earth’s climate system every second of every day. The total amount of warming that will occur in response to a climate forcing is determined by a variety of feedbacks, which either amplify or dampen the initial warming. For example, as Earth warms, polar snow and ice melt, allowing the darker colored land and oceans to absorb more heat—causing Earth to become even warmer, which leads to more snow and ice melt, and so on (see Figure 9). Another impor- tant feedback involves water vapor. The amount of water vapor in the atmosphere increases as the ocean surface and the lower atmosphere warm up; warm- ing of 1°C (1.8°F) increases water vapor by about 7%. Because water vapor is also a greenhouse gas, this increase causes additional warming. Feedbacks that reinforce the initial climate forcing are referred to in the scientific community as positive, or ampli- fying, feedbacks. There is an inherent time lag in the warming that is caused by a given climate forcing. This lag occurs because it takes time for parts of Earth’s climate systems—especially the massive oceans—to warm or cool. Even if we could hold all human-produced forcing agents at present-day values, Earth would continue to warm well beyond the 1.4°F already ob- served because of human emissions to date. Warming and Cooling Influences on Earth Since 1750 The warming and cooling influences (measured in Watts per square meter) of various cli- mate forcing agents during the Industrial Age (from about 1750) from human and natural sources has been calculated. Human forcing agents include increases in greenhouse gases and aerosols, and changes in land use. Major volcanic eruptions produce a temporary cooling effect, but the Sun is the only major natural factor with a long-term effect on climate. The net ef- fect of human activities is a strong warming influence of more than 1.6 Watts per square meter. Source: Na- tional Research Council, 2010a (Depiction courtesy U.S. Global Climate Research Program) Energy (Watts/m2) FIGURE 8 TEMPERATURES RISE AS REFLECTIVE ICE DISAPPEARS, DARKER OCEAN WATER ABSORBS MORE HEAT ARCTIC SEA ICE MELTS Climate Feedback Loops The amount of warming that occurs because of increased greenhouse gas emissions depends in part on feedback loops. Positive (amplifying) feedback loops increase the net temperature change from a given forcing, while negative (damping) feedbacks offset some of the temperature change associated with a climate forcing. The melting of Arctic sea ice is an example of a positive feedback loop. As the ice melts, less sunlight is reflected back to space and more is absorbed into the dark ocean, causing further warming and further melting of ice. Source: National Research Council, 2011d FIGURE 9 11 Another way to test a scientific theory is to in- vestigate alternative explanations. Because the Sun’s output has a strong influence on Earth’s temperature, scientists have examined records of solar activity to determine if changes in solar output might be responsible for the observed global warm- ing trend. The most direct measurements of solar output are satellite readings, which have been avail- able since 1979. These satellite records show that the Sun’s output has not shown a net increase dur- ing the past 30 years (Figure 10) and thus cannot be responsible for the warming during that period. Prior to the satellite era, solar energy output had to be estimated by more indirect methods, such as records of the number of sunspots observed each year, which is an indicator of solar activity. These indirect methods suggest there was a slight increase in solar energy reaching Earth during the first few Measures of the Sun’s Energy Satellite measurements of the Sun’s energy incident on Earth, available since 1979, show no net increase in solar forcing during the past 30 years. They show only small periodic variations associated with the 11-year solar cycle. Source: National Research Council, 2010a Warming Patterns in the Layers of the Atmosphere Data from weather balloons and satellites show a warming trend in the troposphere, the lower layer of the atmosphere, which extends up about 10 miles (lower graph), and a cooling trend in the stratosphere, which is the layer immediately above the troposphere (upper graph). This is exactly the pattern expected from increased greenhouse gases, which trap energy closer to the Earth’s surface. Source: National Research Council, 2010a FIGURE 11 FIGURE 10 EvidEncE for Human-Caused Climate CHangeHow do we know the current warming trend isn’t caused by the Sun? 12 decades of the 20th century. This increase may have contributed to global temperature increases during that period, but does not explain warming in the latter part of the century. Further evidence that current warming is not a result of solar changes can be found in the temperature trends in the different layers of the atmosphere. These data come from two sources: weather balloons, which have been launched twice daily from hundreds of sites worldwide since the late 1950s, and satellites, which have monitored the temperature of different layers of the atmosphere since the late 1970s. Both of these data sets have been heavily scrutinized, and both show a warming trend in the lower layer of the atmosphere (the troposphere) and a cooling trend in the upper layer (the stratosphere) (Figure 11). This is exactly the vertical pattern of temperature changes expected from increased greenhouse gases, which trap energy closer to the Earth’s surface. If an increase in solar output were responsible for the recent warming trend, the vertical pattern of warming would be more uniform through the layers of the atmosphere. How do we know that the current warming trend is not caused by natural cycles? Detecting climate trends is complicated by the fact that there are many natural variations in temperature, precipitation, and other climate variables. These natural variations are caused by many different processes that can occur across a wide range of timescales—from a particularly warm summer or snowy winter to changes over many millions of years. Among the most well-known short-term cli- matic fluctuations are El Niño and La Niña, which are periods of natural warming and cooling in the tropical Pacific Ocean. Strong El Niño and La Niña events are associated with significant year-to-year changes in temperature and rainfall patterns across many parts of the planet, including the United States. These events have been linked to a number of extreme weather events, such as the 1992 flood- ing in midwestern states and the severe droughts in southeastern states in 2006 and 2007. Globally, temperatures tend to be higher during El Niño periods, such as 1998, and lower during La Niña years, such as 2008. However, these up-and- down fluctuations are smaller than the 20th cen- tury warming trend; 2008 was still quite a warm year in the long-term record. Natural climate variations can also be forced by slow changes in the Earth’s orbit around the Sun that affect the solar energy received by Earth, as is the case with the Ice Age cycle (see pp. 18-19) or by short-term changes in the amount of volca- nic aerosols in the atmosphere. Major eruptions, like that of Mount Pinatubo in 1991, spew huge amounts of particles into the stratosphere that cool Earth. However, surface temperatures typically rebound in 2-5 years as the particles settle out of the atmosphere. The short-term cooling effects of several large volcanic eruptions can be seen in the 20th century temperature record, as can the global temperature variations associated with several 13 strong El Niño and La Niña events, but an overall warming trend is still evident (Figure 12). In order to put El Niño and La Niña events and other short-term natural fluctuations into perspec- tive, climate scientists examine trends over several decades or longer when assessing the human influ- ence on the climate system. Based on a rigorous as- sessment of available temperature records, climate forcing estimates, and sources of natural climate variability, scientists have concluded that there is a more than 90% chance that most of the observed global warming trend over the past 50 to 60 years can be attributed to emissions from the burning of fossil fuels and other human activities. Such statements that attribute climate change to human activities also rely on information from FIGURE 12 Short-term Temperature Effects of Natural Climate Variations Natural factors, such as volcanic eruptions and El Niño and La Niña events, can cause average global temperatures to vary from one year to the next, but cannot explain the long-term warming trend over the past 60 years. Image courtesy of the Marian Koshland Science Museum EvidEncE for Human-Caused Climate CHangeFIGURE 13 Model Runs With and Without Human Influences Model simulations of 20th-century surface temperatures more closely match observed temperature when both natural and human influences are included in the simulations. The black line shows an estimate of observed surface temperatures changes. The blue line shows results from models that only include natural forcings (solar activity and volcanoes). The red-shaded regions show results from models that include both natural and human forcings. Source: Meehl et al, 2011 climate models (see Box 4). Scientists have used these models to simulate what would have happened if humans had not modified Earth’s climate during the 20th century—that is, how global tempera- tures would have evolved if only natural factors (volcanoes, the Sun, and internal climate variability) were influencing the climate system. These “undis- turbed Earth” simulations predict that, in the ab- sence of human activities, there would have been negligible warming, or even a slight cooling, over the 20th century. When greenhouse gas emissions and other activities are included in the models, how- ever, the resulting surface temperature changes more closely resemble the observed changes (Figure 13). 14 What are climate models? For several decades, scientists have used the world’s most ad- vanced computers to simulate the Earth’s climate. These models are based on a series of mathemati- cal equations representing the basic laws of physics—laws that govern the behavior of the atmo- sphere, the oceans, the land surface, and other parts of the climate system, as well as the interactions among different parts of the system. Climate models are important tools for understanding past, present, and future climate change. Climate models are tested against observations so that scientists can see if the models correctly simulate what actually happened in the recent or distant past. Image courtesy Marian Koshland Science Museum Box 4 EVAPORATION CONDENSATION & CONVECTION EXCHANGE OF HEAT & GASES BETWEEN ATMOSPHERE, SEA ICE & OCEAN GLACIER MELT RADIATIVE EXCHANGE TERRESTRIAL CARBON CYCLE OCEAN CARBON CYCLE OCEAN CIRCULATION WATER STORAGE IN ICE & SNOW SURFACE RUN-OFF AIR POLLUTION WIND CLOUDS & WATER VAPOR EVAPOTRANSPIRATION R ising temperatures due to increasing greenhouse gas concentrations have produced distinct pat- terns of warming on Earth’s surface, with stronger warming over most land areas and in the Arctic. There have also been significant seasonal differences in observed warming. For example, the second half of the 20th century saw intense winter warming across parts of Canada, Alaska, and northern Europe and Asia, while summer warming was particularly strong across the Mediterranean and Middle East and some other places, including parts of the U.S. west (Figure 15). Heat waves and record high tem- peratures have increased across most regions of the world, while cold snaps and record cold tempera- tures have decreased. Global warming is also having a significant im- pact on snow and ice, especially in response to the strong warming across the Arctic. For example, the average annual extent of Arctic sea ice has dropped by roughly 10% per decade since satel- lite monitoring began in 1978 (Figure 16). This melting has been especially strong in late summer, leaving large parts of the Arctic Ocean ice-free for weeks at a time and raising questions about effects on ecosystems, commercial shipping routes, oil and gas exploration, and national defense. Many of the world’s glaciers and ice sheets are melting in response to the warming trend, and long-term av- erage winter snowfall and snowpack have declined in many regions as well, such as the Sierra Nevada mountain range in the western United States. Much of the excess heat caused by human-emit- ted greenhouse gases has warmed the world’s oceans during the past several decades. Water ex- pands when it warms, which leads to sea-level rise. Water from melting glaciers, ice sheets, and ice caps also contributes to rising sea levels. Measurements made with tide gauges and augmented by satellites show that, since 1870, global average sea level has risen by about 8 inches (0.2 meters). It is estimated 15 EvidEncE for Human-Caused Climate CHangeFIGURE 15 Patterns of Warming in Winter and Summer Twenty-year average temperatures for 1986-2005 compared to 1955-1974 show a distinct pattern of winter and summer warming. Winter warming has been intense across parts of Canada, Alaska, northern Europe, and Asia, and summers have warmed across the Mediterranean and Middle East and some other places, including parts of the U.S. west. Projections for the 21st century show a similar pattern. Source: National Research Council, 2011a What other climate changes and impacts have already been observed? 16 that roughly one-third of the total sea-level rise over the past four decades can be attributed to ocean expansion, with most of the remainder due to ice melt (Figure 17). Because CO2 reacts in seawater to form carbonic acid, the acidification of the world’s oceans is an- other certain outcome of elevated CO2 concentra- tions in the atmosphere (Figure 18). It is estimated that the oceans have absorbed between one-quarter and one-third of the excess CO2 from human activi- ties, becoming nearly 30% more acidic than during preindustrial times. Geologically speaking, this large change has happened over a very short timeframe, and mounting evidence indicates it has the poten- tial to radically alter marine ecosystems, as well as the health of coral reefs, shellfish, and fisheries. Another example of a climate change observed during the past several decades has been changes in the frequency and distribution of precipitation. Total precipitation in the United States has increased by about 5% over the past 50 years, but this has not been geographically uniform—conditions are gener- ally wetter in the Northeast, drier in the Southeast, and much drier in the Southwest. Warmer air holds more water vapor, which has led to a measurable increase in the intensity of precipita- FIGURE 16 Loss of Arctic Sea Ice Satellite-based measurements show a steady decline in the amount of September (end of summer) Arctic sea ice extent from 1979 to 2009 (expressed as a percentage difference from 1979- 2000 average sea ice extent, which was 7.0 million square miles). The data show substantial year-to-year variability, but a long-term decline in sea ice of more than 10% per decade is clearly evident, highlighted by the dashed line. Source: National Research Council, 2010a FIGURE 17 Contributors to Sea-Level Rise Sea level has risen steadily over the past few decades due to various contributors: thermal expansion in the upper 700 meters of ocean (red) and deeper ocean layers (orange), meltwater from Antarctic and Green- land ice sheets (blue), meltwater from glaciers and ice caps (purple), and water storage on land (green). Source: National Research Council, 2011a 17 tion events. In the United States, for example, the fraction of total precipitation falling in the heaviest 1% of rainstorm increased by about 20% over the past century, with the northeastern states experienc- ing an increase of 54%. This change has increased the risk of flooding and puts additional stress on sewer and stormwater management systems. As the climate has changed, many species have shifted their range toward the poles and to higher altitudes as they try to stay in areas with the same ambient temperatures. The timing of different seasonal activities is also changing. Several plant species are blooming earlier in Spring, and some birds, mammals, fish, and insects are migrating earlier, while other species are altering their seasonal breeding patterns. Global analyses show these behaviors occurred an average of 5 days earlier per decade from 1970 to 2000. Such changes can disrupt feeding patterns, pollination, and other vital interactions between species, and they also affect the timing and severity of insects, disease outbreaks, and other disturbances. In the western United States, climate change has increased the population of forest pests such as the pine beetle. The next section describes how observed climate trends and impacts are predicted to continue if emissions of human-produced greenhouse gases are maintained during the next century and beyond. EvidEncE for Human-Caused Climate CHangeFIGURE 18 Evidence of Ocean Acidification With excess CO2 building up in the atmo- sphere, scientists wanted to know if it was also accumulating in the ocean. Studies that began in the mid-1980s show that the concentration of CO2 in ocean water (in blue, calculated from the partial pressure of CO2 in seawater) has risen in parallel with the increase in atmospheric CO2 (in red, part of the Keeling curve). At the same time, the ocean has become more acidic, because the CO2 reacts with seawater to form carbonic acid. The orange dots are direct measurements of pH in surface seawater (lower pH being more acidic), and the green dots are calculated based on the chemical properties of seawater. Source: National Research Council, 2010d Climate change has increased the population of forest pests in the western United States. The red trees in this photo of Dillon Reservoir in Colorado have died from an infestation of mountain pine beetle. 18 Perhaps the most dramatic example of natural climate variability over long time periods is the Ice Age cycle. Detailed analyses of ocean sediments, ice cores, and other data show that for at least 800,000 years, and probably for the past 4 to 5 million years, the Earth has gone through extended periods when temperatures were much lower than today and thick blankets of ice covered large areas of the Northern Hemisphere. These long cold spells, which typically lasted for around 100,000 years, were interrupted by shorter warm “interglacial” periods, including the past 10,000 years (Figure 14). Through a convergence of theory, observations, and modeling, scientists have deduced that the ice ages are caused by slight recurring variations in Earth’s orbit that alter the amount and seasonal distribution of solar energy reaching the Northern Hemisphere. These relatively small changes in solar energy are reinforced over thousands of years by gradual changes in Earth’s ice cover (the cryosphere) and ecosystems (the biosphere), eventually leading to large changes in global the iCe ages 800,000 Years of Temperature and Carbon Dioxide Records As ice core records from Vostok, Antarctica, show, the tempera- ture near the South Pole has varied by as much as 20°F (11°C) during the past 800,000 years. The cyclical pattern of tempera- ture variations constitutes the ice age/interglacial cycles. During these cycles, changes in carbon dioxide concentrations (in red) track closely with changes in temperature (in blue), with CO2 lagging behind temperature changes. Because it takes a while for snow to compress into ice, ice core data are not yet available much beyond the 18th century at most locations. However, atmo- spheric carbon dioxide levels, as measured in air, are higher today than at any time during the past 800,000 years. Source: National Research Council, 2010a FIGURE 14 19 temperature. The average global temperature change during an ice age cycle, which occur over about 100,000 years, is on the order of 9°F ± 2°F (5°C ± 1°C). The data show that in past ice age cycles, changes in temperature have led—that is, started prior to—changes in CO2. This is because the changes in temperature induced by changes in Earth’s orbit around the Sun lead to gradual changes in the biosphere and the carbon cycle, and thus CO2, reinforcing the initial temperature trend. In contrast, the relatively rapid release of CO2 and other greenhouse gases since the start of the Industrial Revolution from the burning of fossil fuel has, in essence, reversed the pattern: the additional CO2 is acting as a climate forcing, with temperatures increasing afterward. The ice age cycles nicely illustrate how climate forcing and feedback effects can alter Earth’s temperature, but there is also direct evidence from past climates that large releases of carbon dioxide have caused global warming. One of the largest known events of this type is called the Paleocene-Eocene Thermal Maximum, or PETM, which occurred about 55 million years ago, when Earth’s climate was much warmer than today. Chemical indicators point to a huge release of carbon dioxide that warmed Earth by another 9°F and caused widespread ocean acidification. These climatic changes were accompanied by massive ecosystem changes, such as the emergence of many new types of mammals on land and the extinction of many bottom- dwelling species in the oceans. The U.S. Geological Survey National Ice Core Lab stores ice cores samples taken from polar ice caps and mountain glaciers. Ice cores provide clues about changes in Earth’s climate and atmosphere going back hundreds of thousands of years. EvidEncE for Human-Caused Climate CHange 20 Fortunately, scientists have made great strides in predicting the amount of temperature change that can be expected for different amounts of future greenhouse gas emissions and in understanding how increments of globally averaged temperatures—increases of 1°C, 2°C, 3°C and so forth—relate to a wide range of impacts. Many of these projected impacts pose serious risks to human societies and things people care about, including water resources, coastlines, infrastructure, human health, food security, and land and ocean ecosystems. Warming, Climate Changes and impacts in the 21st Century and Beyond Part II In order to respond effectively to the risks posed by future climate change, decision makers need information on the types and severity of impacts that might be expected. 21 The biggest factor in determining future global warming is projecting future emissions of CO2 and other greenhouse gases—which in turn depend on how people will produce and use energy, what national and international policies might be imple- mented to control emissions, and what new tech- nologies might become available. Scientists try to account
Scientific evidence for warming of the climate system is unequivocal.
- Intergovernmental Panel on Climate Change
The current warming trend is of particular significance because most of it is extremely likely (greater than 95 percent probability) to be the result of human activity since the mid-20th century and proceeding at a rate that is unprecedented over decades to millennia.
Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.
The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century. Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must cause the Earth to warm in response.
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