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CHAPTER SIX
Changes in the Climate System

Scientific understanding of the factors and processes that govern the evolution of Earth’s climate has increased markedly over the past several decades, as has the ability to simulate and project future changes in the climate system. As noted in Chapter 2, this knowledge has been regularly assessed, synthesized, and summarized by the Intergovernmental Panel on Climate Change (IPCC), the U.S. Global Climate Research Program (USGCRP, referred to as the U.S. Climate Change Science Program from 2000 to 2008), and other groups to provide a thorough and detailed description of what is known about past, present, and projected future changes in climate and related human and environmental systems. This chapter provides an updated overview of the current state of knowledge about the climate system, followed by a list of some of the key scientific advances needed to further improve our understanding.


To help frame the sections that follow, it is useful to consider some questions that decision makers are asking or will be asking about changes in the climate system:

  • How are temperature and other aspects of climate changing?

  • How do we know that humans are responsible for these changes?

  • How will temperature, precipitation, severe weather, and other aspects of climate change in my city/state/region over the next several decades?

  • Will these changes be steady and gradual, or abrupt?

  • Will seasonal and interannual climate variations, like El Niño events, continue the same way or will they be different?

  • Why is there so much uncertainty about future changes?

This chapter attempts to answer these questions or explain what additional research would be needed to answer them. The chapters that follow focus on the impacts of climate change on a range of human and environmental systems, the role of these systems in driving climate change, and the state of scientific knowledge regarding actions that could potentially be taken to adapt to or limit the magnitude of climate change in those systems. All of the chapters in Part II follow a similar structure and are more detailed and extensively referenced than the concise overview of climate change science found in Chapter 2. However, these chapters represent only highlights of a broad and extensive collection of scientific research; readers desiring further detail are encouraged to consult other recent assessment reports and the primary literature.



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CHAPTER SIX Changes in the Climate System S cientific understanding of the factors and processes that govern the evolution of Earth’s climate has increased markedly over the past several decades, as has the ability to simulate and project future changes in the climate system. As noted in Chapter 2, this knowledge has been regularly assessed, synthesized, and summarized by the Intergovernmental Panel on Climate Change (IPCC), the U.S. Global Climate Research Program (USGCRP, referred to as the U.S. Climate Change Science Program from 2000 to 2008), and other groups to provide a thorough and detailed description of what is known about past, present, and projected future changes in climate and re- lated human and environmental systems. This chapter provides an updated overview of the current state of knowledge about the climate system, followed by a list of some of the key scientific advances needed to further improve our understanding. To help frame the sections that follow, it is useful to consider some questions that decision makers are asking or will be asking about changes in the climate system: • How are temperature and other aspects of climate changing? • How do we know that humans are responsible for these changes? • How will temperature, precipitation, severe weather, and other aspects of climate change in my city/state/region over the next several decades? • Will these changes be steady and gradual, or abrupt? • Will seasonal and interannual climate variations, like El Niño events, continue the same way or will they be different? • Why is there so much uncertainty about future changes? This chapter attempts to answer these questions or explain what additional research would be needed to answer them. The chapters that follow focus on the impacts of climate change on a range of human and environmental systems, the role of these systems in driving climate change, and the state of scientific knowledge regarding actions that could potentially be taken to adapt to or limit the magnitude of climate change in those systems. All of the chapters in Part II follow a similar structure and are more detailed and extensively referenced than the concise overview of climate change science found in Chapter 2. However, these chapters represent only highlights of a broad and extensive collection of scientific research; readers desiring further detail are encouraged to consult other recent assessment reports and the primary literature. 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E FACTORS INFLUENCING EARTH’S CLIMATE The Greenhouse Effect The Earth’s physical climate system, which includes the atmosphere, oceans, cryo- sphere, and land surface, is complex and constantly evolving. Nevertheless, the laws of physics, chemistry, and biology ultimately govern the system and can be used to un- derstand how and why climate varies from place to place and over time. For example, the energy balance of the Earth as a whole is determined by the difference between incoming and outgoing energies at the top of the atmosphere. The only significant incoming energy is radiation from the sun, which is concentrated at short wavelengths (visible and ultraviolet light), while the outgoing energy includes both infrared (long- wavelength) radiation emitted by the Earth and the portion of incoming solar radia- tion (about 30 percent on average) that is reflected back to space by clouds, small particles in the atmosphere, and the Earth’s surface. If the outgoing energy is slightly lower than the incoming energy for a period of time, then the climate system as a whole will warm until the outgoing radiation from the Earth balances the incoming radiation from the sun. The temperature of the Earth’s surface and lower atmosphere depends on a broader range of factors, but the transfer of radiation again plays an important role, as does the composition of the atmosphere itself. Nitrogen (N2) and oxygen (O2) make up most of the atmosphere, but these gases have almost no effect on either the incom- ing radiation from the sun or the outgoing radiation emitted by the Earth’s surface. Certain other gases, however, absorb and reemit the infrared radiation emitted by the surface, effectively trapping heat in the lower atmosphere and keeping the Earth’s surface much warmer—roughly 59°F (33°C) warmer—than it would be if greenhouse gases were not present.1 This is called the greenhouse effect, and the gases that cause it—including water vapor, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)—are called greenhouse gases (GHGs). GHGs only constitute a small fraction of the Earth’s atmosphere, but even relatively small increases in the amount of these gases in the atmosphere can amplify the natural greenhouse effect, warming the Earth’s surface (see Figure 2.1). 1 This difference includes the greenhouse effect associated with clouds, which are composed of water droplets, but it assumes that the total reflectivity of the Earth—including the reflection by clouds—does not change. 

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Changes in the Climate System Carbon Dioxide The important role played by CO2 in the Earth’s energy balance has been appreciated since the late 19th century, when Swedish scientist Svante Arrhenius first proposed a link between CO2 levels and temperature. At that time, humans were only begin- ning to burn fossil fuels—which include coal, oil, and natural gas—on a wide scale for energy. The combustion of these fuels, or any material of organic origin, yields mostly CO2 and water vapor, but also small amounts of other by-products, such as soot, carbon monoxide, sulfur dioxide, and nitrogen oxides. All of these substances occur naturally in the atmosphere, and natural fluxes of water and CO2 between the atmo- sphere, oceans, and land surface play a critical role in both the physical climate system and the Earth’s biosphere. However, unlike water vapor molecules, which typically remain in the lower atmosphere for only a few days before they are returned to the surface in the form of precipitation, CO2 molecules are only exchanged slowly with the surface. The excess CO2 emitted by fossil fuel burning and other human activities will thus remain in the atmosphere for many centuries before it can be removed by natural processes (Solomon et al., 2009). A number of agencies and groups around the world, including the Carbon Dioxide Information Analysis Center at Oak Ridge National Laboratory and the International Energy Agency, produce estimates of how much CO2 is released to the atmosphere ev- ery year by human activities. The most recent available estimates indicate that, in 2008, human activities released over 36 Gt (gigatons, or billion metric tons) of CO2 into the atmosphere—including 30.6 ± 1.7 Gt from fossil fuel burning, plus an additional 4.4 ± 2.6 Gt from land use changes and 1.3 ± 0.1 Gt from cement production (Le Quéré et al., 2009). Emissions from fossil fuels have increased sharply over the last two decades, ris- ing 41 percent since 1990 (Figure 6.1). CO2 emissions due to land use change—which are dominated by tropical deforestation—are estimated based on a variety of meth- ods and data sources, and the resulting estimates are both more uncertain and more variable from year-to-year than fossil fuel emissions. Over the past decade (2000– 2008), Le Quéré et al. (2009) estimate that land use changes released 5.1 ± 2.6 Gt of CO2 each year, while fossil fuel burning and cement production together released on average 28.2 ± 1.7 Gt of CO2 per year. Up until the 1950s, most scientists thought the world’s oceans would simply absorb most of the excess CO2 released by human activities. Then, in a series of papers in the late 1950s (e.g., Revelle and Suess, 1957), American oceanographer Roger Revelle and several collaborators hypothesized that the world’s oceans could not absorb all the excess CO2 being released from fossil fuel burning. To test this hypothesis, Revelle’s colleague C. D. Keeling began collecting canisters of air at the Mauna Loa Observatory 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E Total 30 Liquid Carbon Dioxide Solid Gigatons of 20 Gas Cement 10 0 1750 1800 1850 1900 1950 2000 Year FIGURE 6.1 Estimated global CO2 emissions from fossil fuel sources, in gigatons (or billion metric tons). Based on data from Boden et al. (2009; available at.1.pdf 6 http://cdiac.ornl.gov/trends/emis/tre_glob.html). in Hawaii, far away from major industrial and population centers, and analyzing the composition of these samples to determine whether CO2 levels in the atmosphere were increasing. Similar in situ measurements continue to this day at Mauna Loa as well as at many other sites around the world. The resulting high-resolution, well- calibrated, 50-year-plus time series of highly accurate and precise atmospheric CO2 measurements (Figure 6.2), commonly referred to as the Keeling curve, is both a major scientific achievement and a key data set for understanding climate change. The Keeling curve shows that atmospheric CO2 levels have risen by more than 20 percent since 1958; as of January 2010, they stood at roughly 388 ppm, rising at an av- erage annual rate of almost 2.0 ppm per year over the past decade (Blasing, 2008; Tans, 2010). When multiplied by the mass of the Earth’s atmosphere, this increase corre- sponds to 15.0 ± 0.1 Gt CO2 added to the atmosphere each year, or roughly 45 percent of the excess CO2 released by human activities over the last decade. The remaining 55 percent is absorbed by the oceans and the land surface. The size of these CO2 “sinks” is estimated via both modeling and direct observations of CO2 uptake in the oceans and on land. These estimates indicate that the oceans absorbed on average 8.4 ± 1.5 Gt CO2 annually over the last decade (or 26 percent of human emissions), while the land surface took up 11.0 ± 3.3 Gt per year (29 percent), with a small residual of 0.3 Gt (Le Quéré et al., 2009). A careful examination of the Keeling curve reveals that atmospheric CO2 concentra- tions are currently increasing twice as fast as they did during the first decade of the re- cord (compare the slope of the black line in Figure 6.2). This acceleration in the rate of CO2 rise can be attributed in part to the increases in CO2 emissions due to increasing 

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Changes in the Climate System Scripps Institution of Oceanography FIGURE 6.2 Atmospheric CO2 concentrations (in parts per million [ppm]) at Mauna Loa Observatory 6.2.pdf in Hawaii. The red curve, which represents the monthly averaged data, includes a seasonal cycle associ- ated with regular changes in the photosyntheticbitmap plants, which are more widespread in the activity in Northern Hemisphere. The black curve, which represents the monthly averaged data with the seasonal cycle removed, shows a clear upward trend. SOURCE: Tans (2010; available at http://www.esrl.noaa. gov/gmd/ccgg/trends/). energy use and development worldwide (as indicated in Figure 6.1). However, recent studies suggest that the rate at which CO2 is removed from the atmosphere by ocean and land sinks may also be declining (Canadell et al., 2007; Khatiwala et al., 2009). The reasons for this decline are not well understood, but, if it continues, atmospheric CO2 concentrations would rise even more sharply, even if global CO2 emissions remain the same. Improving our understanding and estimates of current and projected future fluxes of CO2 to and from the Earth’s surface, both over the oceans and on land, is a key research need (research needs are discussed at the end of the chapter). To determine how CO2 levels varied prior to direct atmospheric measurements, scientists have studied the composition of air bubbles trapped in ice cores extracted from the Greenland and Antarctic ice sheets. These remarkable data, though not as accurate and precise as the Keeling curve, show that CO2 levels were relatively con- stant for thousands of years preceding the Industrial Revolution, varying in a narrow band between 265 and 280 ppm, before rising sharply starting in the late 19th cen- tury (Figure 6.3). The current CO2 level of 388 ppm is thus almost 40 percent higher 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E FIGURE 6.3 CO2 variations during the last 1,000 years, in parts per million (ppm), obtained from analysis of air bubbles trapped in an ice core extracted from Law Dome in Antarctica. The data show a sharp rise 6.3.pdf in atmospheric CO2 starting in the late 19th century, coincident with the sharp rise in CO2 emissions illus- trated in Figure 6.1. Similar data from other ice bitmap cores indicate that CO2 levels remained between 260 and 285 ppm for the last 10,000 years. SOURCE: Etheridge et al. (1996). than preindustrial conditions (usually taken as 280 ppm). As discussed in further detail in the next section, data from even longer ice cores extracted from the hearts of the Greenland and Antarctic ice sheets—the bottoms of which contain ice that was formed hundreds of thousands of years ago—indicate that the current CO2 levels are higher than they have been for at least 800,000 years. Collectively, the in situ measurements of CO2 over the past several decades, ice core measurements showing a sharp rise in CO2 since the Industrial Revolution, and de- tailed estimates of CO2 sources and sinks provide compelling evidence that CO2 levels are increasing as a result of human activities. There is, however, an additional piece of evidence that makes the human origin of elevated CO2 virtually certain: measure- ments of the isotopic abundances of the CO2 molecules in the atmosphere—a chemi- cal property that varies depending on the source of the CO2—indicate that most of the excess CO2 in the atmosphere originated from sources that are millions of years old. The only source of such large amounts of “fossil” carbon are coal, oil, and natural gas (Keeling et al., 2005). 

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Changes in the Climate System Climate Forcing Changes in the radiative balance of the Earth—including the enhanced greenhouse effect associated with rising atmospheric CO2 concentrations—are referred to as climate forcings (NRC, 2005d). Climate forcings are estimated by performing detailed calculations of how the presence of a forcing agent, such as excess CO2 from human activities, affects the transfer of radiation through the Earth’s atmosphere.2 Climate forcings are typically expressed in Watts per square meter (W/m2, or energy per unit area), with positive forcings representing warming, and are typically reported as the change in forcing since the start of the Industrial Revolution (usually taken to be the year 1750). Figure 6.4 provides a graphical depiction of the estimated globally aver- aged strength of the most important forcing agents for recent climate change. Each of these forcing agents are discussed below. Well-Mixed Greenhouse Gases Carbon dioxide (CO2). The CO2 emitted by human activities is the largest single climate forcing agent, accounting for more than half of the total positive forcing since 1750 (see Figure 6.4). As of the end of 2005, the forcing associated with human-induced atmospheric CO2 increases stood at 1.66 ± 0.17 W/m2 (Forster et al., 2007). This number may seem small relative to the total energy received by the Earth from the sun (which averages 342 W/m2, of which 237 W/m2 is absorbed by the Earth system, after account- ing for reflection of 30 percent of the solar energy back to space). When multiplied by the surface area of the Earth, however, the CO2 forcing is roughly 850 terawatts, which is more than 50 times the total power consumed by all human activities. Human activities have also led to increases in the concentrations of a number of other “well-mixed” GHGs—those that are relatively evenly distributed because their molecules remain in the atmosphere for at least several years on average. Many of these gases are much more potent warming agents, on a molecule-for-molecule basis, than CO2, so even small changes in their concentrations can have a substantial influ- ence. Collectively, they produce an additional positive forcing (warming) of 1.0 ± 0.1 W/m2, for a total well-mixed GHG-induced forcing (including CO2) of 2.63 ± 0.26 W/m2 2 As discussed in NRC (2005e): “Radiative forcing traditionally has been defined as the instantaneous change in energy flux at the tropopause resulting from a change in a component external to the climate system. Many current applications [including the radiative forcing values discussed in this chapter] use an ‘adjusted’ radiative forcing in which the stratosphere is allowed to relax to thermal steady state, thus focusing on the energy imbalance in the Earth and troposphere system, which is most relevant to surface temperature change.” 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E FIGURE 6.4 Radiative forcing of climate between 1750 and 2005 due to both human activities and natural processes, expressed in Watts per square meter (energy per unit area). Positive values correspond to warming. See text for details. SOURCE: Forster et al. (2007). (Forster et al., 2007) (see Figure 6.4). Forcing estimates for all of the well-mixed GHGs are quite accurate because we have precise measurements of their concentrations, their influence on the transfer of radiation through the atmosphere is well understood, and they become relatively evenly distributed across the global atmosphere within a year or so of being emitted. Methane (CH4). Methane is produced from a wide range of human activities, including natural gas management, fossil fuel and biomass burning, animal husbandry, rice cul- tivation, and waste management (Houweling et al., 2006). Natural sources of CH4— which are smaller than human sources—include wetlands and termites, and both of 0

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Changes in the Climate System 6.5.pdf FIGURE 6.5 Atmospheric CH4 concentrations in parts per billion (ppb), (left) during the past millennium, 2 bitmaps measurements. as measured in Antarctic ice cores, and (right) since 1979, based on direct atmospheric SOURCES: Etheridge et al. (2002) and NOAA/ESRL (2009). these sources are actually influenced to some degree by changes in land use. Recent measurements have suggested that plants and crops may also emit trace amounts of CH4 (Keppler et al., 2006), although the size of this source has been questioned (Dueck et al., 2007). 6.5.pdf The atmospheric2concentration of CH4 rose sharply through the late 1970s before bitmaps starting to level off, ultimately reaching a relatively steady concentration of around 1775 ppb—which is more than two-and-a-half times its average preindustrial concen- tration—from 1999 to 2006 (Figure 6.5). There have been several theories proposed for the apparent leveling off of CH4 concentrations, including a decline in industrial emissions during the 1990s and a slowdown of natural wetland-related emissions (Dlugokencky et al., 2003). As discussed at the end of the chapter, there are also con- cerns that warming temperatures could lead to renewed rise in CH4 levels as a result of melting permafrost across the Arctic (Schuur et al., 2009) or, less likely, the destabiliza- tion of methane hydrates3 on the seafloor (Archer and Buffet, 2005; Overpeck and Cole, 2006). The causes of the recent uptick in concentrations in 2007 and 2008 are currently being studied (Dlugokencky et al., 2009). Unlike CO2, which is only removed slowly from the atmosphere by processes at the land surface, the atmospheric concentration of CH4 is limited mainly by a chemical reaction in the atmosphere that yields CO2 and water vapor. As a result, molecules of CH4 spend on average less than 10 years in the atmosphere. However, CH4 is a much 3Methane hydrates are crystalline structures composed of methane and water molecules that can be found in significant quantities in sediments on the ocean floor. 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E FIGURE 6.6 N2O concentrations in the atmosphere, in parts per billion (ppb), (left) during the last millen- nium, and (right) since 1979. SOURCES: Etheridge et al. (1996) and NOAA/ESRL (2009). 6.6.pdf 2 bitmaps more potent warming agent, on a molecule-for-molecule basis,4 than CO2, and its rela- tive concentration in the atmosphere has risen by almost four times as much as CO2. Hence, the increases in CH4 since 1750 are associated with a climate forcing of roughly 0.48 ± 0.05 W/m2 (Forster et al., 2007), or around 18 percent of the total forcing by well-mixed GHGs. Nitrous oxide (N2O). Concentrations of nitrous oxide in the atmosphere have increased around 15 percent since 1750, primarily as a result of agricultural activities (especially the application of chemical fertilizers) but also as a by-product of fossil fuel combus- tion and certain industrial process. The average atmospheric concentration of N2O continues to grow at a steady rate of around 0.8 ppb per year and, as of the end of 2008, stood at just over 322 ppb (Figure 6.6) (see also NASA, 2008). N2O is an extremely potent warming agent—more than 300 times as potent as CO2 on a molecule-by-mol- ecule basis—and its molecules remain in the atmosphere more than 100 years on av- erage. Thus, even though N2O concentrations have not increased nearly as much since 1750 as CH4 or CO2, N2O still contributes a climate forcing of 0.16 ± 0.02 W/m2 (Forster et al., 2007), or around 6 percent of total well-mixed GHG forcing. N2O and its decom- position in the atmosphere also have a number of other environmental effects—for example, N2O is now the most important stratospheric ozone-depleting substance being emitted by human activities (Ravishankara et al., 2009). Halogenated gases. Over a dozen halogenated gases, a category that includes ozone- depleting substances such as chlorofluorocarbons (CFCs), hydrofluorocarbons, per- 4 The relative (molecule-by-molecule) radiative forcing of a GHG over a particular time scale (usually taken as 100 years), compared to carbon dioxide, is sometimes expressed as the global warming potential of the gas. Another common comparative metric is carbon dioxide equivalent (CO2-eq), which describes the equivalent amount of carbon dioxide that would produce the same forcing. 

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Changes in the Climate System fluorocarbons, and sulfur hexafluoride, also contribute to the positive climate forcing associated with well-mixed GHGs. Although relatively rare—their concentrations are typically measured in parts per trillion—many of the halogenated gases have very long residence times in the atmosphere and are extremely potent forcing agents on a molecule-by-molecule basis (Ravishankara et al., 1993). Collectively they contribute an additional 0.33 ± 0.03 W/m2 of climate forcing. Most halogenated gases do not have any natural sources (see, e.g., Frische et al., 2006) but rather arise from a variety of industrial activities. Emissions of many of these ozone-depleting compounds have declined sharply over the past 15 years because of the Montreal Protocol (see below). As a result, their atmospheric concentrations, and hence climate forcing, are now de- clining slightly each year as they are slowly removed from the atmosphere by natural processes (Figure 6.7) (NASA, 2008). It has been estimated that the forcing associated with halogenated gases would be 0.2 W/m2 higher than it is today if emissions reduc- tions due to the Montreal Protocol had not taken place (Velders et al., 2007; see also Chapter 17). Other Greenhouse Gases Ozone (O3). Ozone plays a number of important roles in the atmosphere, depending on location, and its concentration varies substantially, both vertically and horizontally. The highest concentrations of ozone are found in the stratosphere—the layer of the atmo- sphere extending from roughly 10 to 32 miles (15 to 50 km) in height (Figure 6.8)— where it is produced naturally by the dissociation of oxygen molecules by ultraviolet light. This chemical reaction, along with the photodissociation of ozone itself, plays the beneficial role of absorbing the vast majority of incoming ultraviolet radiation, which is harmful to most forms of life, before it reaches the Earth’s surface. Levels of ozone in the stratosphere have been declining over the past several decades, especially over Antarctica. Scientific research has definitively shown that CFCs, along with a few other related man-made halogenated gases (see above), are responsible for these ozone losses in the stratosphere; thus, halogenated gases contribute to both global warming and stratospheric ozone depletion. The Montreal Protocol, which was originally signed in 1987 and has now been revised several times and ratified by 196 countries, has resulted in a rapid phase-out of these gases (see Figure 6.7). Recent evidence suggests that ozone levels in the stratosphere are starting to recover as a result, although it may be several more decades before the ozone layer recovers completely (CCSP, 2008a). Near the Earth’s surface, ozone is considered a pollutant, causing damage to plants and animals, including humans, and it is one of the main components of smog (see Chapter 11). Most surface ozone is formed primarily when sunlight strikes air that 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E FIGURE 6.21 Projected changes in number of very hot days in the United States for lower- and higher-emissions scenario. The number of very hot days will increase sub- stantially across virtually the entire country, in some places doubling or even trebling the number of days above 90°F. SOURCE: USGCRP (2009a). 1 that warmer air can hold more moisture, is that the fraction .22.pdf falling in the 6 of rainfall bitmap form of heavy precipitation events will increase in many regions (Meehl et al., 2007a). These and other projected changes in precipitation, and the impact of these changes on freshwater resources, are explored in Chapter 8. Later chapters also explore how changes in temperature, precipitation, and other aspects of the physical climate system are likely to affect ecosystems (see Chapter 9), agriculture (Chapter 10), human health (Chapter 11), the urban environment (Chapter 12), transportation (Chapter 13) and energy systems (Chapter 14), and national security (Chapter 16). 

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Changes in the Climate System Key Uncertainties in Projections of Future Climate A great deal is known about past, present, and projected future climate change, es- pecially at large (continental to global) scales. For example, there is high confidence that global temperatures will continue to rise, that the rate and magnitude of future temperature change depends strongly on current and future rates of GHG emissions, and that climate change—in interaction with other global and regional environmental changes—poses significant risks for a number of human and natural systems. Global climate models and, increasingly, regional techniques are also starting to provide use- ful information about future climate and climate-related changes on local to regional scales. Some of these projections—such as increases in extreme heat events and Arc- tic sea ice—are quite robust, while others are somewhat more speculative. There are, however, several aspects of future climate change that remain more un- certain, and these represent some of the most important and active areas of current scientific research (see Research Needs at the end of this chapter). The uncertainties in climate projections can be categorized into two main sources: (1) uncertainties in future climate forcing and (2) uncertainties in how the climate system will respond to forcing, which includes both the known limitations of global climate models (such as an inability to resolve individual clouds) and the fact that the climate system is complex and might exhibit novel or unanticipated behavior in response to ongoing climate change. The first of these categories, uncertainties in future climate forcing, was discussed in the Future Climate Scenarios section earlier in the chapter. The spread among the three colored curves in Figure 6.20 provides a rough indication of the importance of this uncertainty in terms of the magnitude of future climate change. As discussed above, and described in further detail in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c), future climate forcing depends strongly on the choices that current and future human societies make, especially regarding energy production and use. However, actions that might be taken to limit the magnitude of future climate change, or adapt to its impacts, have not yet been fully and systemati- cally integrated into climate forcing scenarios and evaluated across a range of differ- ent climate models to determine how they might ultimately affect both climate and other aspects of the Earth system. As an illustration of some of the uncertainties present in climate model projections, Figure 6.22 shows projections of temperature change over North America from 21 dif- ferent models, each using the same scenario of future climate forcing. Several robust features emerge from these projections—for example, all of the models project a 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E FIGURE 6.22 Projected warming for the 21st century (difference between 2080-2099 temperature and 1980-1999 temperature) for the North American region using 21 different climate models, all using the same scenario of future GHG emissions. The mean (average) of the 21 model experiments is also shown in 6.22.pdf the bottom right panel. Several robust features are evident, including enhanced warming over land areas bitmap and higher latitudes. Differences among the 21 projections are indicative of some of the uncertainties associated with model projections. SOURCE: Christensen et al. (2007). 

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Changes in the Climate System substantial overall temperature increase, with stronger warming over land areas and at higher latitudes. Most of the models show somewhat less warming over the south- eastern United States and a slight cooling, or at least less warming, over the western North Atlantic Ocean south of Greenland. In other regions, however, the exact pattern and magnitude of projected warming varies considerably among models. Typically, the average of many climate model simulations represents a more robust projection than any individual projection (Randall et al., 2007), so the average of these model calculations (shown in the bottom right panel) can be thought of as the most reli- able prediction of future temperature change over North America. Differences among models indicate some (but not all) of the uncertainty in this “multimodel mean” projec- tion. Analyses of the differences among models—such as CMIP3 and previous model intercomparison projects—are also a key tool for model development. The other main type of “known” uncertainty in model-based projections of future cli- mate change is associated with processes that are either not resolved or not very well simulated in the current generation of global climate models. These processes, which are discussed in further detail in the Research Needs section at the end of this chapter, include clouds and aerosols, the carbon cycle, ocean mixing processes, ice sheet dy- namics, ecosystem processes, land use–related changes, and extreme weather events such as hurricanes, tornadoes, and droughts. Another key research area is the relation- ship between regional modes of variability and global climate change, including the possibility that regional variability modes may shift in response to either regional or global human activities. Abrupt Changes and Other Climate Surprises Confounding all projections of future climate is the possibility that abrupt changes or other climate “surprises” may occur. Abrupt changes in the climate system can occur when (1) there is a rapid change in forcing, such as a rapid increase in atmospheric GHG concentrations or reduction in aerosol forcing, or (2) thresholds for stability (or “tipping points”) are crossed, such that small changes in the climate state are rein- forced, leading to rapid shifts until the climate enters another stable state and stabil- ity is restored. Paleoclimate records indicate that the climate can go through abrupt changes in as little as a single decade (NRC, 2002a). For example, Greenland ice cores indicate that about 13,000 years ago, during the recovery from the last Ice Age, local temperatures fell more than 10°F (6°C) within a few decades and remained low for more than a millennium before jumping up more than 16°F (10°C) in about a decade (CCSP, 2007b). Since the Earth’s temperature is now demonstrably higher than it has been for at least 400 years and possibly more than 1,000 years (NRC, 2006b), and GHG 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E concentrations are now higher than they have been in many hundreds of thousands of years, it is possible that we may be nearing other stability thresholds. However, we have only a limited understanding of what those thresholds might be or when the climate system might be approaching them. One example of a potential abrupt change mechanism is the possibility that GHGs stored in permafrost (frozen soils) across the Arctic could be released in large quanti- ties as high-latitude warming continues. Permafrost contains huge amounts of car- bon that have been locked away from the active carbon cycle for millennia, and it has been demonstrated that thawing permafrost releases some of this carbon to the atmosphere in the form of CH4 and CO2 (Shakova et al., 2010). If the release of these GHGs accelerates as the Arctic continues to warm, this could potentially accelerate the warming, leading to a positive feedback on the warming associated with GHGs re- leased through human activities (Lawrence and Slater, 2005; Schuur et al., 2009; Zimov et al., 2006). In a related example, high-latitude warming can also alter the types of ecosystems covering the land (for instance, a shift from tundra to forest), which in turn changes the reflective characteristics of the land surface and thus potentially exerts a further positive feedback on warming (Field et al., 2007a). Other potential abrupt changes include rapid disintegration of the major ice sheets (see Chapter 7), irreversible drying and desertification in the subtropics as a result of shifts in circulation patterns (see Chapter 8), changes in the meridional overturning circulation in the ocean (Broecker, 1997, 2002; Stocker, 2000; Stocker and Schmittner, 1997), or the rapid release of CH4 from destabilized methane hydrates in the oceans (Archer and Buffet, 2005; Overpeck and Cole, 2006), all of which could dramatically alter the rate of both regional and global climate change. Other surprises that may be associated with future climate change include so-called “low-probability, high-impact” events, such as an unprecedented heat wave or drought, or when multiple climate changes interact with each other or with other environmental stresses to yield an unexpectedly severe impact on a human or environmental system. Some of these po- tential—or in some cases already observed—surprises are discussed in later chapters. RESEARCH NEEDS Advances in our understanding of the climate system have been and will continue to be a critical underpinning for evaluating the risks and opportunities posed by climate change as well as evaluating and improving the effectiveness of different actions taken to respond. Hence, even as actions are taken to limit the magnitude of future climate change and adapt to its impacts, it is important that continued progress be 

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Changes in the Climate System made in observing all aspects of the climate system, in understanding climate system processes, and in projecting the future evolution of the climate system, and as well as its interactions with other environmental and human systems (which are explored in the chapters that follow). The following are some of the most critical basic research needs in these areas. Expand and maintain comprehensive and sustained climate observations to provide real-time information about climate change. Regular and sustained ob- servations of climate variables are needed to monitor the progress of climate change, inform climate-related decision making, and to monitor the effectiveness of actions taken to respond to climate change. Observations are also critical for developing and testing climate models, projections of future climate forcing, and other tools for understanding and projecting climate change, as well as for supporting decision-sup- port activities. As discussed in Chapter 3, a comprehensive climate observing system is needed to provide regular monitoring of biological, chemical, geological, and physical properties in the atmosphere, oceans, land, and cryosphere, as well as related biologi- cal, ecological, and socioeconomic processes. Expanded historical and paleoclimatic records would also be valuable for understanding natural climate variations on all time scales and how these modes of variability interact with global climate change. Finally, a comprehensive data assimilation system is also needed to bring these dis- parate observations into a common framework, so that the state of the whole Earth system can be assessed and impending feedbacks that could alter the rate of climate change can be identified. Research is especially needed on how to better integrate physical indicators with emerging indicators of ecosystem health and human well-be- ing, as discussed in other chapters. Continue to improve understanding of climate variability and its relationship to climate change. Great strides have been made in improving our understanding of the natural variability in the climate system over the past several decades. These improve- ments have translated directly into advances in detecting and attributing human- induced climate change, simulating past and future climate in models, and under- standing the links between the climate system and other environmental and human systems. For example, the ability to realistically simulate natural climate variations, such as the El Niño-Southern Oscillation, is a critical test for climate models. Improved understanding of regional variability modes is also critical for improving regional climate projections, as discussed below. Understanding the impacts associated with natural climate variations also provides insight into the possible impacts of human-in- duced climate change. Continued research on the mechanisms and manifestations of natural climate variability in the atmosphere and oceans on a wide range of space and 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E time scales, including events in the distant past, can be expected to yield additional progress. Develop more informative and comprehensive scenarios of drivers of future cli- mate forcing and socioeconomic vulnerability and adaptive capacity. Uncertainty in projections of the future is inevitable. However, the development of scenarios allows better understanding of the dynamics of the interconnected human-environment system and in particular how the dynamics will change depending on the choices we make. Scenarios are also critical for helping decision makers establish targets for future GHG emissions and concentration levels as well as helping make plans to adapt to the future projected impacts of climate change, topics addressed in many of the chapters that follow. Developing and improving assessments of the potential influence of vari- ous policy choices on emission profiles and adaptive capacity is particularly important in the context of supporting climate-related decision making—especially “overshoot” scenarios, which have the potential to cause irreversible changes to the climate system. Influences of shorter-lived forcing agents (including short-lived GHGs and aerosols) are also of high importance in the near term and could benefit from more near-term emphasis. Developing enhanced scenarios and linking them to a variety of Earth system and so- cioeconomic models is an inherently interdisciplinary and integrative activity requir- ing contributions from many different scientific fields as well as processes that link sci- entific analysis with decision making and, ideally, public deliberation about desirable futures. The new “Representative Concentration Pathways” described earlier represent a few common, transparent, thoroughly documented representative scenarios of key variables over time. A number of research needs and developments are required to develop new socioeconomic scenarios that explore both mitigation and adaptation issues. It is particularly important to explore methods for coupling scenarios across geographic scales (from global to regional to local), to further develop methods for downscaling climate scenarios and providing regional climate information, and to de- velop data and information systems for pairing socioeconomic and climate scenarios for use in impacts research and to support the needs of particular decision makers. Improve understanding of climate system forcing, feedbacks, and sensitivity. The past several decades have seen tremendous progress in quantifying human influences on climate and assessing the response of the climate system to these influences. This progress has been critical both in establishing the current level of confidence in hu- man-induced climate change and in developing reliable projections of future changes. Key uncertainties remain, however, and continued research on the basic mechanisms 0

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Changes in the Climate System and processes of climate change can be expected to yield additional progress. Some critical areas for further study include the following: • Continued research to improve estimates of climate sensitivity, including theo- retical, modeling, and observationally based approaches; • Improved understanding of cloud processes, aerosols and other short-lived forcing agents, and their interactions, especially in the context of radiative forcing, climate feedbacks, and precipitation processes; • Continued theoretical and experimental research on carbon cycle processes in the context of climate change, especially as they relate to strategies for limit- ing climate change (CCSP, 2007a; NRC, 2010j); • Improve understanding of the relationship between climate change and other biogeochemical changes, especially acidification of the ocean (see Chapter 9); • Improve understanding of the hydrologic cycle, especially changes in precipi- tation (see also Chapter 8); • Improved understanding of the mechanisms, causes, and dynamics of changes in the cryosphere, especially changes in major ice sheets (see Chapter 7) and sea ice. Overall, the need for improved understanding of climate forcing, feedbacks, and sensi- tivity was summarized well in the NRC report Understanding Climate Change Feedbacks (NRC, 2003b); these suggestions remain highly relevant today: The physical and chemical processing of aerosols and trace gases in the at- mosphere, the dependence of these processes on climate, and the influence of climate-chemical interactions on the optical properties of aerosols must be elucidated. A more complete understanding of the emissions, atmospheric burden, final sinks, and interactions of carbonaceous and other aerosols with clouds and the hydrologic cycle needs to be developed. Intensive regional measurement campaigns (ground-based, airborne, satellite) should be con- ducted that are designed from the start with guidance from global aerosol models so that the improved knowledge of the processes can be directly applied in the predictive models that are used to assess future climate change scenarios. The key processes that control the abundance of tropospheric ozone and its interactions with climate change also need to be better understood, including but not limited to stratospheric influx; natural and anthropogenic emissions of precursor species such as NOx, CO, and volatile organic carbon; the net export of ozone produced in biomass burning and urban plumes; the loss of ozone at the surface, and the dependence of all these processes on climate change. The 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E chemical feedbacks that can lead to changes in the atmospheric lifetime of methane also need to be identified and quantified. Improve model projections of future climate change. Numerous decisions about climate change, including setting emissions targets and developing and implement- ing adaptation plans, require information that is underpinned by models of the physical climate system. There are a number of scientific and technological advances needed to improve model projections of future changes in the Earth system, espe- cially changes over the next several decades and at the local and regional levels where many climate-related decisions occur. While this research should not be expected to eliminate uncertainties, especially given the inherent uncertainty in projections of future climate forcing, efforts to expand and improve model simulations of future climate changes can be expected to yield more, more robust, and more relevant in- formation for decision making, including the effectiveness of various actions that can be taken to respond to climate change. It should also be noted that improvements in modeling go hand-in-hand with improvements in understanding and observation. The core of the nation’s climate modeling enterprise is the development and testing of global Earth system models, many of which already or are now beginning to in- corporate some of the key forcing and feedback processes noted above, including an explicit carbon cycle, certain biogeochemical and ecological processes, and improved parameterizations for clouds, aerosols, and ocean mixing. While these important activi- ties should continue, the nation should also initiate a strategy for developing the next generation of ultra-high-resolution global models; models that can explicitly resolve clouds and other small-scale processes, include explicit representations of ice sheets and terrestrial and marine ecosystems, and allow for integrated exploration of forcing and feedback processes from local to global scales (Shapiro et al., in press). It may be valuable to consider the merits of coordinating the development of climate models with the development of weather models through “seamless prediction” paradigms that could potentially improve the simulation of extreme events as well as lower development costs (Tebaldi and Knutti, 2007). Expanded computing resources and hu- man capital are needed to support all of these activities. Climate modelers in the United States and around the world have also begun to de- vise strategies for improving the utility of climate models. Decadal-scale climate pre- diction, in which climate models are initialized with present-day observations and run forward in time at fairly high resolution for three to four decades, is another emerging strategy to provide decision makers with information to support near-term decision making (Meehl et al., 2009b). Extending or coupling current models to models of human and environmental systems, including both ecosystems models and models 

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Changes in the Climate System of human activities, would foster the development of more robust and integrated as- sessments of key impacts of climate change (see Chapter 4). Finally, the usefulness of climate model experiments to decision making would be improved if they could be used to comprehensively assess a wider variety of climate response strategies, includ- ing specific GHG emissions-reduction strategies, adaptation strategies, and solar radia- tion management strategies (see Chapter 15). Improve regional climate modeling, observations, and assessments. Given the importance of local and regional information to decision makers, and the fact that it might take decades to develop global models with sufficient resolution to resolve local-scale processes, it is essential to continue improving regional climate informa- tion, including observations and assessments of regional climate and climate-related changes as well as models that can project interannual, decadal, and multidecadal climate change, including extreme events, at regional to local scales across a range of future global climate change scenarios. Improvements in regional climate obser- vations, modeling, and assessment activities often go hand in hand—for example, local and regional-scale observations are needed to verify regional models or down- scaled estimates of precipitation. Models also require a variety of information, for example the regional climate forcing associated with aerosols and land use change, that is also useful to decision makers for planning climate response strategies and for other reasons (such as monitoring air quality). It will also be important to improve our understanding and ability to model regional climate dynamics, including atmo- spheric circulation in complex terrain as well as modes of natural climate variability on all time scales, especially how their intensity and geographic patterns may change under different scenarios of global climate change. Several strategies for improving regional climate models are described in this chapter, including statistical and dynami- cal approaches. As with the development of global climate models, further progress in regional modeling will require expanded computing resources, improvements in data assimilation and parameterization, and both national and international coordination. Advance understanding of thresholds, abrupt changes, and other climate “sur- prises.” Some of the largest potential risks associated with future climate change come not from the relatively smooth changes in average climate conditions that are reasonably well understood and resolved in current climate models, but from extreme events, abrupt changes, and surprises that might occur when thresholds in the climate system (or related human or environmental systems) are crossed. While the paleo- climate record indicates that abrupt climate changes have occurred in the past, and we have many examples of extreme events and nonlinear interactions among differ- ent components of the human-environment system that have resulted in significant impacts, our ability to predict these kinds of events or even estimate their likelihood 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E is limited. Improving our ability to identify potential thresholds and evaluate the potential risks from unlikely but high-impact events will be important for evaluating proposed climate targets and developing adaptation strategies that are robust in the face of uncertainty. Sustained observations will be critical for identifying the signs of possible thresholds and for supporting the development of improved representations of extreme events and nonlinear processes in climate models. Expanded historical and paleoclimatic records would also be valuable for understanding the impacts as- sociated with abrupt changes in the past. Finally, since some abrupt changes or other climate surprises may result from complex interactions among different components of the coupled human-environment system, improved understanding is needed on multiple stresses and their potential intersection with future climate shifts.