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CHAPTER TWO
What We Know About Climate Change and Its Interactions with People and Ecosystems

Over the past several decades, the international and national research communities have developed a progressively clearer picture of how and why Earth’s climate is changing and of the impacts of climate change on a wide range of human and environmental systems. Research has also evaluated actions that could be taken—and in some cases are already being taken—to limit the magnitude of future climate change and adapt to its impacts. In the United States, a series of reports by the U.S. Global Change Research Program (USGCRP, also known as the Climate Change Science Program from 2001 to 2008) have synthesized the information specific to the nation, culminating in the report Global Climate Change Impacts in the United States (USGCRP, 2009a). Internationally, scientific information about climate change is periodically assessed by the Intergovernmental Panel on Climate Change (IPCC), most recently in 2007. Much has been learned, and this knowledge base is continuously being updated and expanded with new research results.


Our assessment of the current state of knowledge about global climate change, which is summarized in this chapter and described in detail in Part II of the report, leads to the following conclusion.


Conclusion 1: Climate change is occurring, is caused largely by human activities, and poses significant risks for—and in many cases is already affecting—a broad range of human and natural systems.


This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of the IPCC’s Fourth Assessment Report (IPCC, 2007a-d), recent assessments by the USGCRP (e.g., USGRP, 2009a), and other recent assessments of the state of scientific knowledge on climate change. Both our assessment and these previous assessments place high or very high confidence1 in the following findings:

1

As discussed in Appendix D, high confidence indicates an estimated 8 out of 10 or better chance of a statement being correct, while very high confidence (or a statement than an ourcome is “very likely”) indicates a 9 out of 10 or better chance.



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CHAPTER TWO What We Know About Climate Change and Its Interactions with People and Ecosystems O ver the past several decades, the international and national research commu- nities have developed a progressively clearer picture of how and why Earth’s climate is changing and of the impacts of climate change on a wide range of human and environmental systems. Research has also evaluated actions that could be taken—and in some cases are already being taken—to limit the magnitude of future climate change and adapt to its impacts. In the United States, a series of reports by the U.S. Global Change Research Program (USGCRP, also known as the Climate Change Science Program from 2001 to 2008) have synthesized the information specific to the nation, culminating in the report Global Climate Change Impacts in the United States (USGCRP, 2009a). Internationally, scientific information about climate change is peri- odically assessed by the Intergovernmental Panel on Climate Change (IPCC), most re- cently in 2007. Much has been learned, and this knowledge base is continuously being updated and expanded with new research results. Our assessment of the current state of knowledge about global climate change, which is summarized in this chapter and described in detail in Part II of the report, leads to the following conclusion. Conclusion 1: Climate change is occurring, is caused largely by human activities, and poses significant risks for—and in many cases is already affecting—a broad range of human and natural systems. This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of the IPCC’s Fourth Assessment Report (IPCC, 2007a-d), recent assessments by the USGCRP (e.g., USGRP, 2009a), and other recent assessments of the state of scientific knowledge on climate change. Both our assessment and these previous assessments place high or very high confidence1 in the following findings: 1 As discussed in Appendix D, high confidence indicates an estimated 8 out of 10 or better chance of a statement being correct, while very high confidence (or a statement than an ourcome is “very likely”) indicates a 9 out of 10 or better chance. 

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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 • Earth is warming. Detailed observations of surface temperature assembled and analyzed by several different research groups show that the planet’s average surface temperature was 1.4ºF (0.8ºC) warmer during the first decade of the 21st century than during the first decade of the 20th century, with the most pronounced warming over the past three decades. These data are cor- roborated by a variety of independent observations that indicate warming in other parts of the Earth system, including the cryosphere (the frozen portions of Earth’s surface), the lower atmosphere, and the oceans. • Most of the warming over the last several decades can be attributed to human activities that release carbon dioxide (CO2) and other heat-trapping green- house gases (GHGs) into the atmosphere. The burning of fossil fuels—coal, oil, and natural gas—for energy is the single largest human driver of climate change, but agriculture, forest clearing, and certain industrial activities also make significant contributions. • Natural climate variability leads to year-to-year and decade-to-decade fluctua- tions in temperature and other climate variables, as well as substantial re- gional differences, but cannot explain or offset the long-term warming trend. • Global warming is closely associated with a broad spectrum of other changes, such as increases in the frequency of intense rainfall, decreases in Northern Hemisphere snow cover and Arctic sea ice, warmer and more frequent hot days and nights, rising sea levels, and widespread ocean acidification. • Human-induced climate change and its impacts will continue for many de- cades, and in some cases for many centuries. Individually and collectively, and in combination with the effects of other human activities, these changes pose risks for a wide range of human and environmental systems, including fresh- water resources, the coastal environment, ecosystems, agriculture, fisheries, human health, and national security, among others. • The ultimate magnitude of climate change and the severity of its impacts depend strongly on the actions that human societies take to respond to these risks. The following sections elaborate on these statements and provide a concise, high- level overview of the current state of scientific knowledge about climate change in 12 critical areas of interest to a broad range of stakeholders: • Changes in the climate system; • Sea level rise and risk in the coastal environment; • Freshwater resources; • Ecosystems, ecosystem services, and biodiversity; 

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What We Know About Climate Change • Agriculture, fisheries, and food production; • Public health; • Cities and the built environment; • Transportation systems; • Energy systems; • Solar radiation management; • National and human security; and • Designing, implementing, and evaluating climate policies. The research progress in each of these topics is explored in additional detail in Part II of the report, but even those chapters are too brief to provide a comprehensive review of the very large body of research on these issues. Likewise, this report does not cover all scientific topics of interest in climate change research, only those of most immedi- ate interest to decision makers. Readers interested in additional information should consult the extensive assessment reports completed by the USGCRP,2 the IPCC,3 the National Research Council (NRC),4 and other groups, as well as the numerous scientific papers that have been published since their completion. CHANGES IN THE CLIMATE SYSTEM5 Earth’s physical climate system, which includes the atmosphere, oceans, cryosphere, and land surface, is complex and constantly evolving. Nevertheless, the laws of physics and chemistry ultimately govern the system, and can be used to understand how and why climate varies from place to place and over time. The Greenhouse Effect is a Natural Phenomenon That Is Critical for Life as We Know It GHGs—which include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others—are present in relatively low concentrations in the atmo- sphere, but, because of their ability to absorb and re-radiate infrared energy, they trap heat near the Earth’s surface, keeping it much warmer than it would otherwise be (Figure 2.1). The atmospheric concentrations of GHGs have increased over the past two centuries as a result of human activities, especially the burning of the fossil 2 http://www.globalchange.gov/publications/reports/scientific-assessments/us-impacts 3 http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm 4 http://national-academies.org/climatechange/ 5 For additional discussion and references, see Chapter 6 in Part II of the report. 

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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 2.1 The greenhouse effect. SOURCE: Marian Koshland Science Museum of the National Academy of Sciences. fuels—coal, oil, and natural gas—for energy. The increasing concentrations of GHGs are amplifying the natural greenhouse effect, causing Earth’s surface temperature to rise. Human activities have also increased the number of aerosols (small liquid drop- lets or particles suspended in the atmosphere). Aerosols have a wide range of environ- mental effects, but on average they increase the amount of sunlight that is reflected back to space, a cooling effect that offsets some, but not all, of the warming induced by increasing GHG concentrations. Earth Is Warming There are many indications—both direct and indirect—that the climate system is warming. The most fundamental of these are thermometer measurements, enough of which have been collected over both land and sea to estimate changes in global average surface temperature since the mid- to late 19th century. A number of inde- 0

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What We Know About Climate Change FIGURE 2.2 Global surface temperature change from 1880 to 2009 in degrees Celsius. The black curve Fig. 2.2 and 6.12.eps shows annual average temperatures, the red curve shows a 5-year running average, and the green bars in- bitmap dicate the estimated uncertainty in the data during different periods of the record. For further details see Figure 6.13. SOURCE: NASA GISS (2010; based on Hansen et al., 2006, updated through 2009 at http://data. giss.nasa.gov/gistemp/graphs/). pendent research teams collect, analyze, and correct for errors and biases in these data (for example, accounting for the “urban heat island” effect and changes in the instru- ments and methods used to measure ocean surface temperatures). Each group uses slightly different analysis techniques and data sources, yet the temperature estimates published by these groups are highly consistent with one another. Surface thermometer measurements show the first decade of the 21st century was 1.4°F (0.8°C) warmer than the first decade of the 20th century (Figure 2.2). This warm- ing has not been uniform, but rather it is superimposed on natural year-to-year and even decade-to-decade variations. Because of this natural variability, it is important to focus on trends over several decades or longer when assessing changes in the heat balance of the Earth. Physical factors also give rise to substantial spatial variations in the pattern of observed warming, with much stronger warming over the Arctic than over tropical latitudes and over land areas than over the ocean. Other measurements of global temperature changes come from satellites, weather balloons, and ships, buoys, and floats in the ocean. Like surface thermometer measure- ments, these data have been analyzed by a number of different research teams around the world, corrected to remove errors and biases, and calibrated using independent observations. Ocean heat content measurements, which are taken from the top sev- 

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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 eral hundred meters of the world’s oceans, show a warming trend over the past several decades that is similar to the atmospheric warming trend in Figure 2.2. Up until a few years ago, scientists were puzzled by the fact that the satellite-based record of atmospheric temperature trends seemed to disagree slightly with the data obtained from weather balloon-based measurements, and both seemed to be slightly inconsistent with surface temperature observations. Recently, researchers identified several small errors in both the satellite and weather balloon-based data sets, includ- ing errors caused by instrument replacements, changes in satellite orbits, and the ef- fect of sunlight on the instruments carried by weather balloons. After correcting these errors, temperature records based on satellite, weather balloon, and ground-based measurements now agree within the estimated range of uncertainty associated with each type of observation. The long-term trends in many other types of observations also provide evidence that Earth is warming. For example: • Hot days and nights have become warmer and more frequent; • Cold snaps have become milder and less frequent; • Northern Hemisphere snow cover is decreasing; • Northern Hemisphere sea ice is declining in both extent and average thickness; • Rivers and lakes are freezing later and thawing earlier; • Glaciers and ice caps are melting in many parts of the world (as described in more detail below); and • Precipitation, ecosystems, and other environmental systems are changing in ways that are consistent with global warming (many of these changes are also described below). Based on this diverse, carefully examined, and well-understood body of evidence, scientists are virtually certain that the climate system is warming. In addition, scientists have collected a wide array of “proxy” evidence that indicates how temperatures and other climate properties varied before direct measurements were available. These proxy data come from ice cores, tree rings, corals, lake sediments, boreholes, and even historical documents and paintings. A recent assessment of these data and the tech- niques used to analyze them concluded that the past few decades have been warmer than any other comparable period for at least the last 400 years, and possibly for the last 1,000 years or longer (NRC, 2006b). 

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What We Know About Climate Change The Climate System Exhibits Substantial Natural Variability Earth’s climate varies naturally on a wide range of timescales, from seasonal varia- tions (such as a particularly wet spring, hot summer, or snowy winter) to geological timescales of millions or even billions of years. Careful statistical analyses have demon- strated that it is very unlikely6 that natural variations in the climate system could have given rise to the observed global warming, especially over the last several decades. However, natural processes produce substantial seasonal, year-to-year, and even de- cade-to-decade variations that are superimposed on the long-term warming trend, as well as substantial regional differences. Improving understanding of natural variability patterns, and determining how they might change with increasing GHG emissions and global temperatures, is an important area of active research (see the end of this sec- tion and Chapter 6). Natural climate variations can also be influenced by volcanic eruptions, changes in the output from the Sun, and changes in Earth’s orbit around the Sun. Large volcanic erup- tions, such as the eruption of Mount Pinatubo in 1991, can spew copious amounts of aerosols into the upper atmosphere. If the eruption is large enough, these aerosols can reflect enough sunlight back to space to cool the surface of the planet by a few tenths of a degree for several years. The Sun’s output has been measured precisely by satellites since 1979, and these measurements do not show any overall trend in solar output over this period. Prior to the satellite era, solar output was estimated by several methods, including methods based on long-term records of the number of sunspots observed each year, which is an indirect indicator of solar activity. These indirect methods suggest that there was a slight increase in solar energy received by the Earth during the first few decades of the 20th century, which may have contributed to the global temperature increase during that period (see Figure 2.2). Perhaps the most dramatic example of natural climate variability is the ice age cycle. Detailed analyses of ocean sediments, ice cores, geologic landforms, and other data show that for at least the past 800,000 years, and probably the past several million years, the Earth has gone through long periods when temperatures were much colder than today and thick blankets of ice covered much of the Northern Hemisphere (in- cluding the areas currently occupied by the cities of Chicago, New York, and Seattle). These very long cold spells were punctuated by shorter, warm “interglacial” periods, including the last 10,000 years. Through a convergence of theory, observations, and 6 As discussed in Appendix D, very unlikely indicates a less than 1 in 10 chance of a statement being incorrect. 

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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 modeling, scientists have deduced that the ice ages were initiated by small recurring variations in the Earth’s orbit around the Sun. GHG Emissions and Concentrations Are Increasing Human activities have increased the concentration of CO2 and certain other GHGs in the atmosphere. Detailed worldwide records of fossil fuel consumption indicate that fossil fuel burning currently releases over 30 billion tons of CO2 into the atmosphere every year (Figure 2.3, blue curve). Tropical deforestation and other land use changes release an additional 3 to 5 billion tons every year. Precise measurements of atmospheric composition at many sites around the world indicate that CO2 levels are increasing, currently at a pace of almost 2 parts per mil- lion (ppm) per year. We know that this increase is largely the result of human activities because the chemical signature of the excess CO2 in the atmosphere can be linked to the composition of the CO2 in emissions from fossil fuel burning. Moreover, analyses of bubbles trapped in ice cores from Greenland and Antarctica reveal that atmospheric CO2 levels have been rising steadily since the start of the Industrial Revolution (usually taken as 1750; see Figure 2.3, red curve). The current CO2 level (388 ppm as of the end of 2009) is higher than it has been in at least 800,000 years. 35 385 370 30 Dioxide Emissions (Gt) Concentration (ppm) 355 Fossil Fuel Carbon 25 Carbon Dioxide 340 20 325 310 15 295 10 280 5 265 250 0 1800 1800 1800 1800 2000 Year FIGURE 2.3 CO2 emissions due to fossil fuel burning (blue line and right axis) from 1800 to 2006 and 2.3.pdf atmospheric CO2 concentrations (red line and left axis) from 1847 to 2008. For further details see Figures 6.2, 6.3, and 6.4. Based on data from Boden et al. (2009), Keeling et al. (2009), and Neftel et al. (1994). 

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What We Know About Climate Change Only 45 percent of the CO2 emitted by human activities remains in the atmosphere; the remainder is absorbed by the oceans and land surface. Current estimates, which are based on a combination of direct measurements and models that simulate ecosys- tem processes and biogeochemical cycles, indicate that roughly twice as much CO2 is taken up annually by ecosystems on the land surface as is released by deforestation; thus, the land surface is a net “carbon sink.” The oceans are also a net carbon sink, but only some of the CO2 absorbed by the oceans is taken up and used by marine plants; most of it combines with water to form carbonic acid, which (as described below) is harmful to many kinds of ocean life. The combined impacts of rising CO2 levels, tem- perature change, and other climate changes on natural ecosystems and on agriculture are described later in this chapter and in further detail in Part II of the report. Human Activities Are Associated with a Net Warming Effect on Climate Human activities have led to higher concentrations of a number of GHGs as well as other climate forcing agents. For example, the human-caused increase in CO2 since the beginning of the Industrial Revolution is associated with a warming effect equivalent to approximately 1.6 Watts of energy per square meter of the Earth’s surface (Figure 2.4). Although this may seem like a small amount of energy, when multiplied by the surface area of the Earth it is 50 times larger than the total power consumed by all hu- man activities. In addition to CO2, the concentrations of methane (CH4), nitrous oxide (N2O), ozone (O3), and over a dozen chlorofluorocarbons and related gases have increased as a result of human activities. Collectively, the total warming associated with GHGs is esti- mated to be 3.0 Watts per square meter, or almost double the forcing associated with CO2 alone. While CO2 and N2O levels continue to rise (due mainly to fossil fuel burning and agricultural processes, respectively), concentrations of several of the halogenated gases are now declining as a result of action taken to protect the ozone layer, and the concentration of CH4 also appears to have leveled off (see Chapter 6 for details). Human activities have also increased the number of aerosols, or particles, in the atmo- sphere. While the effects of these particles are not as well measured or understood as the effects of GHGs, recent estimates indicate that they produce a net cooling effect that offsets some, but not all, of the warming associated with GHG increases (see Figure 2.4). Humans have also modified Earth’s land surface, for example by replacing forests with cropland. Averaged over the globe, it is estimated that these land use and land cover changes have increased the amount of sunlight that is reflected back to 

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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 2.4 Climate forcing due to both human activities and natural processes, expressed in Watts per square meter (energy per unit area). Positive forcing corresponds to a warming effect. See Chapter 6 for further details. SOURCE: Forster et al. (2007). space, producing a small net cooling effect. Other human activities can influence local and regional climate but have only a minor influence on global climate. Feedback Processes Determine How the Climate System Responds to Forcing The response of the climate system to GHG increases and other climate forcing agents is strongly influenced by the effects of positive and negative feedback processes in the climate system. One example of a positive feedback is the water vapor feedback. Water vapor is the most important GHG in terms of its contribution to the natural green- 

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What We Know About Climate Change house effect (see Figure 6.1), but changes in water vapor are not considered a climate forcing because its concentration in the lower atmosphere is controlled mainly by the (natural) processes of evaporation and precipitation, rather than by human activi- ties. Because the rate of evaporation and the ability of air to hold water vapor both increase as the climate system warms, a small initial warming will increase the amount of water vapor in the air, reinforcing the initial warming—a positive feedback loop. If, on the other hand, an initial warming were to cause an increase in the amount of low- lying clouds, which tend to cool the Earth by reflecting solar radiation back to space (especially when they occur over ocean areas), this would tend to offset some of the initial warming—a negative feedback. Other important feedbacks involve changes in other kinds of clouds, land surface properties, biogeochemical cycles, the vertical , profile of temperature in the atmosphere, and the circulation of the atmosphere and oceans—all of which operate on different time scales and interact with one another in scales addition to responding directly to changes in temperature. The collective effect of all feedback processes determines the sensitivity of the climate eedback s the system, or how much the system will warm or cool in response to a certain amount of forcing. A variety of methods have been used to estimate climate sensitivity,, which . is typically expressed as the temperature change expected if atmospheric CO2 levels reach twice their preindustrial values and then remain there until the climate system reaches equilibrium, with all other climate forcings neglected. Most of these estimates indicate that the expected warming due to a doubling of CO2 is between 3.6°F and 8.1°F (2.0°C and 4.5°C), with a best estimate of 5.4°F (3.0°C). Unfortunately, the diversity and complexity of processes operating in the climate system means that, even with , even continued progress in understanding climate feedbacks, the exact sensitivity of the climate system will remain somewhat uncertain. Nevertheless, estimates of climate somewhat uncertain. sensitivity are a useful metric for evaluating the causes of observed climate change and estimating how much Earth will ultimately warm in response to human activities. Global Warming Can Be Attributed to Human Activities Many lines of evidence support the conclusion that most of the observed warming since the start of the 20th century, and especially over the last several decades, can be attributed to human activities, including the following: 1. Earth’s surface temperature has clearly risen over the past 100 years, at the same time that human activities have resulted in sharp increases in CO2 and other GHGs. 2. Both the basic physics of the greenhouse effect and more detailed calcula- 

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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 able and nuclear technology have the potential to provide a large fraction of U.S. elec- tricity supply, but there are a number of distribution, cost, risk, and public acceptance issues that remain to be addressed. Capture and Storage of CO2 During or After Combustion Fossil fuels will probably remain an important part of the U.S. energy system for the near future, in part because of their abundance and the legacy of infrastructure invest- ments. Carbon capture and storage (CCS) technology could be used to remove CO2 from the exhaust gases of power plants fueled by fossil fuels or biomass (as well as exhaust gases from industrial facilities) and sequester it away from the atmosphere in depleted oil and gas reservoirs, coal beds, or deep saline aquifers. Research to evalu- ate the technical, economic, and environmental impacts, and legal aspects, of CCS is a key research need. A number of methods and strategies have also been proposed to capture and sequester CO2 from ambient air. Some of these, such as iron fertilization of the oceans, were mentioned above. Other direct carbon capture technologies, such as air filtration, are in early phases of study. Effects of Climate Change on Energy Systems Climate change is expected to affect energy system operations in several ways. For example, increases in energy demands for cooling and decreases in energy demands for heating can be expected across most parts of the country. This could drive up peak electricity demand, and thus capacity needs, but could also reduce the use of heating oil and natural gas in winter. Water limitations in parts of the country, and increased demand for water for other uses, could result in less water for use in cooling at thermal electric plants. Increased water temperatures may also reduce the cooling capacity of available water resources. Water flows at hydropower sites may increase in some areas and decrease in others. Changes in extreme weather events—including hurricanes, floods, and droughts—may disrupt a wide range of energy system operations, includ- ing transmission lines, oil and gas platforms, ports, refineries, wind farms, and solar installations. Research on Adapting to Climate Change in the Energy Sector Actions to help the energy sector adapt to the effects of climate change include increasing regional electric power generating capacity; accounting for changes in patterns of demand; hardening infrastructure to withstand extreme events; develop- 

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What We Know About Climate Change ing electric power generation strategies that use less water; instituting contingency planning for reduced hydropower generation; increasing resilience of fuel and elec- tricity delivery systems; and increasing energy storage capacity. Research is needed to develop and improve analytical frameworks and metrics for identifying the most vulnerable infrastructure and most effective response options. Research Needs for Advancing Science in the Energy Supply and Consumption Sector Because energy is a dominant component of human GHG emissions, major invest- ments are needed in both the public and private sectors to accelerate research, development, and deployment of climate-friendly energy technologies. Research is also needed on behavioral and institutional barriers to adoption of new energy tech- nologies. It is critical that energy research not be conducted in an isolated manner, but rather using integrated approaches and analyses that investigate energy supply and use within the greater context of efforts to achieve sustainable development goals and other societal concerns. Some specific research needs, discussed in further detail in Chapter 14, include the following: • Develop new energy technologies along with effective implementation strategies. • Develop improved understanding of behavioral impediments at both the individual and institutional level to reducing energy demand and adopting energy efficient technologies. • Develop analytical frameworks to evaluate trade-offs and synergies between efforts to limit the magnitude and adapt to climate change. SOLAR RADIATION MANAGEMENT17 The term geoengineering refers to deliberate, large-scale manipulations of Earth’s environment designed to offset some of the harmful consequences of GHG-induced climate change. Geoengineering encompasses two different classes of approaches: carbon dioxide removal (CDR) and solar radiation management (SRM) (see Figure 2.9). CDR approaches (also referred to as postemission GHG management, atmospheric remediation, or carbon sequestration methods), several of which were discussed in the sections above, involve removal and long-term sequestration of atmospheric CO2 (or other GHGs) in forests, agricultural systems, or through direct air capture and geologic 17 For additional discussion and references, see Chapter 15 in Part II of the report. 

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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 storage. Additional details about these techniques and their implications can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c). SRM approaches, the focus of this section, are those designed to increase the reflec- tivity of Earth’s atmosphere or surface in an attempt to offset some of the effects of GHG-induced climate change. SRM approaches seek to either reduce the amount of sunlight reaching Earth’s surface or reflect additional sunlight back into space. There is a limited body of research on this topic. While some SRM approaches may be techno- logically and economically feasible (only considering direct deployment costs), they all involve considerable risk and potential for unintended (albeit currently understudied) side effects. It is unclear at the present time, therefore, whether SRM could actually re- duce the overall risk associated with climate change and whether it could realistically be employed as quickly as is technically possible, especially in light of the full range of environmental and sociopolitical complexities involved. Although few, if any, voices are promoting SRM as a near-term alternative to GHG emissions-reduction strategies, the concept has recently been gaining more serious attention as a possible “backstop” measure, because strategies attempted to date have failed to yield significant emissions reductions, and climate trends may become sig- nificantly disruptive or dangerous. Further research is necessary to better understand the physical science of the impacts and feasibility of SRM as well as issues related to governance, ethics, social acceptability, and political feasibility of planetary-scale, intentional manipulation of the climate system. Proposed Solar Radiation Management Approaches The SRM approaches proposed to date can be divided into four broad categories: space, stratosphere, cloud, and surface based. Space-based proposals involve plac- ing satellites with reflective surfaces in space. However, to counteract GHG-induced warming, 10 square miles of reflective surface would need to be put into orbit each day for as long as CO2 emissions continue to increase at current rates. The most widely discussed option for stratosphere-based SRM is the injection of sulfate aerosols, which would reflect some amount of incoming solar radiation back to space, offsetting some of the warming associated with GHGs. Another SRM option is to “whiten” clouds, or make them more reflective, by increasing the number of water droplets in the clouds. This could potentially be achieved over remote parts of the ocean by distributing a fine seawater spray in the air. Surface-based options include whitening roofs in the built environment, and planting more reflective crops. While these proposals merit 

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FIGURE 2. Various geoengineering options, including both solar radiation management and carbon dioxide removal. For further details see 2.9.pdf Figure 15.1. SOURCE: Lenton and Vaughn (2009). bitmap, landscape 

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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 further research, their efficacy and environmental consequences are not currently well understood. Potential Drawbacks and Unintended Consequences The overall environmental impacts of SRM approaches are not well characterized, and all proposals have the potential for unintended negative consequences. For example, approaches that are intended to offset globally averaged warming may still lead to local- or regional-scale imbalances in climate forcing that could produce large re- gional changes. Several analyses also suggest that a sudden increase in stratospheric sulfate aerosol could potentially enhance losses of stratospheric ozone for several decades, especially in the Arctic. Additionally, since aerosols remain in the atmosphere for a much shorter time than GHGs, abandonment of aerosol injection could cause warming at a rate far greater than what is estimated in the absence of SRM. These and other issues, including the impact of SRM on precipitation and the hydrologic cycle, are not well understood. Finally, it should be noted that a major shortcoming of SRM approaches is that, while they have the potential to offset GHG-induced warming of the atmosphere, they would not offset ocean acidification or other impacts of elevated CO2. Governance Issues Due to the global nature of SRM, and especially considering the drawbacks and poten- tial negative impacts, an international framework is needed to govern SRM research, development, and possible deployment. Important components of such a framework include a clear definition of “climate emergency” that would trigger deployment and criteria for whether, when, and how SRM approaches should be tested and/or deployed. Unilateral SRM testing or deployment could lead to international tension, distrust, or even conflict. Public involvement in SRM-related decision making, includ- ing research activities, is likewise important since public acceptance is a key issue in informing governance decisions. Ethical Issues Intentional climate alteration, including SRM, raises significant issues with respect to ethics and responsibility. A key consideration in the deployment of SRM, as with other responses to climate change, is the distribution of risks among population groups in 

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What We Know About Climate Change the present generation, as well as future generations. Some have suggested that SRM research efforts may also pose a “moral hazard” by detracting from efforts to reduce GHG emissions or to adapt to the impacts of climate change. SRM and other geoengi- neering approaches also raise deep questions about humans’ relationship with nature, many of which are beyond the scope of this report. Research Needs for Advancing Solar Radiation Management It is beyond the scope of this report to design a research program on SRM, or even to determine the scope, scale, priorities, or goals of such a program. However, the various SRM proposals and their consequences need to be examined, as long as such research does not replace or reduce research on fundamental understanding of climate change or other approaches to limiting climate change or adapting to its impacts. Some key SRM-related research needs, discussed in Chapter 15, include the following: • Improve understanding of the physical potential and technical feasibility of SRM and other geoengineering approaches. • Evaluate the potential consequences of SRM approaches on other aspects of the Earth system, including ecosystems on land and in the oceans. • Develop and evaluate systems of governance that would provide a model for how to decide whether, when, and how to intentionally intervene in the climate system. • Measure and evaluate public attitudes and develop approaches that effec- tively inform and engage the public in decisions regarding SRM. NATIONAL AND HUMAN SECURITY18 Climate change will influence human and natural systems that are linked throughout the globe, creating important implications for bilateral and multilateral relations and for national, international, and human security. Changes in temperature, sea level, precipitation patterns, and other elements of the physical climate system can add substantial stresses to infrastructure and especially to the food, water, energy, and eco- system resources that societies use. Key concerns regarding the interactions between climate change and security include direct impacts on military operations; potential impacts to regional strategic priorities; causal links between environmental scarcity and conflict; the role of environmental conservation and collaboration in promoting peace; and relationships between environmental quality, resource abundance, and 18 For additional discussion and references, see Chapter 16 in Part II of the report. 

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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 human security. In general, these areas are much less well understood than the causes and more direct consequences of climate change. Military Operations Climate change may affect military assets and operations directly: through physical stresses on military systems and personnel, severe weather constraints on operations due to increased frequency and intensity of storms and floods, increased uncertainty about the effects of Arctic ice and ice floes on navigation safety both on and below the ocean surface, or risks to coastal infrastructure due to sea level rise. Climate change is expected to increase heavy rainfalls and floods, droughts, and fires in many parts of the world and could lead to changing storm patterns. This may generate a change in military missions because the U.S. military has substantial logistical, engineering, and medical capabilities that have been used to respond to emergencies in the United States and abroad. Finally, the U.S. military is a major consumer of fossil fuels and could potentially play a major role in reducing U.S. GHG emissions. International Relations and National Security Climate change has the potential to disrupt international relations and raise security challenges through impacts on specific assets and resources. For example, loss of Arctic sea ice will increase the value of Arctic navigation routes. The legal status of the Northwest Passage in particular has long been contested, but the prospect of it becoming more widely usable raises the stakes substantially. Another possible disrup- tion to international relations is the prospect of substantial mineral reserves under the Arctic Ocean. Climate change will also affect shorelines and in some cases “exclu- sive economic zones” and baselines used for projecting national boundaries seaward. Boundaries that could be affected include those in the South China Sea and between the United States and Cuba. Climate-related changes in precipitation and the hydro- logic cycle will likely result in changes in flow regimes in international river systems, and this raises the possibility of challenges to interstate relationships, even conflict, over shared water resources. Finally, climate-related changes in food supply and sea level rise-related land losses could potentially result in intra- and interstate migration and refugee-related conflicts. 

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What We Know About Climate Change Treaty Verification The prospect of binding international agreements on GHG emissions will have impor- tant implications for treaty verification and compliance. In particular, measurements of GHG concentrations and emissions are needed to inform national and international policy aimed at regulating emissions, to verify compliance with emissions-reduction policies, and to ascertain their effectiveness. Measurements of GHGs for treaty verifica- tion or for financial transactions (carbon trading) will require a higher level of scrutiny than that used in the research domain. Key concerns in such a regime are data security, authentication, reliability, and transparency. Human Security The impact of climate change may increase the probability of conflict, and this has become a prominent argument for considering climate change in security analyses. The concept of human security, however, goes far beyond the traditional concerns of national security and conflict and instead includes considerations of access to suf- ficient food, water, and health care infrastructure, as well as freedom from repression and freedom and capacity to determine one’s life path. Analysts have moved toward a more integrative conception of security and threats, one that reflects the lived reali- ties that individuals and communities face. Nevertheless, there are still multiple ways of thinking about human security and no agreement on a policy agenda. Research efforts in this area to date have focused on issues of equity, fairness, vulnerability, and human dignity, and have identified conditions that are critical to maintaining or restoring human security: effective governance systems, healthy and resilient ecosys- tems, comprehensive and sustained disaster risk-management efforts, empowerment of individuals and local institutions, and supportive values. Research Needs for Advancing Science on National and Human Security Implications of Climate Change Scientific understanding of the national and human security implications of climate change are considerably less well understood than many of the other impacts of climate change. As a result, there are a wide variety of research needs for improving understanding of the relationship between climate change and security, including the following: 

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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 • Develop improved observations, models, and vulnerability assessments for regions of importance in terms of military infrastructure. • Build understanding of observations and monitoring requirements for treaty verification. • Identify areas of potential human insecurity and vulnerability in response to climate change impacts interacting with other social and environmental changes. DESIGNING, IMPLEMENTING, AND EVALUATING CLIMATE POLICIES1 Analyzing different policy options that might be used to limit the magnitude of climate change or promote successful adaptation is a key area of scientific research. Indeed, the ability to comprehensively assess the potential consequences of various climate policies—including the costs, benefits, trade-offs, co-benefits, and uncertain- ties associated with their implementation—is paramount to informing public- and private-sector decision making on climate change. Despite a broad range of research focusing on policy making and evaluation in general, policy-oriented research focused specifically on climate change and its interaction with natural and social systems has been relatively limited. Because climate change is becoming an increasingly impor- tant public policy concern in the United States and many other countries, additional research to support climate policy design and implementation is needed. International Policies for Limiting the Magnitude and Adapting to the Impacts of Climate Change At the international level, examples of climate policies include the United Nations Framework Convention on Climate Change, the Kyoto Protocol, and the Copenhagen Accord. Policy options available at the national, regional, and local levels include direct regulation, taxes, cap-and-trade systems for emissions permits, incentive structures and subsidies for voluntary action, technical aid and incentives for the creation and implementation of new technology portfolios, and adaptation options and planning. Research in this area finds that direct regulation, when enforced, can effectively re- duce emissions. It also finds that while taxes are cost-effective, they do not guarantee specific emissions-reduction levels and may be hard to adjust, and that the efficacy of tradable permits depends on the structure of the policy. Voluntary agreements can play a role in accelerating technology adoption, but they are less effective in reducing 19 For additional discussion and references, see Chapter 17 in Part II of the report. 0

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What We Know About Climate Change emissions. Finally, whereas incentives and subsidies to develop cleaner technologies maybe be slow and costly, they can complement other emissions-reduction policies. Monitoring Compliance with Emissions-Reduction Policies International agreements and policies, to be effective, need to be enforced, verified, and monitored. Standards and certification mechanisms for reducing GHG emissions also need to be created and implemented. Constraints to monitoring compliance with and the effectiveness of such policies include lack of adequate and reliable methods for measuring GHG emissions, lack of mechanisms for accurately accounting for GHG emissions and for offsets, and lack of technical capacity to monitor and enforce poli- cies nationally and across international borders. Assessing Benefits and Costs of Climate Action Benefit-cost analyses seek to translate climate change impacts, including lost or gained ecosystem services, into a monetary metric so that they can be compared to estimates of the costs and benefits associated with policies to limit the magnitude or adapt to the impacts of climate change. Alternatively, cost-effectiveness analysis is of- ten used when the costs and benefits of action differ greatly in character, or when the benefits are subject to greater uncertainty or controversy. Cost-effectiveness analysis allows analytically based comparisons of decisions without requiring that all im- pacts—in this case, damages from climate change and costs of emissions reduction— be reduced to a single metric. Both approaches can be powerful tools for informing decisions, but disagreements about (1) how to value ecosystem services or other resources for which market prices do not exist; (2) how to handle low-probability-high- consequence events, discount rates, and risk aversion; (3) prospects for technological innovation; and (4) how to incorporate distributional and intergenerational equity concerns lead to wide ranges in estimates of the social value of climate actions. Dealing with Complex and Interacting Policies, Multilevel Governance, and Equity Effective climate policy making requires analyses that consider the complexity of real policies, how institutions interact across levels of government from global to local, and equity issues. Climate policies are not made in a vacuum. They interact with other climate and nonclimate policies and are often nested across different scales from local to global. In the United States, rapidly emerging local and state climate policy agen- 

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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 das interact with federal policy. It is not yet clear how these interactions will play out and what the net effect will be. The multilevel and hybrid character of climate policy (both for limiting and adapting to climate change) presents opportunities (such as for synergistic outcomes) and challenges (such as one level of decision making constrain- ing or negating the other). One of the most critical challenges is dealing fairly with the distributional effects of climate change impacts. Three main sources of equity con- cerns shape climate policy debates: historical responsibility for the problem of climate change, who will bear the brunt of its negative impacts, and who will be responsible for solving it. Scientific research cannot answer these questions, but it can provide relevant information to policy makers as they attempt to do so. Research Needs Related to Climate Policy Development and Implementation Research needs in this area, explored in further detail in Chapter 17, include the following: • Continue to improve understanding of what leads to the adoption and imple- mentation of international agreements on climate and other environmental issues and what mechanisms are most effective at achieving their goals. • Develop and evaluate protocols, institutions, and technologies for monitoring and verifying compliance with international agreements. • Continue to improve methods for estimating costs, benefits, and cost-effectiveness. • Develop methods for analyzing complex, hybrid policies. • Develop further understanding of how institutions interact in the context of multilevel governance and adaptive risk management. • Develop analyses that examine climate policy from a sustainability perspec- tive, taking account of the full range of effects of climate policy on human well-being, including unintended consequences and equity effects.