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CHAPTER FOUR
Integrative Themes for Climate Change Research

One of the main tasks assigned to the Panel on Advancing the Science of Climate Change was to identify the additional science needed to improve our understanding of climate change and its interactions with human and environmental systems, including the scientific advances needed to improve the effectiveness of actions taken to respond to climate change. An examination of the research needs identified in the technical chapters of Part II of the report reveals that there is indeed still much to learn. However, our analysis suggests that the most crucial research needs of the coming decades can be captured in seven crosscutting research themes, whether one is interested in sea level rise, agriculture, human health, national security, or other topics of concern. For example, nearly every chapter in Part II calls for improved understanding of human behaviors and institutions, more detailed information about projected future changes in climate, and improved methods for assessing the economic, social, and environmental costs, benefits, co-benefits, and unintended consequences of actions taken in response to climate change.


Box 4.1 lists the seven crosscutting research themes that the panel has identified, grouped into three general categories: research for improving understanding of coupled human-environment systems, research for improving and supporting more effective responses to climate change, and tools and approaches needed for both of these types of research. These seven crosscutting themes are not intended to represent a comprehensive or exclusive list of research needs, nor do the numbers indicate priority order. Rather, they represent a way of categorizing and, potentially, organizing some of the nation’s most critical climate change research activities. Most of these themes are integrative—they require collaboration across different fields of study, including some fields that are not typically part of the climate change science enterprise. Moreover, there are important synergies among the seven themes, and they are not completely independent. For example, research focused on improving responses to climate change will clearly benefit from increased understanding of both human systems and the Earth system, and advances in observations, models, and scientific understanding often go hand in hand. Finally, because most of the themes include research that contributes both to fundamental scientific understanding and to more informed decision making, research under all seven themes would benefit from



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CHAPTER FOUR Integrative Themes for Climate Change Research O ne of the main tasks assigned to the Panel on Advancing the Science of Climate Change was to identify the additional science needed to improve our understanding of climate change and its interactions with human and environmental systems, including the scientific advances needed to improve the effectiveness of actions taken to respond to climate change. An examination of the research needs identified in the technical chapters of Part II of the report reveals that there is indeed still much to learn. However, our analysis suggests that the most crucial research needs of the coming decades can be captured in seven crosscutting research themes, whether one is interested in sea level rise, agriculture, human health, national security, or other topics of concern. For example, nearly every chapter in Part II calls for improved understanding of human behaviors and institutions, more detailed informa- tion about projected future changes in climate, and improved methods for assessing the economic, social, and environmental costs, benefits, co-benefits, and unintended consequences of actions taken in response to climate change. Box 4.1 lists the seven crosscutting research themes that the panel has identified, grouped into three general categories: research for improving understanding of coupled human-environment systems, research for improving and supporting more effective responses to climate change, and tools and approaches needed for both of these types of research. These seven crosscutting themes are not intended to repre- sent a comprehensive or exclusive list of research needs, nor do the numbers indicate priority order. Rather, they represent a way of categorizing and, potentially, organizing some of the nation’s most critical climate change research activities. Most of these themes are integrative—they require collaboration across different fields of study, including some fields that are not typically part of the climate change science enter- prise. Moreover, there are important synergies among the seven themes, and they are not completely independent. For example, research focused on improving responses to climate change will clearly benefit from increased understanding of both human systems and the Earth system, and advances in observations, models, and scientific understanding often go hand in hand. Finally, because most of the themes include research that contributes both to fundamental scientific understanding and to more informed decision making, research under all seven themes would benefit from 

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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 BOX 4.1 Crosscutting Themes for the New Era of Climate Change Research Research to Improve Understanding of Human-Environment Systems 1. Climate Forcings, Feedbacks, Responses, and Thresholds in the Earth System 2. Climate-related Human Behaviors and Institutions Research to Support Effective Responses to Climate Change 3. Vulnerability and Adaptation Analyses of Coupled Human-Environment Systems 4. Research to Support Strategies for Limiting Climate Change 5. Effective Information and Decision-Support Systems Research Tools and Approaches to Improve Both Understanding and Responses 6. Integrated Climate Observing Systems 7. Improved Projections, Analyses, and Assessments increased dialogue with decision makers across a wide range of sectors and scales. As discussed in Chapter 5, these characteristics all point to the need for an expanded and enhanced climate change science enterprise—an enterprise that is comprehensive, integrative, interdisciplinary, and better supports decision making both in the United States and around the world. In the following sections, the seven integrative, crosscutting research themes identi- fied by the panel are discussed in detail. Our intent is to describe some of the more important scientific issues that could be addressed within each theme, to show how they collectively span the most critical areas of climate change research, and to dem- onstrate the vital importance of research progress in all of these areas to the health and well-being of citizens of the United States as well as people and natural systems around the world. Issues related to the implementation of these themes are explored in the next chapter. THEME 1: CLIMATE FORCINGS, FEEDBACKS, RESPONSES, AND THRESHOLDS IN THE EARTH SYSTEM Scientific understanding of climate change and its interactions with other environ- mental changes is underpinned by empirical and theoretical understanding of the Earth system, which includes the atmosphere, land surface, cryosphere, and oceans, 

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Integrative Themes for Climate Change Research as well as interactions among these components. Numerous decisions about cli- mate change, including setting emissions targets and developing and implement- ing adaptation plans, rest on understanding how the Earth system will respond to greenhouse gas (GHG) emissions and other climate forcings. While this understanding has improved markedly over the past several decades, a number of key uncertainties remain. These include the strength of certain forcings and feedbacks, the possibility of abrupt changes, and the details of how climate change will play out at local and regional scales over decadal and centennial time scales. While research on these topics cannot be expected to eliminate all of the uncertainties associated with Earth system processes (and uncertainties in future human actions will always remain), efforts to improve projections of climate and other Earth system changes can be expected to yield more robust and more relevant information for decision making, as well as a bet- ter characterization of remaining uncertainties. Research on forcing, feedbacks, thresholds, and other aspects of the Earth system has been ongoing for many years under the auspices of the U.S. Global Change Research Program (USGCRP) and its predecessors (see Appendix E). Our analysis—the details of which can be found in Part II of the report—indicates that additional research, sup- ported by expanded observational and modeling capacity, is needed to better un- derstand climate forcings, feedbacks, responses, and thresholds in the Earth system. A list of some of the specific research needs within this crosscutting theme is included in Table 4.1, and the subsections below and the chapters of Part II include additional discussion of these topics. Many of these needs have also been articulated, often in greater detail, in a range of recent reports by the USGCRP, the National Research Coun- cil, federal agencies, and other groups. Climate Variability and Abrupt Climate Change Great strides have been made in improving our understanding of the natural variability in the climate system (see, e.g., Chapter 6 of this report and USGCRP, , 2009b). These improvements have translated directly into advances in detecting and attributing human-induced climate change, simulating past and future climate in models, and understanding 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, has been a critical driver for, and test of, the development of climate models (see Theme 7). Improved (see . understanding of natural variability modes is also critical for improving regional climate projections, especially on decadal time scales. Research on the impacts of . natural climate variations can also provide insight into the possible impacts of human- 

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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 TABLE 4.1 Examples of Research Needs Related to Improving Fundamental Understanding of Climate Forcings, Feedbacks, Responses, and Thresholds in the Earth System • Extend understanding of natural climate variability on a wide range of space and time scales, including events in the distant past. • Improve understanding of transient climate change and its dependence on ocean circulation, heat transport, mixing processes, and other factors, especially in the context of decadal-scale climate change. • Improve estimates of climate sensitivity, including theoretical, modeling, and observationally based approaches. • Expand observations and understanding of aerosols, especially their radiative forcing effects and implications for strategies that might be taken to limit the magnitude of future climate change; • Improve understanding of cloud processes, and cloud-aerosol interactions, especially in the context of radiative forcing, climate feedbacks, and precipitation processes. • Improve understanding of ice sheets, including the mechanisms, causes, dynamics, and relative likelihood of ice sheet collapse versus ice sheet melting. • Advance understanding of thresholds and abrupt changes in the Earth system. • Expand understanding of carbon cycle processes in the context of climate change and develop Earth system models that include improved representations of carbon cycle processes and feedbacks. • Improve understanding of ocean dynamics and regional rates of sea level rise. • Improve understanding of the hydrologic cycle, especially changes in the frequency and intensity of precipitation and feedbacks of human water use on climate. • Improve understanding and models of how agricultural crops, fisheries, and natural and managed ecosystems respond to changes in temperature, precipitation, CO2 levels and other environmental and management changes. • Improve understanding of ocean acidification and its effects on marine ecosystems and fisheries. SOURCE: These research needs (and those included in each of the other six themes in this chapter) are compiled from the detailed lists of key research needs identified in the technical chapters of Part II of this report. induced climate change. Continued research on the mechanisms and manifesta- tions of natural climate variability in the atmosphere and oceans on a wide range of space and time scales, including events in the distant past, can be expected to yield , additional progress. Some of the largest risks associated with climate change are associated with the potential for abrupt changes or other climate “surprises” (see Chapters 3 and 6). The paleoclimate record indicates that such abrupt changes have occurred in the past, but our ability to predict future abrupt changes is constrained by our limited understand- 

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Integrative Themes for Climate Change Research ing of thresholds and other nonlinear processes in the Earth system. An improved understanding of the likelihood and potential consequences of these changes will be important for setting GHG emissions-reduction targets and for developing adapta- tion strategies that are robust in the face of uncertainty. Sustained observations will be critical for identifying abrupt changes and other climate surprises if and when they occur, and for supporting the development of improved abrupt change simulations in climate models. Finally, since some abrupt changes or other climate surprises may result from complex interactions within or among different components of coupled human-environment systems, improved understanding is needed on multiple stresses and their potential role in future climate shifts (NRC, 2002a). Improved understanding of forcings, feedbacks, and natural variability on regional scales is also needed. Many decisions related to climate change impacts, vulnerability, and adaptation could benefit from improvements in regional-scale information, es- pecially over the next several decades. As discussed in Theme 7, these improvements require advances in understanding regional climate dynamics, including atmospheric circulation in complex terrain as well as modes of natural variability on all time scales. It is especially important to understand how regional variability patterns may change under different scenarios of global climate change and the feedbacks that regional changes may in turn have on continental- and global-scale processes. Regional climate models, which are discussed later in this chapter, are a key tool in this area of research. The Atmosphere Many research needs related to factors that influence the atmosphere and other components of the physical climate system are discussed in the chapters of Part II, and many of these needs have also been summarized in other recent reports. For example, many of the conclusions and research recommendations in Understanding Climate Change Feedbacks (NRC, 2003b) and Radiative Forcing of Climate Change (NRC, 2005d), such as those highlighted in the following two paragraphs, 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- 

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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 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 chemical feedbacks that can lead to changes in the atmospheric lifetime of CH4 also need to be identified and quantified. (NRC, 2003b) Two particularly important—and closely linked—research topics related to forc- ing and feedback processes in the physical climate system are clouds and aerosols. Aerosols and aerosol-induced changes in cloud properties play an important role in offsetting some of the warming associated with GHG emissions and may have im- portant implications for several proposed strategies for limiting the magnitude of climate change (see Theme 4). Cloud processes modulate future changes in tempera- ture and in the hydrologic cycle and thus represent a key feedback. As noted later in this chapter, the representation of cloud and aerosol processes in climate models has been a challenge for many years, in part because some of the most important cloud and aerosol processes play out at spatial scales that are finer than global climate models are currently able to routinely resolve, and in part because of the complex- ity and limited understanding of the processes themselves. Continued and improved observations, field campaigns, process studies, and experiments with smaller-domain, high-resolution models are needed to improve scientific understanding of cloud and aerosol processes, and improved parameterizations will be needed to incorporate this improved understanding into global climate models. The Cryosphere Changes in the cryosphere, especially the major ice sheets on Greenland and Ant- arctica, represent another key research area in the physical climate system. Compre- hensive, simultaneous, and sustained measurements of ice sheet mass and volume changes and ice velocities are needed, along with measurements of ice thickness and bed conditions, both to quantify the current contributions of ice sheets to sea level rise (discussed below) and to constrain and inform ice sheet model development. These measurements, which include satellite, aircraft, and in situ observations, need 

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Integrative Themes for Climate Change Research to overlap for several decades in order to enable the unambiguous isolation of ice melt, ice dynamics, snow accumulation, and thermal expansion. Equally important are investments in improving ice sheet process models that capture ice dynamics as well as ice-ocean and ice-bed interactions. Efforts are already underway to improve modeling capabilities in these critical areas, but fully coupled ice-ocean-land models will ultimately be needed to reliably assess ice sheet stability, and considerable work remains to develop and validate such models. Glaciers and ice caps outside Greenland and Antarctica are also expected to remain significant contributors to sea level rise in the near term, so observations and analysis of these systems remain critical for under- standing decadal and century-scale sea level rise. Finally, additional paleoclimate data from ice cores, corals, and ocean sediments would be valuable for testing models and improving our understanding of the impacts of sea level rise. The Oceans A variety of ocean processes are important for controlling the timing and characteris- tics of climate change. For a given climate forcing scenario, the timing of atmospheric warming is strongly dependent on the north-south transport of heat by ocean cur- rents and mixing of heat into the ocean interior. Changes in the large-scale meridional overturning circulation could also have a significant impact on regional and global cli- mate and could potentially lead to abrupt changes (Alley et al., 2003; NRC, 2002a). The relative scarcity of ocean observations, especially in the ocean interior, makes these factors among the more uncertain aspects of future climate projections. Changes in ocean circulations and heat transport are also connected to the rapid disappearance of summer sea ice in the Arctic Ocean. A better understanding of the dependence of ocean heat uptake on vertical mixing and the abrupt changes in polar reflectivity that follow the loss of summer sea ice in the Arctic are some of the most critical improve- ments needed in ocean and Earth system models. Ice dynamics and thermal expansion are the main drivers of rising sea levels on a global basis, but ocean dynamics and coastal processes lead to substantial spatial vari- ability in local and regional rates of sea level rise (see Chapters 2 and 7). Direct, long- term monitoring of sea level and related oceanographic properties via tide gauges, ocean altimetry measurements from satellites, and an expanded network of in situ measurements of temperature and salinity through the full depth of the ocean wa- ter column are needed to quantify the rate and spatial variability of sea level change and to understand the ocean dynamics that control global and local rates of sea level rise. In addition, oceanographic, geodetic, and coastal models are needed to predict the rate and spatial dynamics of ocean thermal expansion, sea level rise, and coastal 

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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 inundation. The need for regionally specific information creates additional challenges. For example, coastal inundation models require better bathymetric data, better data on precipitation rates and stream flows, ways of dealing with storm-driven sediment transport, and the ability to include the effects of built structures on coastal wind stress patterns (see Chapter 7). Such improvements in projections of sea level changes are critical for many different decision needs. The Hydrosphere There is already clear evidence that changes in the hydrologic cycle are occurring in response to climate change (see, e.g., Trenberth et al., 2007; USGCRP, 2009a). Improved regional projections of changes in precipitation, soil moisture, runoff, and groundwa- ter availability on seasonal to multidecadal time scales are needed to inform water management and planning decisions, especially decisions related to long-term infra- structure investments. Likewise, projections of changes in the frequency and intensity of severe storms, storm paths, floods, and droughts are critical both for water manage- ment planning and for many adaption decisions. Developing improved understand- ing and projections of hydrological and water resource changes will require new multiscale modeling approaches, such as nesting cloud-resolving climate models into regional weather models and then coupling these models to land surface models that are capable of simulating the hydrologic cycle, vegetation, multiple soil layers, ground- water, and stream flow. Improved data collection, data analysis, and linkages with water managers are also critical. See Chapter 8 for additional details. Ecosystems on Land Climate change interacts with ecosystem processes in a variety of ways, including direct and indirect influences on biodiversity, range and seasonality shifts in both plants and animals, and changes in productivity and element cycling processes, among others (NRC, 2008b). Research is needed to understand how rapidly species and ecosystems can or cannot adjust in response to climate-related changes and to understand the implications of such adjustments for ecosystem services. In addition, improved analyses of the interactions of climate-related variables—especially temper- ature, moisture, and CO2—with each other and in combination with other natural and human-caused changes (e.g., land use change, water diversions, and landscape-scale management choices) are needed, as such interactions are more relevant than any individual change acting alone. Climate change-related changes in fire, pest, and other disturbance regimes have also not been well assessed, especially at regional scales. 

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Integrative Themes for Climate Change Research Research is needed to identify the ecosystems, ecosystem services, species, and people reliant on them that are most vulnerable. See Chapter 9 for additional details. The Carbon Cycle Changes in the carbon cycle and other biogeochemical cycles play a key role in modulating atmospheric and oceanic concentrations of CO2 and other GHGs. Scien- tists have learned a great deal over the past 50 years about the exchange of carbon between the atmosphere, ocean, and biosphere and the effects of these changes on temperature and other climate change (CCSP, 2007a). However, key uncertainties re- main. For example, we have an incomplete understanding of how interacting changes in temperature, precipitation, CO2, and nutrient availability will change the processing of carbon by land ecosystems and, thus, the amount of CO2 emitted or taken up by ecosystems in the decades ahead (see Chapter 9). As noted in Chapters 2 and 6, some of these feedbacks have the potential to dramatically accelerate global warming (e.g., the possibility that the current warming of permafrost in high-latitude regions will lead to melting of frozen soils and release huge amounts of CO2 and CH4 into the at- mosphere). Changes in biogeochemical processes and biodiversity (including changes in reflectance characteristics due to land use changes) also have the potential to feed back on the climate system on a variety of time scales. Models and experiments that integrate knowledge about ecosystem processes, plant physiology, vegetation dynam- ics, and disturbances such as fire are needed, and such models should be linked with climate models. As the ocean warms and ocean circulation patterns change, future changes in the ocean carbon cycle are also uncertain. For example, it is unclear whether the natural “biological pump,” which transports enormous amounts of carbon from the surface to the deep ocean, will be enhanced (Riebesell et al., 2007) or diminished (Mari, 2008) by ocean acidification and by changes in ocean circulation. Recent observational and modeling results suggest that the rate of ocean uptake of CO2 may in fact be declining (Khatiwala et al., 2009). Because the oceans currently absorb over 25 percent of hu- man-caused CO2 emissions (see Chapter 6), changes in ocean CO2 uptake could have profound climate implications. Results from the first generation of coupled carbon-cli- mate models suggest that the capacity of the oceans and land surface to store carbon will decrease with global warming, which would represent a positive feedback on warming (Friedlingstein et al., 2006). Improved understanding and representation of the carbon cycle in Earth system models is thus a critical research need. 

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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 Interactions with Managed Systems and the Built Environment Feedbacks and thresholds within human systems and human-managed systems, and between the climate system and human systems, are a closely related research need that spans both this research theme and several of the other research themes described in this chapter. For example, crops respond to multiple and interacting changes in temperature, moisture, CO2, ozone, and other factors, such as pests, dis- eases, and weeds. Experimental studies that evaluate the interactions of multiple factors are needed, especially in ecosystem-scale experiments and in environments where temperature is already close to optimal for crops. Of particular concern are water resources for agriculture, which are influenced at regional scales by competition from other uses as well as by changing frequency and intensity of rainfall. Assessments that evaluate crop response to climate-related variables should explicitly include interactions with other resources that are also affected by climate change. Design- ing effective agricultural strategies for limiting and adapting to climate change will require models and analyses that reflect these complicated interactions and that also incorporate the response of farmers and markets not only to production and prices but to policies and institutions (see Themes 3, 4, and 7 below). In fisheries, sustainable yields require matching catch limits with the growth of the fishery. Climate variability already makes forecasting the growth of fish populations difficult, and future climate change will increase this uncertainty. There is considerable uncertainty about—and considerable risk associated with—the sensitivity of fish spe- cies to ocean acidification. Further studies of connections between climate and marine population dynamics are needed to enhance model frameworks for effective fisher- ies management. Most fisheries are also subject to other stressors, such as increasing levels of pollution, and the interactions of these other stresses should be analyzed and incorporated into models. Finally, all of these efforts should be linked to the analysis of effective institutions and policies for managing fisheries. (See Chapter 9 for additional details of links between climate change and agriculture and fisheries.) The role of large built environments (including the transportation and energy systems associated with them) in shaping GHG emissions, aerosol levels, ground-level air pol- lution, and surface reflectivity need to be examined in a systematic and comparative way to develop a better understanding of their role in climate forcing. This should include attention to the extended effect of urban areas on other areas (such as de- position of urban emissions on ocean and rural land surfaces) as well as interactions between urban and regional heat islands and urban vegetation-evapotranspiration feedbacks to climate. Examination of both local and supralocal institutions, markets, and policies will be required to understand the various ways urban centers drive 00

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Integrative Themes for Climate Change Research climate change and to identify leverage points for intervention. (See Chapter 10 and Theme 4 later in this chapter for additional details.) Finally, the identification and evaluation of unintended consequences of proposed or already-initiated strategies to limit the magnitude of climate change or adapt to its impacts will need to be evaluated as part of the overall evaluation of the efficacy of such approaches. This topic is explored in more detail later in the chapter, but it depends on a robust Earth system research enterprise. THEME 2: CLIMATE-RELATED HUMAN BEHAVIORS AND INSTITUTIONS Knowledge gained from research involving physical, chemical, and ecological pro- cesses has been critical for establishing that climate change poses sufficiently serious risks to justify careful consideration and evaluation of alternative responses. Emerging concerns about how best to respond to climate change also bring to the fore ques- tions about human interactions with the climate system: how human activities drive climate change; how people understand, decide, and act in the climate context; how people are affected by climate change; and how human and social systems might respond. Thus, not surprisingly, many of the research needs that emerge from the detailed analyses in Part II focus on human interactions with climate change (see Table 4.2). Human and social systems play a key role in both causing and responding to climate change. Therefore, in the context of climate change, a better understanding of hu- man behavior and of the role of institutions and organizations is as fundamental to effective decision making as a better understanding of the climate system. Such knowledge underlies the ability to solve focused problems of climate response, such as deciding how to prioritize investments in protecting coastal communities from sea level rise, choosing policies to meet federal or state targets for reducing GHG emis- sions, and developing better ways to help citizens understand what science can and cannot tell them about potential climate-driven water supply changes. Such funda- mental understanding provides the scientific base for making informed choices about climate responses in much the same way that a fundamental understanding of the physical climate system provides the scientific base for projecting the consequences of climate change. Research investments in the behavioral and social sciences would expand this knowl- edge base, but such investments have been lacking in the past (e.g., NRC, 1990a, 1999a, 2003a, 2004b, 2005a, 2007f, 2009k). Barriers and institutional factors, both in research funding agencies and in academia more broadly, have also constrained progress in 0

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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 proaches for integrated analysis. Rather, it provides examples of the kinds of ap- proaches that need to be developed, improved, and used more extensively to improve scientific understanding of climate change and make this scientific knowledge more useful for decision making. Scenario Development Scenarios help improve understanding of the key processes and uncertainties associ- ated with projections of future climate change. Scenarios are critical for helping deci- sion makers establish targets or budgets for future GHG emissions and devise plans to adapt to the projected impacts of climate change in the context of changes in other human and environmental systems. Scenario development is an inherently interdisci- plinary and integrative activity requiring contributions from many different scientific fields as well as processes that link scientific analysis with decision making. Chapter 6 describes some recent scenario development efforts as well as several key outstand- ing research needs. Climate Models Climate models simulate how the atmosphere, oceans, and land surface respond to increasing concentrations of GHGs and other climate drivers that vary over time (see Chapter 6). These models are based on numerical representations of fundamen- tal Earth system processes, such as the exchange of energy, moisture, and materials between the atmosphere and the underlying ocean or land surface. Climate models have been critically important for understanding past and current climate change and remain an essential tool for projecting future changes. They have also been steadily increasing in detail, sophistication, and complexity, most notably by improving spatial resolution and incorporating representations of atmospheric chemistry, biogeochemi- cal cycling, and other Earth system processes. These improvements represent an im- portant integrative tool because they allow for the evaluation of feedbacks between the climate system and other aspects of the Earth system. As discussed in Chapter 6, there are a number of practical limitations, gaps in under- standing, and institutional constraints that limit the ability of climate models to inform climate-related decision making, including the following • The ability to explicitly simulate all relevant climate processes (for example, individual clouds) on appropriate space and time scales; • Constraints on computing resources; 0

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Integrative Themes for Climate Change Research • Uncertainties and complexities associated with data assimilation and parameterization; • Lack of a well-developed framework for regional downscaling; • Representing regional modes of variability; • Projecting changes in storm patterns and extreme weather events; • Inclusion of additional Earth system processes, such as ice sheet dynamics and fully interactive ecosystem dynamics; • Ability to simulate certain nonlinear processes, including thresholds, tipping points, and abrupt changes; and • Representing all of the processes that determine the vulnerability, resilience, and adaptability of both natural and human systems. As discussed in Chapter 6, climate modelers in the United States and around the world have begun to devise strategies, such as decadal-scale climate predictions, for improv- ing the utility of climate model experiments. These experimental strategies may in- deed yield more decision-relevant information, but, given the importance of local- and regional-scale information for planning responses to climate change, continued and expanded investments in regional climate modeling remain a particularly pressing priority. Expanded computing resources and human capital are also needed. Progress in both regional and global climate modeling cannot occur in isolation. Expanded observations are needed to initialize models and validate results, to de- velop improved representations of physical processes, and to support downscaling techniques. For example, local- and regional-scale observations are needed to verify regional models or downscaled estimates of precipitation, and expanded ocean ob- servations are needed to support decadal predictions. Certain human actions and ac- tivities, including agricultural practices, fire suppression, deforestation, water manage- ment, and urban development, can also interact strongly with climate change. Without models that account for such interactions and feedbacks among all important aspects of the Earth system and related human systems, it is difficult to fully evaluate the costs, benefits, trade-offs and co-benefits associated with different courses of action that might be taken to respond to climate change (the next subsection describes model- ing approaches that address some of these considerations). An advanced generation of climate models with explicit and improved representations of terrestrial and marine ecosystems, the cryosphere, and other important systems and processes, and with improved representations and linkages to models of human systems and actions, will be as important as improving model resolution for increasing the value and utility of climate and Earth system models for decision making. 

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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 Models and Approaches for Integrated Assessments Integrated assessments combine information and insights from the physical and biological sciences with information and insights from the social sciences (including economics, geography, psychology, and sociology) to provide comprehensive analyses that are sometimes more applicable to decision making than analyses of human or environmental systems in isolation. Integrated assessments—which are done through either formal modeling or through informal linkages among relevant disciplines— have been used to develop insights into the possible effectiveness and repercussions of specific environmental policy choices (including, but not limited to, climate change policy) and to evaluate the impacts, vulnerability, and adaptive capacity of both hu- man and natural systems to a variety of environmental stresses. Several different kinds of integrated assessment approaches are discussed in the paragraphs below. Integrated Assessment Models In the context of climate change, integrated assessment models typically incorporate a climate model of moderate or intermediate complexity with models of the economic system (especially the industrial and energy sectors), land use, agriculture, ecosystems, or other systems or sectors germane to the question being addressed. Rather than focusing on precise projections of key system variables, integrated assessment models are typically used to compare the relative effectiveness and implications of different policy measures (see Chapter 17). Integrated assessment models have been used, for instance, to understand how policies designed to boost production of biofuels may actually increase tropical deforestation and lead to food shortages (e.g., Gurgel et al., 2007) and how policies that limit CO2 from land use and energy use together lead to very different costs and consequences than policies that address energy use alone (e.g., Wise et al., 2009a). Another common use of integrated assessments and inte- grated assessment models is for “impacts, adaptation, and vulnerability” or IAV assess- ments, which evaluate the impacts of climate change on specific systems or sectors (e.g., agriculture), including their vulnerability and adaptive capacity, and explore the effectiveness of various response options. IAV assessments can aid in vulnerability and adaptation assessments of the sort described in Theme 3 above. An additional and valuable role of integrated assessment activities is to help deci- sion makers deal with uncertainty. Three basic approaches to uncertainty analysis have been employed by the integrated assessment community: sensitivity analysis, stochastic simulation, and sequential decision making under uncertainty (DOE, 2009b; Weyant, 2009). The aim of these approaches is not to overcome or reduce uncertainty, 

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Integrative Themes for Climate Change Research but rather to characterize it and help decision makers make informed and robust decisions in the face of uncertainty (Schneider and Kuntz-Duriseti, 2002), for instance by adopting an adaptive risk-management approach to decision making (see Box 3.1). Analytic characterizations of uncertainty can also help to determine the factors or pro- cesses that dominate the total uncertainty associated with a specific decision and thus potentially help identify research priorities. For example, while uncertainties in climate sensitivity and future human energy production and consumption are widely appreci- ated, improved methods for characterizing the uncertainty in other socioeconomic drivers of environmental change are needed. In addition, a set of fully integrated models capable of analyzing policies that unfold sequentially, while taking account of uncertainty, could inform policy design and processes of societal and political judg- ment, including judgments of acceptable risk. Enhanced integrated assessment capability, including improved representation of diverse elements of the coupled human-environment system in integrated assess- ment models, promises benefits across a wide range of scientific fields as well as for supporting decision making. A long-range goal of integrated assessment models is to seamlessly connect models of human activity, GHG emissions, and Earth system pro- cesses, including the impacts of climate change on human and natural systems and the feedbacks of changes in these systems on climate change. In addition to improved computational resources and improved understanding of human and environmental systems, integrated assessment modeling would also benefit from model intercom- parison and assessment techniques similar to those employed in models that focus on Earth system processes. Life-Cycle Assessment Methods4 The impacts of a product (or process) on the environment come not only when the product is being used for its intended purpose, but also as the product is manufac- tured and as it is disposed of at the end of its useful life. Efforts to account for the full set of environmental impacts of a product, from production of raw materials through manufacture and use to its eventual disposition, are referred to as life-cycle analysis (LCA). LCA is an important tool for identifying opportunities for reducing GHG emis- sions and also for examining trade-offs between GHG emissions and other environ- mental impacts. LCA has been used to examine the GHG emissions and land use requirements of renewable energy technologies (e.g., NRC, 2009) and other technolo- 4 This subsection was inadvertently left out of prepublication copies 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 gies that might reduce GHG emissions (e.g., Jaramillo et al., 2009, Kubiszewski et al., 2010, Lenzen, 2008, Samaras and Meisterling, 2008). LCA of corn-based ethanol and other liquid fuels derived from plant materials (e.g., Davis et al., 2009; Kim et al., 2009; Robertson et al., 2008; Tilman et al., 2009) illustrate both the value of the method and some of the complexities in applying it. Because corn ethanol is produced from sugars created by photosynthesis, which removes CO2 from ambient air, it might be assumed that substituting corn ethanol for gasoline pro- duced from petroleum would substantially reduce net GHG emissions. However, LCA shows that these emissions reductions are much smaller (and in some cases may even result in higher GHG emissions) when the emissions associated with growing the corn, processing it into ethanol, and transporting it are accounted for. A substantial shift to corn-based ethanol (or other biofuels) could also lead to significant land use changes and changes in food prices. LCA also points out the importance of farming practices in shaping agricultural GHG emissions and to the potential for alternative plant inputs, such as cellulose, as a feedstock for liquid fuels. The utility and potential applications of LCA have been recognized by government agencies in the United States and around the world (EPA, 2010a; European Commis- sion Joint Research Centre, 2010) and by the private sector. For example, Walmart is emphasizing LCA in the sustainability assessment it is requiring of all its suppliers.5 Useful as it is, LCA, like any policy analysis tool, has limitations. For example, the bound- aries for the analysis must be defined, materials used for multiple purposes must be allocated appropriately, and the databases typically consulted to estimate emissions at each step of the analysis may have uncertainties. There is currently little standardiza- tion of these databases or of methods for drawing boundaries and allocating impacts. LCA may also identify multiple environmental impacts. For example, nuclear reactors or hydroelectric systems produce relatively few GHG emissions but have other envi- ronmental impacts (see, e.g., NRC, 2009d; NRC, 2009f ), and it is not clear how to weight trade-offs across different types of impacts (but see Huijbregts et al., 2008). Finally, LCA is not familiar to most consumers and policy makers so its ultimate contribution to better decision making will depend on processes that encourage its use. These and other scientific challenges are starting to be addressed by the research community (see, e.g., Finnveden et al., 2009; Horne et al., 2009; Ramaswami et al., 2008); additional research on LCA would allow its application to an expanding range of problems and improve its use as a decision tool in adaptive risk-management strategies. 5 See http://walmartstores.com/Sustainability/9292.aspx. 

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Integrative Themes for Climate Change Research Environmental Benefit-Cost and Cost-Effectiveness Analyses Integrated assessment models are intended to help decision makers understand the implications of taking different courses of action, but when there are many outcomes of concern, the problem of how to make trade-offs remains. Benefit-cost analysis is a common method for making trade-offs across outcomes and thus linking modeling to the decision-support systems (see Chapter 17). Benefit-cost analysis defines each out- come as either a benefit or a cost, assigns a value to each of the projected outcomes, weights them by the degree of certainty associated with the projection of outcomes, and discounts outcomes that occur in the future. Then, by comparing the ratio of ben- efits to costs (or using a similar metric), benefit-cost analysis allows for comparisons across alternative decisions, including across different policy options. As discussed in Chapter 17, the current limits of benefit-cost analysis applied to global climate change decision making are substantial. A research program focused on improvements to benefit-cost analysis and other valuation approaches, especially for ecosystem services (see below), could yield major contributions to improved decision making. Equity and distributional weighting issues, including issues related to weight- ing the interests of present versus future generations, are areas of particular interest. In all, five major research needs are identified in Chapter 17: (1) estimating the social value of outcomes for which there is no market value, such as for many ecosystem services; (2) handling low-probability/high-consequence events; (3) developing better methods for comparing near-term outcomes to those that occur many years hence; (4) incorporating technological change into the assessment of outcomes; and (5) includ- ing equity consideration in the analysis. In contrast to benefit-cost analysis, cost-effectiveness analysis compares costs of actions to predefined objectives, without assigning a monetary value to those objec- tives. Cost-effectiveness analysis, which is also discussed in Chapter 17, can be espe- cially useful when there is only one policy objective, such as comparing alternative policies for pricing GHG emissions to reach a specific emissions budget or concentra- tion target. Cost-effectiveness analysis avoids some of the difficulties of benefit-cost analysis. However, when more than one outcome matters to decision makers, cost- effectiveness analysis requires a technique for making trade-offs. Again, additional research can help to extend and improve such analyses. 

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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 Ecosystem Function and Ecosystem Services Models Dynamic models of ecosystem processes and services translate what is known about biophysical functions of ecosystems and landscapes or water systems into informa- tion about the provision of goods and services that are important to society (Daily and Matson, 2008). Such models are critical in allowing particular land, freshwater, or ocean use decisions to be evaluated in terms of resulting values to decision makers and society; for evaluating the effects of specific policies on the provision of goods and services; or for assessing trade-offs and side benefits of particular choices of land or water use. For example, Nelson et al. (2009) used ecosystem models to determine the potential for policies aimed at increasing carbon sequestration to also aid in spe- cies conservation. Such analyses can yield maps and other methods for conveying complex information in ways that can effectively engage decision makers and allow them to compare alternative decisions and their impacts on the ecosystem services of interest to them (MEA, 2005; Tallis and Kareiva, 2006). Ecosystem process models and other methods for assessing the effects of policies on ecosystem goods and services (MEA, 2005; Turner et al., 1998; Wilson and Howarth, 2002) also provide critical information about the impacts and trade-offs associated with both climate-related and other choices, including impacts that might not other- wise be considered by decision makers (Daily et al., 2009). If and when such informa- tion is available, various market-based schemes and “payments for ecosystem services” approaches have been developed to provide a mechanism for compensating resource managers for the ecosystem services provided to other individuals and communities. The design and evaluation of such mechanisms requires collaboration across disci- plines (including, for example, ecology and economics) and improvements in the abil- ity to link incentives with trade-offs and synergies among multiple services (Jack et al., 2008). Valuation of goods and services that typically fall outside the realm of economic analysis remains a significant research challenge, although a number of approaches have been developed and applied (Farber et al., 2002). Policy-Oriented Heuristic Models Policy-oriented simulation methods can be a useful tool for informing policy makers about the basic characteristics of climate policy choices. These simulation methods can either involve informal linkages between policy choices, climate trajectories, and economic information, or be implemented in a formal integrated modeling frame- work. For example, the C-ROADS model6 divides the countries of the world into blocs 6 See http://www.climateinteractive.org/simulations/C-ROADS. 

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Integrative Themes for Climate Change Research with common situations or common interests (such as the developed nations), takes as input the commitments to GHG emissions reductions each bloc might be willing to make, and generates projected emissions, atmospheric CO2 concentrations, tem- perature, and sea level rise over the next 100 years. The underlying model is simple enough to be used in real time by policy makers to ask “what if” questions that can inform negotiations. It can also be used in combination with gaming simulations in which individuals or teams take on the roles of blocs of countries and negotiate with each other to simulate not only the climate system but also the international negotia- tion process. When such simplified models are used, however, it is important to ensure that the simplified representations of complex processes are backed up, supported, and verified by more comprehensive models that can simulate the full range of critical processes in both the Earth system and human systems. Heuristic models and exercises have also been developed that engage decision mak- ers, scientists, and others in planning exercises and gaming to explore futures. Such tools are particularly well developed for military and business applications but have also been applied to climate change, including in processes that engage citizens (Pou- madère et al., 2008; Toth and Hizsnyik, 2008). Though not predictive, such models and exercises can provide unexpected insights into future possibilities, especially those that involve human interactions. They can also be powerful tools for helping deci- sion makers understand and develop strategies to cope with uncertainty, especially if coupled with improved visualization techniques (Sheppard, 2005; Sheppard and Meitner, 2005). Metrics and Indicators Metrics and indicators are critical tools for monitoring climate change, understand- ing vulnerability and adaptive capacity, and evaluating the effectiveness of actions taken to respond to climate change. While research on indicators has been a focus of attention for several decades (Dietz et al., 2009c; Orians and Policansky, 2009; Parris and Kates, 2003; York, 2009), progress is needed to improve integration of physical indicators with emerging indicators of ecosystem health and human well-being (NRC, 2005c). Developing reliable and valid approaches for measuring and monitoring sustainable well-being (that is, approaches that account for multiple dimensions of human well-being, the social and environmental factors that contribute to it, and the relative efficiency with which nations, regions, and communities produce it) would greatly aid adaptive risk management (see Box 3.1) by providing guidance on the overall effectiveness of actions taken (or not taken) in response to climate change and other risks. 

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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 Development of and improvements in metrics or indicators that span and integrate all relevant physical, chemical, biological, and socioeconomic domains are needed to help guide various actions taken to respond to climate change. Such metrics should focus on the “vitals” of the Earth system, such as freshwater and food availability, ecosystem health, and human well-being, but should also be flexible and, to the extent allowed by present understanding, attempt to identify possible indicators of tipping points or abrupt changes in both the climate system and related human and environmental systems. Many candidate metrics and indicators exist, but additional research will be needed to test, refine, and extend these measures. One key element in this research area is the development of more refined metrics and indicators of social change. For example, gross domestic product (GDP) is a well-de- veloped measure of economic transactions that is often interpreted as a measure of overall human well-being, but GDP was not designed for this use and may not be a good indicator of either collective or average well-being (Hecht, 2005). A variety of ef- forts are under way to develop alternative indicators of both human well-being and of human impact on the environment that may help monitor social and environmental change and the link between them (Frey, 2008; Hecht, 2005; Krueger, 2009; Parris and Kates, 2003; Wackernagel et al., 2002; World Bank, 2006). Certification Systems and Standards A number of certification systems have emerged in recent decades to identify prod- ucts or services with certain environmental or social attributes, assist in auditing compliance with environmental or resource management standards, and to inform consumers about different aspects of the products they consume (Dilling and Farhar, 2007; NRC, 2010d). In the context of climate change, certification systems and stan- dards are sets of rules and procedures that are intended to ensure that sellers of cred- its are following steps that ensure that CO2 emissions are actually being reduced (see Chapter 17). Certification systems typically span a product’s entire supply chain, from source materials or activities to end consumer. Performance standards are frequently set and monitored by third-party certifiers, and the “label” is typically the indicator of compliance with the standards of the system. Natural resource certification schemes, many of which originated in the forestry sec- tor, have inspired use in fisheries, tourism, some crop production, and park manage- ment (Auld et al., 2008; Conroy, 2006). Variants are also used in the health and building sectors and in even more complicated supply chains associated with other markets. Certification schemes are increasingly being used to address climate change issues, 

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Integrative Themes for Climate Change Research especially issues related to energy use, land use, and green infrastructure, as well as broader sustainability issues (Auld et al., 2008; Vine et al., 2001). With such a diversifica- tion and proliferation of certification systems and standards, credibility, equitability, usability, and unintended consequences have become important challenges. These can all be evaluated through scientific research efforts (NRC, 2010d; Oldenburg et al., 2009). For example, research will be needed to improve understanding and analysis of the credibility and effectiveness of specific approaches, including positive and nega- tive unintended consequences. Analysis in this domain, as with many of the others discussed in this chapter, will require integrative and interdisciplinary approaches that span a range of scientific disciplines and also require input from decision makers. CHAPTER CONCLUSION Climate change has the potential to intersect with virtually every aspect of human activity, with significant repercussions for things that people care about. The risks asso- ciated with climate change have motivated many decision makers to begin to take or plan actions to limit climate change or adapt to its impacts. These actions and plans, in turn, place new demands on climate change research. While scientific research alone cannot determine what actions should be taken in response to climate change, it can inform, assist, and support those who must make these important decisions. The seven integrative, crosscutting research themes described in this chapter are criti- cal elements of a climate research endeavor that seeks to both improve understand- ing and to provide input to and support for climate-related actions and decisions, and these themes would form a powerful foundation for an expanded climate change research enterprise. Such an enterprise would continue to improve our understanding of the causes, consequences, and complexities of climate change from an integrated perspective that considers both human systems and the Earth system. It would also inform, evaluate, and improve society’s responses to climate change, including ac- tions that are or could be taken to limit the magnitude of climate change, adapt to its impacts, or support more effective climate-related decisions. Several of the themes in this chapter represent new or understudied elements of climate change science, while others represent established research programs. Prog- ress in all seven themes is needed (either iteratively or concurrently) because they are synergistic. Meeting this expanded set of research requirements will require changes in the way climate change research is supported, organized, and conducted. Chapter 5 discusses how this broader, more integrated climate change research enterprise might be formulated, organized, and conducted, and provides recommendations for the new era of climate change research. 

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