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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties 7 Recommendations The current global mean top-of-the-atmosphere (TOA) radiative forcing concept with adjusted stratospheric temperatures has been used extensively in the climate research literature over the past few decades and has also become a standard tool for policy analysis endorsed by the Intergovernmental Panel on Climate Change (IPCC). It is a useful index for estimating global average surface temperature change resulting from changes in well-mixed greenhouse gases, solar irradiance, surface albedo, and nonabsorbing aerosols. The relative ease of calculating radiative forcing and the associated temperature response has enabled the use of climate models, simpler versions of those models, and chemical transport models to investigate the many factors that may influence climate. In short, the TOA radiative forcing concept still has considerable value and should be retained as a standard metric in future climate research. Nonetheless, the traditional radiative forcing concept has major limitations that have been revealed by recent research on nonconventional forcing agents and regional studies. It is limited in its ability to describe the climate effects of absorbing aerosols, aerosol interactions with clouds, ozone, land-surface modification, and surface biogeochemical effects. Also, it diagnoses only one measure of climate change: equilibrium response of global mean surface temperature. It does not provide information on nonradiative climate effects, spatial or temporal variation of the forcing, or nonlinearity in the relationship between forcings and surface temperature response. Recent extensions of the concept that allow surface temperatures to adjust have refined the radiative forcing concept to address deficiencies in the original approach. Although currently applied to global mean conditions, this method could be extended for regional conditions.
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties The strengths of the traditional radiative forcing concept warrant its continued use in scientific investigations, climate change assessments, and policy applications. At the same time, its limitations call for using additional metrics that account more fully for the nonradiative effects of forcing, the spatial and temporal heterogeneity of forcing, and nonlinearities. The committee believes that these limitations can be addressed effectively through the introduction of additional forcing metrics in climate change research and policy. This chapter provides several recommendations for extending the traditional radiative forcing concept in the scientific and policy arenas. It identifies research needed to improve quantification and understanding of different forcings and their impacts on climate, to better inform climate policy discussions, and to obtain reliable observations of climate forcings and responses in the past and future. A large number of recommendations are provided because many research avenues need to be explored in order to improve understanding of climate forcings. The recommendations that should be undertaken immediately with high priority are identified with the symbol. EXPANDING THE RADIATIVE FORCING CONCEPT Account for the Vertical Structure of Radiative Forcing Recent observations show that radiative forcing calculated at the top of the atmosphere is not always a good index for changes in surface temperature. Indeed, the relationship between TOA radiative forcing and surface temperature is not valid if there is significant variation in the vertical distribution of radiative forcing. For example, the direct radiative forcing of black carbon and other absorbing aerosols leads to a reduction in surface heat input while increasing atmospheric heating. Likewise, land-use changes can modify latent and sensible heat fluxes at the surface. Considering the surface radiative forcing along with the comparable value at the top of the atmosphere would enable quantification of the effects of aerosols and other forcings on the surface energy balance and the net forcing of the atmosphere. It would provide information about the extent to which forcings affect the atmospheric lapse rate, with implications for precipitation and mixing. Associated with expanding the treatment of radiative forcing in this way are several new research needs. In general, climate models have been unable to reproduce the vertical distribution of forcing due to aerosols observed during aircraft campaigns. Nor, in general, do general circulation models (GCMs) have the needed stratospheric processes to adequately model volcanic and solar ultraviolet radiation effects. Little research has addressed how climate response might depend on the vertical structure of the radiative forcing.
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties RECOMMENDATIONS: Test and improve the ability of climate models to reproduce the observed vertical structure of forcing for a variety of locations and forcing conditions. Undertake research to characterize the dependence of climate response on the vertical structure of radiative forcing. Report global mean radiative forcing at both the surface and the top of the atmosphere in climate change assessments. • Develop parameterizations for using surface forcing in integrated assessment and simple climate models. Determine the Importance of Regional Variation in Radiative Forcing The concept of a global mean radiative forcing is an approximation. Even forcings thought to be fairly uniform, such as solar variability and the well-mixed greenhouse gases, have seasonal and latitudinal variability. Other forcings, in particular tropospheric aerosols and landscape changes, have much more spatial and temporal heterogeneity in their distribution. Human modifications to landscape and vegetation dynamics have caused large regional changes in the surface distribution of net absorbed surface radiation into latent and sensible turbulent heat fluxes. To date, there are only limited observational and modeling studies of regional radiative forcing and response. Indeed, there is not yet a consensus on how best to diagnose a regional forcing and response in the observational record. Regional variations in radiative forcing are likely important for understanding regional and global climate responses; however, the relationship between the two is not well understood. Regional climate responses can also be caused by global forcings, making it difficult to disentangle the effects of regional and global forcings. Regional diabatic heating can cause nonlinear, long-distance communication of convergence and divergence fields, often referred to as teleconnections. Thus, regionally concentrated diabatic heating can influence climate thousands of kilometers away from its source region. Improving societally relevant projections of regional impacts will require a better understanding of the magnitude of regional forcings and the associated climate response. RECOMMENDATIONS: Use climate records to investigate relationships between regional radiative forcing (e.g., land-use or aerosol changes) and climate response in the same region, other regions, and globally. • Test and improve model simulations of regional radiative forcing and the surface energy budget using observations from aircraft campaigns, surface networks, and satellites.
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties Quantify and compare climate responses from regional radiative forcings in different climate models and on different timescales (e.g., seasonal, interannual), and report results in climate change assessments. Specific focus should be given to regions in which forcing could interact with modes of climate variability (e.g., El Niño/Southern Oscillation [ENSO], Antarctic Oscillation, Arctic Oscillation) and result in major teleconnections (e.g., forcing over the tropical Pacific from biomass burning affecting ENSO and therefore drought in the United States, Australia, and other distant regions); regions in which there are significant anthropogenic forcings due to anthropogenic emissions or land-use modifications—for example, North America and Europe (industrial emissions, reduction in sulfur dioxide [SO2] emissions), Asia (black carbon emissions, land-use change), and the Amazon (deforestation); and major geopolitical regions with large anticipated socioeconomic changes or vulnerability to climate change and variability. Determine the Importance of Nonradiative Forcings Several types of forcings—most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry—impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received. These aerosol forcings are sometimes referred to as thermodynamic forcings because they affect spatial patterns of diabatic heating. In some cases, aerosols may be able to modify the hydrological cycle without changing the global average surface temperature. Other nonradiative forcings modify the biological components of the climate system by changing the fluxes of trace gases and heat between vegetation, soils, and the atmosphere; the biogeochemistry of vegetation biomass and soils; or plant species composition. Nonradiative forcings have been shown in a few studies to have first-order effects on regional and global climate, although the globally averaged impacts are not yet sufficiently quantified to allow a careful comparison with forcing from greenhouse gases. No metrics for quantifying nonradiative forcing have been accepted. Unlike traditional radiative forcing, which can be directly related to surface temperature, nonradiative forcings are not easily linked to a single climate variable. No single metric will be applicable to all nonradiative forcings. Nonradiative forcings generally do have radiative impacts, so one option would be to compare them by quantifying these radiative impacts. Al-
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties though this approach would enable comparisons with traditional radiative forcings, it would not convey fully the impacts of nonradiative forcings on societally relevant climate variables, such as precipitation or ecosystem functioning. Furthermore, quantifying nonradiative forcings in terms of their radiative effects is not straightforward. Another consideration in identifying potential metrics for nonradiative forcings is their significant regional variation; any new metrics will have to be able to characterize the regional structure in forcing and climate response. Further work is needed to quantify links between regional nonradiative forcing and climate response, whether the response occurs in the region, in a distant region through teleconnections, or globally. RECOMMENDATIONS: Improve understanding and parameterizations of aerosol-cloud thermodynamic interactions and land-atmosphere interactions in climate models in order to quantify the impacts of these nonradiative forcings on both regional and global scales. Develop improved land-use and land-cover classifications at high resolution for the past and present, as well as scenarios for the future. • Develop parameterizations of terrestrial and marine biogeochemistry to investigate the resulting nonradiative forcings. • Identify suitable climate diagnostics, metrics, and monitoring procedures for specific nonradiative forcing processes and responses. ADDRESSING KEY UNCERTAINTIES Whereas the level of understanding associated with radiative forcing by well-mixed greenhouse gases is relatively high, there are major gaps in understanding for the other forcings, as well as for the links between forcings and climate response. Error bars remain large for current estimates of radiative forcing by ozone, and are even larger for estimates of radiative forcing by aerosols. Nonradiative forcings are even less well understood. The potential for large and abrupt climate change triggered by radiative and nonradiative forcings needs to be explored. The following recommendations identify critical research avenues for addressing these key uncertainties. Reduce Uncertainties Associated with Indirect Aerosol Radiative Forcing The interactions between aerosols and clouds can lead to a number of indirect radiative effects, which arguably represent the largest uncertainty in current radiative forcing assessments. In the so-called first indirect aerosol effect, the presence of aerosols leads to clouds with more, but smaller
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties particles; such clouds are more reflective and therefore have a negative radiative forcing. These smaller cloud droplets can also decrease the precipitation efficiency and prolong cloud lifetime; this is known as the second indirect aerosol effect. The so-called semidirect aerosol effect occurs when absorption of solar radiation by soot leads to an evaporation of cloud droplets. The IPCC Third Assessment Report gave an estimated range for the radiative forcing associated with the first indirect aerosol effect (0 to −2 W m−2); this range was larger than the uncertainty attributed to any of the other forcings, reflecting in large part the very low level of scientific understanding. Potential magnitudes of the second indirect effect and the semi-direct effect were not estimated in the report. A number of research avenues hold promise for improving understanding of indirect and semidirect aerosol effects and better constraining estimates of their magnitudes. These include climate modeling, laboratory measurements, field campaigns, and satellite measurements. To improve the representation of the indirect effect in climate models, fundamental research is needed on the physical and chemical composition of aerosols, aerosol activation, cloud microphysics, cloud dynamics, and subgrid-scale variability in relative humidity and vertical velocity. RECOMMENDATIONS: • Conduct integrated and comprehensive field investigations to quantify indirect aerosol radiative forcings—for example, in closure experiments with redundant observational and modeling studies. • Enhance the value of information derived from satellite instruments with targeted field campaigns and greater support for analysis of long-term surface records. Improve understanding and parameterizations of the indirect aerosol radiative and nonradiative effects in GCMs using process models, laboratory measurements, field campaigns, and satellite measurements. • Report the different indirect aerosol radiative forcings in climate change assessments and provide better estimates of the associated uncertainties. Better Quantify the Direct Radiative Effects of Aerosols Aerosols have direct radiative effects in that they scatter and absorb radiation. Knowledge of direct radiative forcing of aerosols is limited to a large extent by uncertainty in the global distribution and mixing states of the aerosols and by the role of different sources in contributing to atmospheric concentrations. Mixing states have major implications for aerosol optical properties that are not well understood and are difficult to parameterize in climate models. Another factor of uncertainty in representing
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties aerosol direct radiative forcings in climate models is the small-scale variability of humidity and temperature, which has a major impact on aerosol optical properties. Describing the rapid growth of particles as humidities approach 100 percent is a particular challenge. Radiative transfer models relating aerosol columns and optical properties to the corresponding radiative forcing are thought to be relatively mature but must be tested further with field closure studies that provide multiple constraints for the models for a range of environments. Assessments of past and future radiative forcings are compromised by the poor characterization of aerosol sources and sinks. Many natural and anthropogenic mechanisms of aerosol production are not understood: Their variability with future changes in population, technology, and climate cannot be accurately predicted. This finding is especially true for sea salt, dust, biomass burning, and the sources of carbonaceous aerosol. Removal of aerosols from the atmosphere occurs mainly by wet deposition, but model parameterizations of this process are highly uncertain and rudimentary in their coupling to the hydrological cycle. RECOMMENDATIONS: • Improve understanding of the global distribution of aerosols and their relationship to sources using data assimilation and inverse modeling approaches. • Improve understanding of the radiative forcing of aerosols by direct measurements of radiative fluxes at the surface and TOA, vertical profiles of aerosols, their scattering and absorption coefficients, and their hygroscopic growth factors. Improve representation in global models of aerosol microphysics, growth, reactivity, and processes for their removal from the atmosphere through laboratory studies, field campaigns, and process models. Better characterize the sources and the physical, chemical, and optical properties of carbonaceous and dust aerosols. • Improve estimates of black carbon and organic carbon emissions for the past (last 100 years) and future (next 100 years), utilizing indicators beyond static population density. Better Quantify Radiative Forcing by Ozone Ozone is a major greenhouse gas. The greatest uncertainty in quantifying this forcing lies in reconstructing the ozone concentration field in the past and projecting it into the future. Simulations of ozone with chemical transport models involve complex photochemical mechanisms coupled to transport on all scales and remain a major challenge, particularly in the troposphere. The inability of models to reproduce ozone trends over the
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties twentieth century is disturbing and suggests that there could be large errors in current estimates of natural ozone levels and the sensitivity of ozone to human influence. In the troposphere these errors could relate to emissions of precursors, heterogeneous and homogeneous chemical processes, and stratospheric influence. Lightning emissions of nitrogen oxides (NOx) are particularly uncertain and yet play a major role for ozone production in the middle and upper troposphere where the radiative effect is maximum. Transport of ozone between the stratosphere and troposphere greatly affects upper tropospheric concentrations in a manner that is still poorly understood. RECOMMENDATIONS: • Improve knowledge of sources of ozone precursors, including in particular nitrogen oxide emissions from lightning. Improve understanding of the transport of ozone in the upper troposphere and lower stratosphere region and the ability of models to describe this transport. • Use observed long-term century trends in ozone concentrations to evaluate and improve global chemical transport models. Investigate the Potential for Abrupt Climate Change Paleoclimate records indicate that climate can change so rapidly and unexpectedly that human or natural systems may have difficulty adapting. Such abrupt climate changes take place when “the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause” (NRC, 2002). The Earth’s climate has experienced abrupt shifts in temperature and precipitation during the preindustrial Holocene. Each of these events appears to have weakened the North Atlantic thermohaline circulation enough to cause abrupt cooling of the northern North Atlantic, Greenland, Iceland, and Europe. The present climate could undergo abrupt changes in the future, not necessarily by the same mechanisms as in the past. Models imply, for example, that greenhouse warming may alter the hydrologic cycle enough to freshen North Atlantic surface waters and shift thermohaline circulation closer to a threshold. Collapse of parts of the Greenland ice sheet could be a risk factor as suggested by evidence that meltwater-induced basal sliding of southern parts of the ice sheet toward the ocean may have begun within the last decade. Knowledge of what triggers abrupt climate changes is still quite limited; more research is needed to determine the possible role of radiative and nonradiative climate forcings, such as human-caused increases in greenhouse gases or land-use changes. Indeed, past abrupt climate
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties changes have been especially common when the climate system itself was being altered. The current understanding of abrupt climate change is discussed in a recent National Research Council report Abrupt Climate Change: Inevitable Surprises (NRC, 2002). That report provides more detailed recommendations for research needed to improve our understanding of and ability to predict possible abrupt changes in the future, including enhanced research on possible causes of abrupt change. Here, a few recommendations that pertain specifically to radiative forcing are identified. The committee notes that the recommended research will require long-term efforts. RECOMMENDATIONS: Investigate the magnitude, spatial patterns, and temporal variation of radiative forcing that may cause the climate system to cross a threshold (e.g., shutdown of the thermohaline circulation). Conduct societal impact studies to investigate the magnitude of future forcing that would cause a crossing of a threshold in societal vulnerability. Determine the probability that future radiative and nonradiative forcings (e.g., reductions in aerosol emissions, continued tropical deforesta tion) could induce an abrupt climate change. IMPROVING THE OBSERVATIONAL RECORD The most important step for improving understanding of forcings is to obtain a robust record of radiative forcing variables, both in the past and into the future. A robust observational record is essential for improved understanding of the past and future evolution of climate forcings and responses. Existing observational evidence from surface-based networks, other in situ data (e.g., aircraft campaigns, ocean buoys), remote sensing platforms, and a range of proxy data sources (e.g., tree rings, ice cores) has enabled substantial progress in understanding, but important shortcomings remain. The observational evidence needs to be more complete both in terms of the spatial and spectral coverage and in terms of the quantities measured. Long-term monitoring of forcing and climate variables at much improved accuracy is necessary to detect and understand future changes. Advance the Attribution of Decadal to Centennial Climate Change Carefully attributing past climate changes to known natural and anthropogenic forcings provides information on how such forcings may impact large-scale climate in the future. Instrumental records of past climate
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties conditions and of the magnitude of various forcings extend back about 150 years at best. Comparisons of observed surface temperatures with those simulated using reconstructions of the past forcings have yielded important insights into the roles of various natural and anthropogenic factors governing climate change. The shortness of the instrumental record and of accurate monitoring of climate forcings, however, limits the confidence with which climate change since preindustrial times can be attributed to specific forcings. Proxy records obtained from ice cores, sediments, tree rings, and other sources provide a critical tool for extending knowledge of both climate forcings and climate response further back in history. Improved historical radiative forcing reconstructions will require new understanding of physical process to better understand the relevance of available historical observations and their relationship to the actual forcings. For example, in the case of solar forcing, separate physical connections must be made of the solar magnetic activity with irradiance and with the near-Earth space environment, which modulates galactic cosmic rays that produce cosmogenic isotopes—the only long-term archive of solar activity and a crucial component of long-term Sun-climate research. The lack of proxy climate data in certain key regions (e.g., large parts of the tropical Pacific and the extratropical oceans) is a major limitation. Such regional information is important in evaluating the potential roles of changes in modes of climate variability, such as ENSO, in past climate changes. Experiments employing fully coupled land-ocean-atmosphere models to study regional past climate change are just now under way. For comparison with model simulations, greater historical knowledge should be sought for a broad array of climate system parameters including the hydrological cycle (e.g., droughts, rainfall, streams), modes of variability (e.g., ENSO, annular modes), land use, the stratosphere, and ozone. RECOMMENDATIONS: • Seek greater historical knowledge for a broad array of climate system parameters including the hydrological cycle (e.g., droughts, rainfall, streams), modes of variability (e.g., ENSO, annular modes), land use, the stratosphere, and ozone. • Improve proxy records of past radiative forcing (e.g., indicators of solar and volcanic forcing). • Enhance physical understanding of how these proxy records relate to the forcings (e.g., relationship between solar activity, irradiance, and cosmogenic isotopes). • Undertake “data archeology” projects to recover long-term instrumental records of climate variables of the past few centuries. • Continue to develop high-quality, high-resolution (or well-dated,
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties lower-resolution) proxy records of past climate change, and synthesize these data into spatially and temporally resolved reconstructions of climate change in past centuries to millennia. Develop a best-estimate climate forcing history for the past century to millennium. Using an ensemble of climate models, simulate the regional and global climate response to the best-estimate forcings and compare to the observed climate record. Conduct Accurate Long-Term Monitoring of Radiative Forcing Variables Geophysical quantities relevant to climate forcing should be known with a level of accuracy that is significantly smaller than the expected changes. The current approach relies primarily on measurement repeatability (precision), using overlapping successive measurements to cross-calibrate their absolute uncertainties. The principles described in Karl et al. (1995) (endorsed in Adequacy of Climate Observing Systems; NRC, 1999) provide a suitable framework for guiding collection of observations of radiative forcing and other climate variables. These principles have been updated in the Strategic Plan for the U.S. Climate Change Science Program (USCCSP, 2003). Ultimately, the measurement accuracies of the geophysical parameters must be tied to irrefutable absolute standards and be tested and validated in perpetuity. Such benchmark measurements of radiative and other climate forcings and climate variables are needed immediately. Because the radiative forcings and the climate responses are highly dependent on wavelength, space-based observations with high spectral resolution are needed to isolate the signatures of the relevant radiative processes and components. Because the forcings and responses that determine any one particular climate state involve a distribution about a mean, the ensemble must be properly characterized and quantified so that changes in the mean can be reliably identified. Ultimately, the specification of the forcings and responses must be integrated to test climate forecast models. Observational networks for the detection of long-term changes in climate variables must be improved. For example, local land-use changes and vegetation dynamics (i.e., microclimate effects) have been shown for some long-term climate monitoring sites to result in surface air temperature trends that are not spatially representative. Photographic and other documentation of monitoring sites and the surrounding landscape is needed to document the integrity of the sites over time. Surface and tropospheric heat content changes may provide in the future a robust evaluation of climate changes. Long-term, globally averaged changes in the heat content of the oceans permit the calculation of the
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties globally averaged radiative forcing over the timescale of the averaging period. Ocean heat storage changes have been shown to be an essential metric that climate model simulations must skillfully reproduce. The accurate assessment of concurrent heat storage changes in the atmosphere, land, and continental glaciers and sea ice would permit the averaging time to be shorter. Measurements of moist enthalpy can be used to characterize the heat content in surface air, providing more information about the surface energy budget than given by surface air temperature alone. A network of surface stations intended to characterize the surface energy budget could help better understand and monitor nonradiative forcings, although care would be needed in determining the siting and density of stations to appropriately account for the impact of landscape heterogeneity. RECOMMENDATIONS: Continue observations of climate forcings and variables without interruption for the future in a manner consistent with established climate monitoring principles (e.g., adequate cross-calibration of successive, overlapping datasets). Develop the capability to obtain benchmark measurements (i.e., with uncertainty significantly smaller than the change to be detected) of key parameters (e.g., sea level altimetry, solar irradiance, and spectrally resolved, absolute radiance to space). • Continuously monitor key radiative forcing parameters with high spectral resolution in order to isolate and understand the physical processes (e.g., solar spectral irradiance; surface, ocean, and atmosphere radiance to space), and ensure continuity and radiometric compatibility with existing and future broadband satellite measurements of shortwave and longwave irradiance. • Conduct a comprehensive review and documentation of current and historical surface observation sites that are used in long-term temperature monitoring. Conduct highly accurate measurements of global ocean heat content and its change over time. • Explore the value of creating a network of surface sites that provide representative monitoring of the surface energy budget. ADDRESSING POLICY NEEDS The concept of radiative forcing has clear policy applications. It has been used by the policy community to compare different forcings and as input to simple climate models used to consider policy options. Control strategies designed to address other environmental issues, such as air pollution or land-use changes, can also impact radiative forcings, a consequence
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties that is rarely considered in developing such strategies. One specific concern is the possibility that reducing aerosol concentrations could enhance radiative warming. In addition, the policy community has focused primarily on global mean radiative forcing and the associated response in surface temperature. Given the increasing realization of the significance of geographically dependent climate forcings, the policy community will need new forcing metrics and guidance on how to apply them. Integrate Climate Forcing Criteria in Environmental Policy Analysis Policies designed to manage air pollution and land use may be associated with unintended impacts on climate. For example, aerosol and ozone have significant impacts on human health, ecosystems, and climate. Emissions of aerosols, aerosol precursors, and ozone precursors are already regulated in the United States and other industrialized nations. Increasing evidence of their health effects makes it likely that aerosols will be the target of further regulations to reduce their concentrations in the future. To date, these control strategies have not considered the potential climatic implications of emissions reductions. Regulations targeting black carbon emissions or ozone precursors would have combined benefits for public health and climate. However, because some aerosols have a negative radiative forcing, reducing their concentrations could actually accelerate radiative warming. Understanding of the effect of aerosols on the hydrological cycle and vegetation is still incomplete, making it is difficult to predict the total effect on climate of reducing aerosol emissions. The assumptions made about the magnitude of climate sensitivity are an important consideration associated with regulations that attempt to reduce aerosols. Several modeling studies have suggested that aerosol direct and indirect forcing may have offset as much as 50 to 75 percent of the greenhouse gas forcing since preindustrial times. At the same time, the IPCC Third Assessment Report and climate modeling studies attribute the large warming witnessed during the recent decades to the increase of concentrations of carbon dioxide (CO2) and other greenhouse gases. These two findings taken together reveal the possibility that climate sensitivity due to radiative forcing is in the upper range of the 1.5 to 4.5 K global-averaged surface warming for a doubling of CO2. This implies that attempts to regulate air pollution, which would reduce aerosol abundances, could inadvertently trigger a strong acceleration of global surface warming in the coming decades. Policies associated with land management practices also need to consider their inadvertent effects on climate. The continued conversion of landscapes by human activity, particularly in the humid tropics, could have
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties complicated and possibly important consequences for regional and global climate change as a result of changes in the surface energy budget. RECOMMENDATIONS: • Improve projections of future emissions of aerosols, aerosol precursors, and ozone precursors. • Improve projections of future land-use changes. Apply climate models to the investigation of scenarios in which aerosols are significantly reduced over the next 10 to 20 years and for a range of cloud microphysics parameterizations. Integrate climate forcing criteria in the development of future policies for air pollution control and land management. Provide Improved Guidance to the Policy Community The radiative forcing concept is used to inform climate policy discussions, in particular to compare the expected relative impacts of forcing agents. For example, integrated assessment models use radiative forcing as input to simple climate models, which are linked with socioeconomic models that predict the economic damages from climate impacts and the costs of various response strategies. This approach has been used to evaluate potential greenhouse gas emissions control strategies for meeting the Kyoto Protocol targets, as well as other policy questions. Many simplified climate models have focused on global mean surface temperature as the primary climate response to forcings, although more recently they have considered regional temperature changes and other societally relevant aspects of climate, such as sea level. It is important that models used for policy analysis incorporate further complexities in the radiative and nonradiative forcing concepts, as identified in this chapter. It is important also to communicate the expanded forcing concepts as described in this report to the policy community and to develop the tools that will make their application useful in a policy context. Many climate policy questions require comparing the climate change effects of different greenhouse gases, aerosols, and other forcings. The concept of global warming potential (GWP) was developed to address this need. Application of the GWP concept has been restricted mainly to the long-lived greenhouse gases. In principle, it could be applied to short-lived forcing agents such as ozone and aerosols or, more specifically, to the emissions of their precursors, but there are a number of complicating factors including (1) the often poorly defined relationship between a precursor and a radiative forcing agent; (2) the inhomogeneity of the forcing; and (3) the much shorter time horizons (decades or less) relevant to the radiative
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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties forcing from these short-lived agents. In addition, the current concept is not useful for evaluating how the rate of technical transformation, which depends on economic and policy drivers, affects the trade-off between two greenhouse gases. For most policy applications, the relationship between radiative forcing and temperature is assumed to be linear, suggesting that radiative forcing from individual positive and negative forcing agents could be summed to determine a net forcing. This assumption is generally reasonable for homogeneously distributed greenhouse gases, but it does not hold for all forcings. Thus, the assumed linearity of radiative forcing has been simultaneously useful and misleading for the policy community. It is important to determine the degree to which global mean TOA forcings are additive and whether one can expect, for example, canceling effects on climate change from changes in greenhouse gases on the one hand and changes in reflective aerosols on the other. RECOMMENDATIONS: Encourage policy analysts and integrated assessment modelers to move beyond simple climate models based entirely on global mean TOA radiative forcing and incorporate new global and regional radiative and nonradiative forcing metrics as they become available. • Devise practical tools to relate new forcing metrics that may be introduced in the future to simple measures of climate change. • Explore ways to extend the GWP concept to account for aerosols and aerosol precursors, regional variation in forcing, and economic and policy factors that might affect the long-term impact of forcings. • Provide guidance to policy analysts on how individual radiative forcings combine to produce a net radiative forcing with an associated uncertainty.
Representative terms from entire chapter: