Executive Summary

It has been more than a century since scientists first predicted that changes in the chemical composition of the atmosphere, particularly increasing concentrations of carbon dioxide from anthropogenic activities, might change the Earth's heat balance and cause a warming of the atmosphere (Arrhenius, 1896). In contrast, it is only recently that scientists began to consider quantitatively how anthropogenic aerosols—very small particles that are suspended in the air—affect global climate (Charlson et al., 1990). Although there is much evidence to suggest that aerosols cause cooling, it is not confidently known just how large such an effect might be. Because the effects of atmospheric aerosols on climate are still poorly understood, and because these effects could play an essential role in explaining past climate trends and in predicting future climate change, the results of aerosol research will likely be highly relevant to current international policy activities involving the United States.

To approach this complex subject systematically, the scientific community has divided the problem into two major parts referred to as climate forcings and climate responses. Climate forcings are the changes in the energy balance of the Earth's environmental system that are imposed upon it. Forcings are calculated or measured in units of heat flux—watts per square meter (W m-2). Climate responses are the meteorological results of these forcings, including changes in temperature, wind, rainfall, and their probability distributions.

Aerosol forcings are divided into two types: direct and indirect. Direct



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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Executive Summary It has been more than a century since scientists first predicted that changes in the chemical composition of the atmosphere, particularly increasing concentrations of carbon dioxide from anthropogenic activities, might change the Earth's heat balance and cause a warming of the atmosphere (Arrhenius, 1896). In contrast, it is only recently that scientists began to consider quantitatively how anthropogenic aerosols—very small particles that are suspended in the air—affect global climate (Charlson et al., 1990). Although there is much evidence to suggest that aerosols cause cooling, it is not confidently known just how large such an effect might be. Because the effects of atmospheric aerosols on climate are still poorly understood, and because these effects could play an essential role in explaining past climate trends and in predicting future climate change, the results of aerosol research will likely be highly relevant to current international policy activities involving the United States. To approach this complex subject systematically, the scientific community has divided the problem into two major parts referred to as climate forcings and climate responses. Climate forcings are the changes in the energy balance of the Earth's environmental system that are imposed upon it. Forcings are calculated or measured in units of heat flux—watts per square meter (W m-2). Climate responses are the meteorological results of these forcings, including changes in temperature, wind, rainfall, and their probability distributions. Aerosol forcings are divided into two types: direct and indirect. Direct

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change forcing relates to the direct interaction of aerosol particles with the incoming solar radiation in, essentially, non-cloudy conditions. Indirect forcing relates to forcing under cloudy conditions, during which the aerosol and the cloud may interact in a number of ways: the aerosol may increase the cloud droplet concentration, thereby influencing the cloud albedo; the aerosol may influence cloud persistence; or it may reduce the possibility of precipitation. Even modestly confident quantification of indirect forcing is not yet possible. CHARGE TO THE PANEL In their letter to the chair of the Board on Atmospheric Sciences and Climate (BASC) of the National Research Council, the director of the Environmental Sciences Division, Department of Energy; the chief of the Climate and Hydrologic Systems Branch, National Aeronautics and Space Administration; the director of the Office of Global Programs, National Oceanic and Atmospheric Administration; and the director of the Division of Atmospheric Sciences, National Science Foundation, requested that In view of the potential significance of climate forcing by both man-made and natural aerosols and the relatively undeveloped ability to describe this forcing in climate models, we are writing to request that BASC advise the government on development of a strategy and potential program plan for a U.S. research effort. Given current agency priorities concerning climate and global change, we feel that the appropriate scientific focus for the U.S. program of aerosol research is the climatic effects of aerosol particles … (see Chapter 1 for the complete Statement of Task). This report summarizes current understanding of the effects of aerosols on climate change and recommends a research program to identify and prioritize the research required to determine the effects of aerosol forcing on the atmosphere's energy balance. FINDINGS The panel's main findings are that (1) anthropogenic aerosols reduce the amount of solar radiation reaching the Earth's surface, (2) anthropogenic aerosols provide a negative climate forcing function for large regions, (3) global models suggest that sulfate aerosols produce a direct forcing in the Northern Hemisphere of the same order of magnitude as that from anthropogenic greenhouse gases, but opposite in sign, and (4) there is substantial uncertainty about the magnitude and spatial distribution of the radiative forcing by aerosols. Reduction in this uncertainty requires a scientifically and administratively integrated research program that could organize the

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change research capabilities of the atmospheric science community in this country and permit cooperation with other national and international research programs. The following sections summarize the management strategy and specific research projects recommended to achieve this goal. MANAGEMENT RECOMMENDATIONS Action 1: Establish leadership by empowering an Interagency Climate-Aerosol Radiative Uncertainties and Sensitivities (ICARUS) Program (see Figure 1 for the proposed ICARUS organizational structure). Action 2: Mobilize ICARUS by developing methods to define research priorities. Action 3: Develop a multiagency, integrated research program. Action 4: Maintain ICARUS's leadership in aerosol research by applying steadily improving sensitivity analysis to research designs. FIGURE 1 Organizational structure of the ICARUS program. NOTE: CENR = Committee on Environment and Natural Resources.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change TECHNICAL RECOMMENDATIONS It is recommended that a research program be developed to reduce uncertainties in calculated aerosol forcing at the top of the atmosphere to within ±15 percent both globally and locally. This limit of uncertainty is equivalent to that required in estimating the greenhouse gas forcing (IPCC, 1995a). Locally, this would imply an uncertainty in forcing of less than ±1.5 W m-2 (by assuming a local aerosol effect of -10 W m-2 in the diurnal average). Six categories of research are recommended to achieve this goal: global climate model development; process studies (modeling, small-scale field studies) and technology development (excluding satellite development); field studies: large, international field studies and systematic ship-and aircraft-based surveys of aerosol properties; satellite observations and continuous measurements from surface-based research measurement networks; technology development; and system integration of the program (continuous coupling of modeling and measurement programs with ongoing assessment of sensitivities and uncertainties). The following specific research undertakings are recommended for each of these categories. SCIENCE DEVELOPMENT Global Climate Model Development Sensitivity and Uncertainty of Aerosol Forcing in Global Climate Models: Using existing global climate and chemistry models, determine the global and local sensitivity of climate predictions to uncertainty in aerosol forcing and associated factors. Development of Global-Scale Aerosol Radiative Forcing Models: Develop aerosol radiative forcing models that incorporate chemical, physical, and meteorological processes for regional to global scales. Process Research Aerosol Formation and Growth by Nucleation and Gas-to-Particle Conversion: Using field, laboratory, and theoretical approaches, determine the extent to which aerosol formation and growth by homogeneous nucleation and gas-to-particle conversion is occurring in the atmosphere.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Aerosol and Cloud Optical Properties: Using field, laboratory, and theoretical approaches, determine the extent to which theoretical treatments of aerosol and cloud optical properties are applicable to ambient aerosols and clouds. The Aerosol-Cloud Condensation Nuclei (CCN)-Cloud Drop Number Concentration (CDNC)-Cloud Albedo Linkage: This project will develop an understanding of ''indirect" aerosol forcing that depends on information on the relationship among aerosol number concentration, CCN, CDNC, and cloud albedo. Aerosol Sinks: Quantification of the rate of removal of aerosols from the atmosphere will be developed from field studies of particle size-specific dry deposition velocities over a variety of terrain, from precipitation scavenging measurements, and from storm-venting experiments. Field Studies and Continuous In Situ Measurements Multiplatform Field Campaigns: The goal of these field studies is to integrate satellite radiation measurements and surface-based column-integrated radiation measurements and to quantitatively understand the processes controlling the formation, transformation, and removal of aerosols in different environments. Mobile Platforms: It is proposed that surface measurements be supplemented with systematic observations from a suitably instrumented aircraft, from ships, and from a balloon program, the primary uses of which would be to supply "ground truth" measurements for comparison with satellite observations and to provide typical values and variabilities of aerosol properties used in the models. Surface-Based Stations for Continuous Research Measurement of Aerosols : Establish a dual-density network of surface-based stations for the research measurement of characteristics of optical depth (dense network) and radiative, chemical, and microphysical properties of aerosols (low-density network). Satellite Observations Satellite Remote Sensing of Aerosols: The primary objective of this project is to provide a geographically and vertically resolved climatology of aerosol extinction throughout the troposphere and stratosphere. TECHNOLOGY DEVELOPMENT Non-Satellite Technology Development Instrumentation Development: The purpose of this project is the development of instrumental systems intended specifically for aerosol

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change forcing/climate research. Instruments include aerosol optical instrumentation, a CCN spectrometer, and in situ chemical analysis samplers. Satellite Technology Development Development of Extinction Profile Measurement Capability: The goal is to develop the capability to measure extinction profiles (vertically resolved aerosol optical depths) on a global scale from a satellite. SYSTEM INTEGRATION AND ASSESSMENT The following projects are necessary to assure that the preceding technical recommendations are integrated and coordinated across the program, and that the results of the research are communicated promptly and authoritatively to those requiring technical guidance for decisionmaking. Coordination: The goal is to maintain communication among projects regarding the technical details of the ongoing efforts. Included are small projects for linking the research efforts, tracking projects, and reporting research results. Integration and Assessment: This small project will organize the data on aerosol forcing generated by the United States into formats suitable for presentation to the Intergovernmental Panel on Climate Change and other scientific and administrative bodies with differing requirements. Integration of U.S. Research on Aerosol Forcing of Climate: While some aspects of the U.S. research of aerosol forcing of climate will be supported directly by the funding agency members of ICARUS, there are others that may not be or that will require some additional coordination. Typically, these efforts include the maintenance of data quality assurance efforts, the construction of program-wide data bases, model and measurement intercomparison studies, and the support of meetings.