Issue No. 17, May 1999	Introduction
	Contributed by Barry J. Huebert, University of Hawaii, USA   "Greenhouse Forecasting Still Cloudy," –Science, 16 May 97   Is it just the headline writers, or do most people expect that scientists should be capable of providing a definite answer to every possible question about how the Earth works? I think this idea that scientists ought to know everything probably comes from primary school, where there is always one kid in class who seems to have every answer. He or she can work the science and math problems nobody else can, and all the other kids feel like he or she makes them look bad. Years later some of these smart kids become Earth scientists, and their classmates still expect them to know everything. Only Earth science questions are a lot harder than those we had to answer in third grade. When society needs to know whether we are causing irreversible climate change and how it will affect rainfall in the world's grain belts, many people think these smart kids ought to be able to give them an answer. You and I know, however, that many geophysical problems are so vast and so complex that we may never have the tools to provide concrete solutions to them. For questions about aerosol forcing of climate change our understanding is improving, but even our most confident answers have large uncertainties. Yet the mass media react as if it is scandalous when scientists disagree or when they admit to not knowing the answers to important environmental questions. The popular conception seems to be that the scientific community is a failure if it can't provide uniform, direct answers to questions about how changes in emissions would change climatic impacts. Ironically, that kind of certainty is the exact opposite of how good science should work. Frank discussions about uncertainties motivate progress. When I need the best possible judgement about an issue, I try to ask someone who is skeptical about our level of understanding. The person who believes our present theories are good enough (even if they are) is not likely to be motivated to look as hard at the weakest points in hypotheses. The doubter, by contrast, will try to calculate just what the major uncertainty is, and how that can be reduced to give us greater confidence in the answer. The U.S. National Research Council convened a Panel on Aerosol Radiative Forcing and Climate Change a few years ago to evaluate uncertainties in the role of aerosols in the climate system and to recommend the best strategies for reducing them. To reduce the uncertainties in calculated aerosol radiative climate forcing to 15% both globally and locally, the Panel (NRC, 1996) recommended six categories of research that need to be conducted in concert: Global climate model development; Process studies (modeling and small-scale field studies); 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 (excluding satellite development); and System integration. Interestingly, the Panel also found that few of these recommendations were being made for the first time. An extensive Appendix quotes from publications as far back as 1971 that have argued for similar work on many of these same issues. At issue now is how to finally implement these recommendations in the most cost-effective manner possible. Several implementation strategies are discussed in this issue of the IGAC newsletter. Since the Panel last met in 1995, there have been advances in our understanding of the impact of aerosols on climate. IGAC has organized three major field programs, ACE—1 (Bates et al., 1998), TARFOX (Russell et al., 1999), and ACE—2 to study aerosol distributions, properties, and processing (in the clean marine troposphere, North American outflow, and European outflow, respectively). In part because of these experiments we have developed a more quantitative understanding of aerosol radiative forcing, but we have also learned much more about how to study the problem most effectively. Our models are more realistic, we have developed new instruments for studying the problem, and our experimental strategies are improved. In the articles that follow, we evaluate some of this progress and reassess strategies for reducing uncertainty in the impact of aerosols on climate. These papers describe strategies for addressing several of the issues the NRC Panel raised: Is the Lagrangian observational strategy a viable one for quantifying process rates in the atmosphere? (Huebert and Lenschow) How should we modify our approach to the indirect forcing question in view of our present understanding of aerosol/CCN/cloud droplet number relationships? (Seinfeld and Flagan) How does the evolution of mineral dust modify the impact of the resulting multi-component aerosol? (Sokolik) Some review the results of recent IGAC experiments. All point us toward strategies we should be using in the future. Some of the strategies are discussed in the context of ACE-Asia, a new field campaign being planned. The international community of aerosol researchers is turning its attention to Asia. The rapid evolution of emissions in the region creates an opportunity to do a stimulus-response experiment. As automotive emissions increase (Elliott et al., 1997), how will the photochemical system respond? When more coal is burned or biomass emissions are reduced, how will these changes modify the air the regions' population breathes and the deposition of particles to the ocean downwind? These are global issues both because they affect the global climate and because in one form or another they are common to each continent. The only way to understand such common problems is to study them together. IGAC is structured to do just that, with a truly democratic structure that allows every country to include its ideas and expertise in the planning process. Through this vehicle, we are planning some experiments that will help everyone do a better job of estimating the impact of their emissions. The more countries, funding agencies, and research groups that decide to participate formally, the more powerful and useful the results will be to people everywhere. The sophistication of Asia's atmospheric scientists makes it possible to mount a broad-reaching study of Asian aerosols and their ultimate fate over the Pacific. They will be working with scientists from Europe, North America, and Australia who will bring state-of-the-art aerosol instruments to study this interesting region. Several years of network operations will be supple-mented by multi-platform intensive experiments during the spring period of outflow to the Pacific. In March and April of 2001, for instance, the goals will be to 1) survey the composition of aerosols at many latitudes, locations, and distances from the coast and 2) study the evolution of those aerosols with time and distance to quantify the processes that control their concentrations and effects. More information can be found at http://saga.pmel.noaa.gov/aceasia/. Since you are reading this, you were probably one of those kids who stood out in grade school as better than most at doing science and math. If so, you now share the burden to provide policymakers in your country with reliable predictions about human-induced climate changes, including realistic error bars. But progress in understanding the Earth system is incremental by nature. When we do an experiment, we learn a little about the Earth and a lot about how to be more successful next time. Our hope is that the strategies summarized in the following papers will help us fine-tune the course of aerosol/climate research, so that our next experiments will convince even us that scientists are pretty smart. References Bates, T.S., B.J. Huebert, J.L. Gras, F.B. Griffiths, and P.A. Durkee, The International Global Atmospheric Chemistry (IGAC) Project's first aerosol characterization experiment (ACE-1) - Overview, J. Geophys. Res., 103, 16,297-16,318, 1998. Elliott, S., et al., Atmospheric effects of the emerging mainland Chinese transportation system at and beyond the regional scale, J. Atmos. Chem., 27 (1), 31-70, 1997. NRC, P.o.A.R.F.a.C.C., Aerosol Radiative Forcing and Climate Change, 161 pp., National Academy Press, Washington, DC, 1996. Russell, P.B., P.V. Hobbs, and L.L. Stowe, Aerosol properties and radiative effects in the United States East Coast haze plume: An overview of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), J. Geophys. Res., 104(D2), 2213-2222, 1999.

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