Fossil fuel consumption and other human activities have increased exponentially worldwide. As a consequence, emissions of greenhouse gases, aerosol particles, and aerosol precursor gases have increased as well. Greenhouse gases decrease the outgoing radiation from the Earth, tending to increase its temperature (a positive radiative forcing), whereas aerosol particles can either increase or decrease the outgoing radiation by scattering and absorption (direct effect) and by changing the reflectivity, duration, and extent of clouds (indirect effect). Current estimates of the global, annually-averaged, direct radiative forcing by anthropogenic aerosols (sulfates, soots, and biomass smokes) range from -0.3 to -1.0 W m-2. Analogous estimates for the indirect effect are 0 to -1.5 W m-2. These values are comparable in magnitude, but opposite in sign, to the current estimates of +2.1 to +2.8 W m-2 for the forcing due to increases in greenhouse gases over the past century. Because of the great spatial variability in aerosol concentrations that results from their short lifetime, there are many regions--principally over and downwind of major industrial areas--where aerosol negative forcing must exceed the greenhouse positive forcing [e.g. Charlson et al., 1992; Kiehl and Briegleb, 1993]. Recent studies have shown that (1) aerosol effects appear to be present in the global and regional twentieth-century temperature record, and (2) inclusion of aerosol effects in numerical model predictions improves agreement with observed temperatures, in both timing and spatial patterns [e.g. Karl et al., 1995; Santer et al., 1995].

Although the potential impact of anthropogenic aerosols on climate is large, there are large uncertainties regarding this impact. This is due to a lack of globally distributed data on aerosol properties and effects. In fact, aerosol effects constitute one of the largest sources of uncertainty in validating current climate models and in predicting future climate. An important step in reducing these uncertainties is to measure the direct radiative forcing by tropospheric aerosols over various regions of the globe while simultaneously measuring the properties of the responsible aerosols. Intensive field programs, when extended with current and future satellite measurements and validated retrieval algorithms, can provide the data needed to reduce the uncertainties and hence improve the performance of climate prediction models. The International Global Atmospheric Chemistry Project (IGAC) is coordinating four such field programs: TARFOX and the Aerosol Characterization Experiments (ACE-1, ACE-2, and ACE-3; see Relationships to other Projects, below).

To illustrate the coverage possible with satellite remote sensing of aerosol properties, Figure 1 shows color maps of seasonal mean aerosol optical thickness derived from the NOAA/AVHRR operational product. These are approximate values, since an aerosol model is used to infer aerosol optical thickness from the measured satellite reflectances. TARFOX will provide a critical assessment of the correctness of these deductions. The June/July/August data in Figure 1 show a well-defined plume of aerosol optical depth extending from the U. S. east coast over the Atlantic Ocean. This plume is well separated from the plume that extends from the west coast of northern Africa, across the Atlantic, and over the Caribbean, which consists predominantly of mineral dust. Hence, the plume extending from the U.S. east coast provides an excellent opportunity to isolate and study aerosols generated by human industrial activity, and to measure the magnitude and uncertainty of the direct radiative forcing due to these aerosols.

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