Scientific Objectives , Measurement Needs , and Challenges Motivating the PARAGON Aerosol Initiative

OCTOBER 2004 AMERICAN METEOROLOGICAL SOCIETY | T he Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) concept (Diner et al. 2004) envisions a coordinated and systematic approach for dramatically improving our understanding of aerosol climate impacts and environmental interactions. Among its scientific motivations is the need to establish, with great confidence, the magnitude of direct anthropogenic aerosol influences on climate and the magnitude and sign of indirect aerosol–cloud effects. Modeling of the industrial era temperature record indicates that net aerosol forcing is slightly less, but opposite in sign, than forcing from anthropogenic greenhouse gases (GHGs). However, the uncertainties in aerosol direct and indirect anthropogenic forcing are of the same magnitude as the effects themselves (Penner et al. 2001; Ramaswamy et al. 2001), and are the dominant sources of uncertainty in current estimates of total anthropogenic climate forcing. A major cause of uncertainty is the tremendous tropospheric aerosol variability in space and time. On regional scales, aerosols can have a much greater effect than GHGs, especially at the earth’s surface. Anthropogenic aerosol radiative forcing of climate change has been assessed with forward radiative transfer calculations, constrained by the amount, distribution, and properties of anthropogenic aerosols, and by inverse calculations, which infer anthropogenic aerosol forcing by subtracting greenhouse gas and other known influences from the observed climate record over the industrial period. Using the latter approach, Crutzen and Ramanathan (2003) estimate that the current global net forcing from the combined effect of aerosols on solar radiation backscattering and cloud brightness is about –1.2 W m-2, using as conScientific Objectives, Measurement Needs, and Challenges Motivating the PARAGON Aerosol Initiative