Aerosols exert an important effect on the Earth's radiative balance by directly absorbing and scattering incoming sunlight. Traditionally, chemical transport and general circulation models enjoyed a monopoly on estimating the role of aerosols in the Earth's climate.
Observations of aerosols from ground-based, airborne or satellite instruments have been used only to validate these models. The prevailing strategy dictates that measurements improve models, and then models, not measurements, answer climate questions.
However, results of model simulations of aerosol effect on the distribution of solar radiation vary considerably because aerosols vary considerably. Each type of aerosol affects sunlight differently by absorbing and scattering the light in varying amounts, in different directions and at different wavelengths. Simulating the wide variety of aerosol effects in a model remains a difficult task. The cumulative effect of these differences in aerosol properties can result in vastly different estimates of global change, and in fact be the difference between whether the planet is warming or cooling.
In this study Remer and Kaufman present an observationally based estimate of the aerosol radiative effect in clear-sky situations over the oceans. They bypass much of the guesswork in assigning aerosol optical properties to different types of aerosols by using a consistent set of information gleaned from the satellite observations. The satellite sensor that they use is the MODerate resolution Imaging Spectroradiometer (MODIS). If used consistently MODIS will provide highly accurate estimates of the reflected sunlight by the particles. An example of MODIS-measured aerosol radiative effect for four seasons is shown in the figure.
Click on the months to see seasonal changes
Global distribution of MODIS-observed aerosol radiative effect at top of atmosphere from the Terra satellite for four seasons: Northern Winter 2003-2004 (upper left), Spring 2004 (upper right), Summer 2004 (lower left) and Fall 2003 (lower right).
