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Aerosol Properties of the Global Atmosphere As Input Data to Climate Models
Reference
Mishchenko, M.I., Geogdzhayev, I.V., Liu, L., Lacis, A.A., Cairns, B. and Travis, L.D. 2009. Toward unified satellite climatology of aerosol properties: What do fully compatible MODIS and MISR aerosol pixels tell us? Journal of Quantitative Spectroscopy & Radiative Transfer 110: 402-408.

Background
The authors write that "because of the global nature of aerosol climate forcings, satellite observations have been and will be an indispensable [our italics] source of information about aerosol characteristics for use in various assessments of climate and climate change," and they say "there have been parallel claims of unprecedented accuracy of aerosol retrievals with the moderate-resolution imaging spectroradiometer (MODIS) and multi-angle imaging spectroradiometer (MISR)."

What was done
If both aerosol retrieval systems are as good as they have been claimed to be, they should agree on a pixel by pixel basis as well as globally. Consequently, and noting that "both instruments have been flown for many years on the same Terra platform, which provides a unique opportunity to compare fully collocated pixel-level MODIS and MISR aerosol retrievals directly," Mishchenko et al. decided to see how they compare in this regard by analyzing eight years of such data.

What was learned
The six scientists from NASA's Goddard Institute for Space Studies report finding what they describe as "unexpected significant disagreements at the pixel level as well as between long-term and spatially averaged aerosol properties." In fact, they say that "the only point on which both datasets seem to fully agree is that there may have been a weak increasing tendency in the globally averaged aerosol optical thickness (AOT) over the land and no long-term AOT tendency over the oceans."

What it means
The bottom line for the NASA scientists is quite succinct: "our new results suggest that the current knowledge of the global distribution of the AOT and, especially, aerosol microphysical characteristics remains unsatisfactory." And since this knowledge is indispensable "for use in various assessments of climate and climate change," it would appear that current assessments of greenhouse-gas forcing of climate made by the very best models in use today may be of very little worth in describing the real world of nature.

Reviewed 22 July 2009