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The ABCs of Global Warming
Ramanathan, V., Ramana, M.V., Roberts, G., Kim, D., Corrigan, C., Chung, C. and Winker, D. 2007. Warming trends in Asia amplified by brown cloud solar absorption. Nature 448: 575-578.

The authors of this important new study write that light-absorbing and light-scattering aerosols "contribute to atmospheric solar heating and surface cooling," and that "the sum of the two climate forcing terms - the net aerosol forcing effect - is thought to be negative and may have masked as much as half of the global warming attributed to the recent rapid rise in greenhouse gases," but they caution that there is "at least a fourfold uncertainty in the aerosol forcing effect."

What was done
In an observational program that studied this phenomenon as it has never been studied before, Ramanathan et al. employed "three lightweight unmanned aerial vehicles that were vertically stacked between 0.5 and 3 km over the polluted Indian Ocean," which "deployed miniaturized instruments measuring aerosol concentrations, soot amount and solar fluxes" within the infamous atmospheric brown clouds (ABCs) that have been demonstrated to envelop "most of Asia and the adjacent oceans" during "the six-month-long tropical dry season," when "convective coupling between the surface and the troposphere is weak [and] aerosol solar heating can amplify the effect of greenhouse gases in warming the atmosphere while simultaneously cooling the surface."

What was learned
The seven scientists found that "atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent" during the period of their study; and they say that general circulation model simulations suggest that, over the Indian Ocean and Asia during the long tropical dry season, "atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends."

What it means
For a huge and important part of the world - where the situation could well get worse before it gets better - only half of the observed lower atmospheric warming trend of the past few decades has been either natural or greenhouse-gas-induced (take your pick), while the other half appears to have come courtesy of the region's infamous ABCs. One can only wonder, therefore, what the case may be over other polluted parts of the planet. Perhaps aerosols there may also be adding to global warming instead of subtracting from it, which would appear to be a real possibility considering the enormous fourfold uncertainty Ramanathan et al. associate with the net aerosol forcing effect.

Reviewed 31 October 2007