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Warming trends in Asia amplified by brown cloud solar absorption


Atmospheric brown clouds are mostly the result of biomass burning and fossil fuel consumption1. They consist of a mixture of light-absorbing and light-scattering aerosols1 and therefore contribute to atmospheric solar heating and surface cooling. 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 gases2. There is, however, at least a fourfold uncertainty2 in the aerosol forcing effect. Atmospheric solar heating is a significant source of the uncertainty, because current estimates are largely derived from model studies. Here we use three lightweight unmanned aerial vehicles that were vertically stacked between 0.5 and 3 km over the polluted Indian Ocean. These unmanned aerial vehicles deployed miniaturized instruments measuring aerosol concentrations, soot amount and solar fluxes. During 18 flight missions the three unmanned aerial vehicles were flown with a horizontal separation of tens of metres or less and a temporal separation of less than ten seconds, which made it possible to measure the atmospheric solar heating rates directly. We found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent. Our general circulation model simulations, which take into account the recently observed widespread occurrence of vertically extended atmospheric brown clouds over the Indian Ocean and Asia3, suggest that atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends. We propose that the combined warming trend of 0.25 K per decade may be sufficient to account for the observed retreat of the Himalayan glaciers4,5,6.

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Figure 1: Vertical distribution of UAV observations.
Figure 2: Colour-coded profiles of 532 nm backscatter return signal from the CALIPSO lidar showing the vertical distribution of ABCs.
Figure 3: The temperature change d T due to ABC solar heating at the 700 hPa (or 700 mb) level (3 km a.s.l.) simulated by CCM3.

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We acknowledge NSF and J. Fein for supporting and guiding the UAV programme and the NSF, the NOAA and NASA for funding the MAC campaign. We also thank D. Fahey, H. Maring, J. Kuettner, C. Jennison and R. Curry for guidance of MAC and Advanced Ceramics for the field support and the government of Maldives for support of the field campaign and MCOH.

Author Contributions V.R. designed MAC, performed the analysis and wrote the paper. M.V.R. was responsible for radiometric observations, validation and data analysis. G.R. was the lead instrument scientist; designed the instrument–UAV integration package; and did the analysis of aerosol and cloud physics data. D.K. conducted the MACR calculations and analysis. C. Corrigan was responsible for UAV absorption measurements and analysis of this data. C. Chung was responsible for GCM simulations and analysis. D.W. is the Principal Investigator for CALIPSO data. All authors reviewed and commented on the paper.

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Correspondence to Veerabhadran Ramanathan.

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Ramanathan, V., Ramana, M., Roberts, G. et al. Warming trends in Asia amplified by brown cloud solar absorption. Nature 448, 575–578 (2007).

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