There is a fascinating review of a very recent Nature paper on the “brown clouds” in Green Car Congress. These clouds have been observed in several parts of the world, especially over the densely populated and polluted countries in Asia, by V. Ramanathan and his colleagues at Scripps. These clouds have been found to be persistent, filled with dust and soot, and linked to increases in a masking of the full magnitude of global warming impact in the region, adverse human health, decreases in food crop cultivation, and also extreme weather events. See some of my other related posts:
A new analysis of pollution-filled “brown clouds” over south Asia by researchers at the Scripps Institution of Oceanography in La Jolla, Calif., suggests that the region may be able to arrest some of the alarming melting of Himalayan and other tropical glaciers by reducing its air pollution. The team, led by atmospheric chemist V. Ramanathan of Scripps, found that atmospheric brown clouds enhanced solar heating of the lower atmosphere by about 50%. The results are in a paper in this week’s issue of the journal Nature. The combined heating effect of greenhouse gases and brown clouds, which contain soot, trace metals and other particles from urban, industrial and agricultural sources, is enough to account for the retreat of Himalayan glaciers in the past half century, the researchers concluded. These glaciers supply water to major Asian rivers, including the Yangtze, Ganges and Indus. These rivers are the chief water supply for billions of people in China, India and other south Asian countries.
If it becomes widespread and continues for several more decades, the rapid melting of these glaciers, the third-largest ice mass on the planet, will have unprecedented effects on southern and eastern Asia.
The scientists based their conclusions on data gathered by a fleet of unmanned aircraft during a landmark field campaign conducted in March 2006, in the skies over the Maldives, an island nation in the Indian Ocean south of India. The Maldives Autonomous unmanned aerial vehicle Campaign (MAC) took place during the region’s dry season when polluted air masses travel south from the continent to the Indian Ocean. (Earlier post.) The air typically contains particles released from industrial and vehicle emissions, and through biomass burning. Such polluted air has a dual effect of warming the atmosphere as particles absorb sunlight, and of cooling Earth’s surface as the particles reduce the amount of sunlight that reaches the ground.
Atmospheric brown clouds are mostly the result of biomass burning and fossil fuel consumption. They consist of a mixture of light-absorbing and light-scattering aerosols 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 gases. There is, however, at least a fourfold uncertainty in the aerosol forcing effect. Atmospheric solar heating is a significant source of the uncertainty, because current estimates are largely derived from model studies.
…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 Asia, suggest that atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends.
—“Warming trends in Asia amplified by brown cloud solar absorption”
During 18 flight missions, the aircraft, flying in stacked formations, made nearly simultaneous measurements of brown clouds from different altitudes, creating a profile of soot concentrations and light absorption that was unprecedented in its level of vertical detail. The researchers validated the data from the aircraft with ground-based measurements taken at a station at the Maldivian island Hanimadhoo.
The main cause of climate change is the buildup of greenhouse gases from the burning of fossil fuels. But brown clouds, whose environmental and economic impacts are beginning to be unraveled by scientists, are complicating and in some cases aggravating their effects. It is likely that in curbing greenhouse gases we can tackle the twin challenges of climate change and brown clouds, and in doing so, reap wider benefits–from reduced air pollution to improved agricultural yields.
—Achim Steiner, United Nations under-secretary general and executive director of the UN Environment Programme (UNEP)
The analysis reveals that the effect of the brown clouds explains temperature changes observed in the region over the last half-century. It also indicates that south Asia’s warming trend is more pronounced at higher altitudes.
The conventional thinking is that brown clouds have masked as much as 50 percent of global warming by greenhouse gases through so-called global dimming. While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are in fact amplifying the atmospheric warming trend caused by greenhouse gases by as much as 50 percent.
In addition to Ramanathan, the report’s authors include Muvva Ramana, Gregory Roberts, Dohyeong Kim, Craig Corrigan, Chul Chung from Scripps Oceanography and David Winkler from NASA’s Langley Research Center. The National Ocean and Atmospheric Administration (NOAA) and NASA also provided funding, as did UNEP, which sponsors the Atmospheric Brown Clouds (ABC) project and operates the Maldives ABC observatory in collaboration with Scripps.
- “Warming trends in Asia amplified by brown cloud solar absorption”; Veerabhadran Ramanathan, Muvva V. Ramana, Gregory Roberts, Dohyeong Kim, Craig Corrigan, Chul Chung and David Winker; Nature 448, 575-578 (2 August 2007) | doi:10.1038/nature06019