Pollution Amplifies Greenhouse Gas Warming Trends to Jeopardize Asian Water Supplies

Scientists have concluded that the global warming trend caused by the buildup of greenhouse gases is a major contributor to the melting of Himalayan and other tropical glaciers. Now a new analysis of pollution-filled “brown clouds” over south Asia by researchers at Scripps Institution of Oceanography at UC San Diego offers hope that the region may be able to arrest some of the alarming retreat of such glaciers by reducing its air pollution.

The team led by Scripps atmospheric chemistry professor V. Ramanathan describes findings that atmospheric brown clouds enhanced solar heating of the lower atmosphere by about 50 percent in a paper to be released in the Aug. 2 edition of the journal Nature. The combined heating effect of greenhouse gases and the brown clouds, which contain soot, trace metals and other particles from a growing cadre of urban, industrial and agricultural sources, is enough to account for the retreat of Himalayan glaciers observed in the past half century, the researchers concluded. The glaciers supply water to major Asian rivers including the Yangtze, Ganges and Indus. These rivers in turn comprise the chief water supply for billions of people in China, India and other south Asian countries.

“The rapid melting of these glaciers, the third-largest ice mass on the planet, if it becomes widespread and continues for several more decades, will have unprecedented downstream effects on southern and eastern Asia,” the Nature article concluded.

“The main cause of climate change is the buildup of greenhouse gases from the burning of fossil fuels,” said Achim Steiner, United Nations under-secretary general and executive director of the UN Environment Programme (UNEP), which helped support the research. “But brown clouds, whose environmental and economic impacts are beginning to be unraveled by scientists, are complicating and in some cases aggravating their effects.

“The new findings should spur the international community to ever greater action, in particular at the next crucial climate change convention meeting in Indonesia this December. For 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,” Steiner added.

The scientists based their conclusions in large part 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. The air typically contains particles released from industrial and vehicle emissions as well as through biomass burning.

Such polluted air has been demonstrated to have a dual effect of warming the atmosphere as particles absorb sunlight and of cooling the earth's surface as the particles curb the amount of sunlight that reaches the ground. The net effect of the two forces remains uncertain but other research by Ramanathan has suggested that the surface dimming might serve to mask global warming, leading scientists and the public to underappreciate the full magnitude of anthropogenic climate change.

The aircraft, flying in stacked formations, made nearly simultaneous measurements of the 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.

When the researchers fed both greenhouse gas and brown cloud data into computer climate models, the simulations yielded an estimate that the region's atmosphere has warmed 0.25 degrees C (0.5 degrees F) per decade since 1950 at altitudes ranging from 2 to 5 kilometers (6,500 to 16,500 feet) above sea level. At those heights are found many of the glaciers in the Himalayas. The amount of heating corresponds to observed levels of glacial retreat.

“In order to understand the processes that can throw the climate out of balance, Ramanathan and colleagues, for the first time ever, used small and inexpensive unmanned aircraft and their miniaturized instruments as a creative means of simultaneously sampling of clouds, aerosols and radiative fluxes in polluted environments, from within and from all sides of the clouds,” said Jay Fein, program director in the National Science Foundation (NSF)'s Division of Atmospheric Sciences. “These measurements, combined with routine environmental observations and a state-of-the science model, led to these remarkable results.”

The analysis revealed that the effect of the brown cloud was necessary to explain temperature changes that have been observed in the region over the last half-century. It also indicated that south Asia's warming trend is more pronounced at higher altitudes than closer to sea level.

“The conventional thinking is that brown clouds have masked as much as 50 percent of the global warming by greenhouse gases through the so-called global dimming,” said Ramanathan, who is lead author of the Nature paper. “While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are intensifying 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, and Chul Chung from Scripps Oceanography and David Winker from the National Aeronautics and Space Administration's (NASA's) Langley Research Center.

The NSF provided the main funding for the research. Additionally, the National Ocean and Atmospheric Administration (NOAA) and NASA provided support as did the UNEP, which sponsors the Atmospheric Brown Clouds (ABC) project and operates the Maldives ABC observatory in collaboration with Scripps.