Shadow of brown clouds
We know how global warming is responsible for the retreat of glaciers. Research now finds another culprit: dark soot deposits or Atmospheric Brown Clouds, writes Veerabhadran Ramanathan.
The retreat of the Himalayan-Hindu Kush (HHK) glaciers is one of the major environmental problems facing Asia. These glaciers feed major river systems including the Indus, Ganges, Brahmaputra, Mekong, Yangtze and Huang He. The livelihood of over two billion Asians are influenced by these rivers. The glacier retreat began in the mid-19th century in response to the termination of the mini Ice Age. The retreat has accelerated since the 1970s and includes major HHK glaciers such as Gangotri and over 90 per cent of the Tibetan glaciers. Glaciologists link this acceleration to the large warming trend of about 0.25°C per decade that has been observed over the elevated HHK regions.
The prevailing understanding is that the warming trend is part of the global warming due to greenhouse gases. But several American and European scientists have speculated that solar heating by soot in Atmospheric Brown Clouds (ABCs) and deposition of dark soot over bright snow surfaces may also be important contributing factors.
New research published in the August 2 issue of Nature offers direct observational evidence for the magnitude of the solar heating of the lower atmosphere by tiny soot particles resulting from fossil fuel combustion, biomass burning and cooking with wood and other bio-fuels. I led a team of American and Indian scientists to the Indian Ocean during March-April, 2006. We launched lightweight (30 kg) unmanned aerial vehicles (UAVs) with miniaturised instruments to sample the brown clouds.
The origins of ABCs are fossil fuel combustion, cooking with wood and other bio-fuels and biomass burning. They start off as urban or rural haze. Fast atmospheric transport spreads the haze far and wide, in about 2-7 days, over an entire subcontinent or an ocean basin. The brown clouds contain tiny particles of sulfates, nitrates, soot and other substances. These particles reflect as well as absorb sunlight and thus they regulate the fundamental driving force for the climate system and other biological processes. Reflection gives the hazy sky appearance, while absorption by soot gives the brownish colour. Both reflection and absorption shield the surface from sunlight and leads to dimming, which has a cooling effect on the ground.
The magnitude of this dimming effect at the surface as well as the widespread nature of ABCs over the entire north Indian Ocean were discovered about 10 years ago during the Indian Ocean experiment (Indoex) conducted by over 200 scientists from India, Europe and the US. As co-chief scientist of Indoex, I flew the last mission on a
C-130 from Maldives to the Bay of Bengal in April 1999. I recall making a resolution that day to spend the rest of my scientific life to unravel the impact of ABCs on my former countrymen. The recent UAV campaign is part of that journey.
Recently, the UN Intergovernmental Panel on Climate Change concluded that as much as 50 per cent of global warming due to man-made greenhouse gases may have been masked by the dimming due to particles in ABCs. The implication is that elimination of the brown clouds can add another 0.8°C warming to the 0.8°C warming already observed during the last century.
Many scientists believe a global warming of about 2-2.5°C will lead to unprecedented, if not dangerous climate changes. Should we then not reduce the Brown Cloud pollution? The key questions — what is the magnitude of the heating of the atmosphere due to soot absorption of sunlight? Where in the atmosphere is this heating happening? — were addressed in our Nature paper.
While our finding about the magnitude of the solar heating and its spatial extent are robust, the model simulations of the atmospheric warming require confirmation by independent studies. This may take a decade or two. What can the public and policy-makers do in the meantime? As a natural scientist, I don’t have the training to take up this issue. Should we hide behind the uncertainties of the complex workings of the physical-chemical-biological feedbacks of the climate system? Or should we give an early warning to policy-makers and the public? It will be several decades or longer before we can make an accurate prediction of future climates. Do we have the luxury of waiting that long? What we can say based on our observations is that greenhouse gases and ABCs are affecting the fundamental forces that regulate the climate system including the Asian monsoon, the Himalayan glaciers and sea level in major ways.
Veerabhadran Ramanathan is Professor of
Climate and Atmospheric Sciences,
Scripps Institution of Oceanography,
University of California, San Diego.