SINGAPORE — A team from the Chinese Academy of Sciences trekked across frigid highlands in Tibet to confirm a significant recent discovery about climate change. They drilled and analyzed five ice cores from various locations on the Tibetan Plateau to find that the concentration of black carbon, or soot, in the ice has increased by two to three times since 1975.
At Zuoqiupu Glacier, on the southern edge of the plateau downwind from the Indian subcontinent, black carbon deposition rose by 30 percent between 1990 and 2003.
What are the implications of these and other related findings in recent years by numerous researchers from different countries and scientific agencies?
International efforts to combat global warming focus on cutting emissions of six greenhouse gases from human activity. A panel of scientists advising the United Nations has concluded that these gases — chiefly carbon dioxide, methane and nitrous oxide — are most likely to be responsible for warming the planet to potentially dangerous levels.
Yet a growing body of research in the past few years points to another potent source of human-induced warming: airborne aerosol particles, especially black carbon — a key component of soot.
When coal, oil and other fossil fuels are burned without enough oxygen to complete combustion, one of the by-products is black carbon. A similar process takes place with the burning of biomass, including wood, cow dung and crop residues, although the by-product is mainly organic carbon, which scientists say has a lesser warming effect than black carbon.
Asia, with its rapid economic growth and agricultural expansion, is now the leading global source of the tiny particles of soot from this incomplete burning. They rise into the atmosphere and mix with different emissions, including nitrates and sulphates, to form aerosols.
Black carbon absorbs sunlight, as do other greenhouse gases. This blanket-effect warms the atmosphere. But particles of sulphate or nitrate alone reflect solar radiation, thereby cooling the planet. Indeed, advocates of geo-engineering to combat global warming have proposed pumping sulphate aerosols into the atmosphere to slow climate change.
However, Kimberly Prather, professor in the Department of Chemistry and Biochemistry at the University of California, San Diego, and a colleague published research last year showing that sulphate and nitrate play a different role when they mix with black carbon.
The two researchers checked atmospheric aerosols over Riverside, Calif., and Mexico City, using an instrument that measures the size, chemical composition and optical properties of aerosols in real time. Their study showed that jagged bits of fresh soot quickly become coated with a spherical shell of other chemicals, particularly sulphate, nitrate and organic carbon, through light-driven chemical reactions.
“The coating acts like a lens and focuses the light into the center of the particle, enhancing warming,” professor Prather says.
The measurements showed that in the atmosphere the aerosol combination increased the warming effect of the coated black carbon particles to 1.6 times that of pure black carbon particles.
North America and Europe have reduced aerosol levels by enacting clean-air regulations and transport fuel standards. Major developing Asian economies have been slower to follow and the extensive soot-laden pollution now shows up in satellite photos as a huge brownish haze stretching over large parts of South and Southeast Asia and China.
This haze is a health hazard as well as an extra source of a global warming. The U.N. Environment Program (UNEP) said in a 2008 report on atmospheric brown clouds that they were also a major threat to water and food security in Asia.
Black carbon does not only warm the atmosphere. As the pollution is carried by prevailing winds in the Northern Hemisphere, it affects other countries that have cleaner air standards such as Japan, and even the U.S. on the other side of the Pacific Ocean.
Aerosols have another warming effect. The coated black carbon particles do not waft around forever. As winds drop, they are deposited on land and sea surfaces, including snow and ice, creating a smudging effect.
The dirty snow and ice absorb more sunlight, thus warming faster than pure snow and ice, which reflect light. The impact of aerosols has been observed as far away as the Arctic, where sea ice is in rapid retreat, and the Greenland ice sheet, which is also melting.
By some measures, black carbon accounts for roughly half the global warming potential of carbon dioxide, the main greenhouse gas. Moreover, while carbon dioxide can stay in the atmosphere for over a century, aerosols only remain for a few weeks at most.
Could tighter international controls on soot emissions provide a quick fix for climate change? A growing body of evidence suggests that black carbon can be controlled more easily and cost-effectively than greenhouse gases like carbon dioxide.
An Indian-Swedish research team has used carbon analysis to conclude that about two-thirds of the soot-laden brown cloud pollution in Asia comes from biomass burning (mainly household cooking and slash-and-burn agriculture) and one-third from fossil fuel combustion (mainly coal burning for industry and power, and use of sulphur-laden diesel fuel in trucks and ships).
They and other scientists have called for a rapid scaling-up of programs to discourage open air burning and spread the use of low-cost but efficient household stoves and biogas, which could also aid poverty reduction. These scientists also propose tighter transport fuel standards.
While reducing carbon dioxide concentrations is important, changes we make today will not be felt for quite a while, whereas changes we make today on soot and sulphate could affect our planet on timescales of months, says Prather. “This could buy time while we grapple with the problems of reducing carbon dioxide and other greenhouse gases.”
Michael Richardson is a visiting senior research fellow at the Institute of South East Asian Studies in Singapore.
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