Soot adds to rise in global warming

Michael Richardson, For The Straits Times 8 Feb 10;

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 analysed five ice cores from various locations on the Tibetan plateau to find that the concentration of black carbon, or soot, in the ice had increased to between two and three times the level in 1975.

At Zuoqiupu glacier, on the southern edge of the plateau downwind from the Indian subcontinent, black carbon deposition rose by 30 per cent between 1990 and 2003.

What are the implications of this and other related findings by numerous researchers from different countries and scientific agencies in recent years?

International efforts to combat global warming focus on cutting six greenhouse gases. A panel of scientists advising the United Nations has concluded that these gases - chiefly carbon dioxide, methane and nitrous oxide - are most likely responsible for warming the planet to potentially dangerous levels.

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 cow dung and crop residues, although the by-product is mainly organic carbon which scientists say has less of a warming effect than black carbon.

Asia 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. 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, Professor Kimberly Prather of the University of California in San Diego and a colleague published research last year showing that sulphate and nitrate play a different role when they mix with black carbon.

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 centre of the particle, enhancing warming,' Prof Prather says. The measurements showed that in the atmosphere the aerosol combination increased the warming effect of the coated black carbon particles 1.6 times over pure black carbon particles.

North America and Europe have reduced aerosol levels by enacting clean air regulations and transport fuel standards. Developing Asian economies have been slower to follow.

The extensive soot-laden pollution Asia produces now shows up in satellite photographs as a gigantic brownish haze stretching over large parts of South and South-east Asia and China. This haze is a health hazard as well as an extra source of global warming.

As the pollution is carried by prevailing winds in the northern hemisphere, it affects other countries that have cleaner air standards such as Singapore, Japan and the United States.

Aerosols have another warming effect. The coated black carbon particles do not waft around forever. As winds drop, they are deposited on earth's land and sea surfaces, including snow and ice, creating a smudging effect. The dirty snow and ice absorb more sunlight than pure snow and ice, which reflect light.

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 remain only 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 than greenhouse gases like carbon dioxide.

An Indian-Swedish research team has concluded 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.

They and other scientists have called for a rapid scaling-up of programmes to discourage open air burning and spread the use of low-cost but efficient household stoves and biogas.

'While reducing carbon dioxide concentrations is extremely 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 Prof Prather.

'This could buy us time...'

The writer is a visiting senior research fellow at the Institute of Southeast Asian Studies.