The effect of black carbon on snow covered areas

Research into black carbon

7 January 2011

Jonathan Leake, Science Editor for the Sunday Times

The world's glaciers and icecaps are shrinking - but it's not just greenhouse gases causing the extra melting. In the first of our guest articles on climate change Jonathan Leake, Science Editor for the Sunday Times, describes the latest research into 'black carbon'.

The Alps and the Arctic may look white but they are actually getting more grey. Scientists have measured how human pollution is darkening the world's glaciers, icecaps and snowfields - with similar effects suspected in the Himalayas.

The effect, caused by microscopic dust particles released by diesel engines; factories; power stations and burning forests, means that such areas are now reflecting significantly less light than in the past.

Instead the light is being absorbed into the ice where it turns to heat - adding to the melting already being caused by greenhouse gas-induced global warming.

Researchers from the Met Office have worked out that each metre of snow-covered land is reflecting a few per cent less light. The effect is strongest in the Alps where dust has cut the light reflected by up to 8% in places.

In the Arctic the effect is smaller, at up to 2% of light, but the area affected is much bigger. The impact on the Himalayas - where a lot of dust generated by Indian and Chinese industry is thought to end up - is subject to research.

Dr Vicky Pope, Head of Climate Science Advice at the Met Office, said black carbon made snow darker causing it to absorb more light and melt faster.

"As it absorbs extra energy the ice and snow melts faster. This is an extra effect, on top of the warming already caused by greenhouse gases. The result is to accelerate the melting."

The effect of black carbon on snow and ice has been suspected for some time, but only now are clear measurements beginning to emerge.

But what is black carbon, and where does it come from?

How tiny soot-like particles created when humans burn fossil fuels are helping to melt the world’s icecaps and glaciers  Confusingly, perhaps, scientists refer to black carbon as an aerosol - but this does not mean it comes from spray cans. The term simply means that it comprises particles so small they can remain suspended in air for some time.

Such particles occur naturally but humans have added massively to the total simply by burning things. Perhaps the largest sources of artificial emissions come from burning forests and grasslands; solid fuels like coal for cooking and heating, and diesel engines.

For climate scientists, the key point about black carbon aerosols is that they are very good at absorbing sunlight - so they help heat the atmosphere. What's more, they also react chemically with other components of the atmosphere - meaning they can interfere with the chemistry underlying cloud formation and dissolution.

The behaviour of black carbon in air is highly complex but the Met Office Hadley Centre has used computer modelling to try to work out the overall heating effect. Their findings suggest it could add 0.3C to the eventual warming of the Earth's surface.

Perhaps the most potent impact of black carbon particles comes, however, when they land on snow or ice.

Nicolas Bellouin, Climate Research Scientist at the Met Office Hadley Centre, said: "Man-made soot particles make snow darker, leading to a warmer surface. This accelerates spring-time snow melt across the Northern Hemisphere, exposing even darker surfaces. Soot particles are more effective than CO2 at melting snow because they are located precisely where it matters, in the snow itself."

Climate response and efficacy of snow albedo forcing in the HadGEM2-AML climate model [367kb] (opens in a new window)

The Met Office's own work focused on the Arctic and the Alps, but there is other work showing the Himalayan glaciers and snowfields are also being affected.

In one 2007 study scientists found that black carbon in the air over the Himalayas was having just as big a warming effect as CO2 emissions.

  Warming trends in Asia amplified by brown cloud solar absorption

Another - published in 2009 by Professor Mark Flanner, an atmospheric scientist at the University of Michigan - showed that black carbon in Himalayan snow was accelerating melting and warming in the region.

  Springtime warming and reduced snow cover from carbonaceous particles

The good news is that some major sources of black carbon are relatively easy to tackle. These include the emissions from power stations and industry transport, which can be reduced by legislation forcing industry to adopt new emissions standards. The burning of forests is also easily remedied if the political will is there.

Look, for example, at the way stubble burning has been outlawed across Europe - just one example of how the region's black carbon emissions have been brought under control in recent years.

Nicolas said: "Black carbon emissions have decreased over Europe compared with 1980 but they are increasing over India and China. So we can reasonably expect a decrease of the strength of the 'snow darkening' effect in the Arctic, where most black carbon came from Europe but an increase of the effect in the Himalayas because of the industrial expansion of India and China."

The growing understanding of black carbon does, however, bring new complexities for scientists and policy makers. There are agreements in place, among the G8 industrialised countries, for example, to cut black carbon emissions. But black carbon is not the only type of manmade dust that affects climate change.

The sulphates and organic aerosols also generated by the very same human activities that produce black carbon also affect climate - but they have a cooling effect. So if we cut black carbon we could also be cutting sulphates - and so cancel out some of the benefits. Working out the overall impact of such changes is an important challenge for researchers.

What the findings illustrate, perhaps above all else, is the sheer complexity of climate research - and the important political role to be played by scientists who can turn their findings into useful unbiased advice for policymakers.

The new research comes as negotiators from more than 180 countries are meeting in Cancun for talks aimed at drawing up a global agreement on cutting greenhouse gas emissions. The issue of dust emissions is, however, not on the table.

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