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Carbon Brief Staff

18.04.2013 | 7:00pm
ScienceClimate scientists highlight urgent need to get to grips with black carbon
SCIENCE | April 18. 2013. 19:00
Climate scientists highlight urgent need to get to grips with black carbon

Soot warms the atmosphere – on a molecule by molecule basis – twice as much as previously thought, according to a recent study. But scientists warn against shifting the focus of emissions reduction efforts toward so-called black carbon and away from carbon dioxide.

What is black carbon?

Burning fossil fuels, biofuels and biomass releases carbon dioxide and water. It can also produces soot – known as black carbon, with the main sources traditional wood-burning cookstoves and diesel engines. Scientists are concerned about black carbon in the atmosphere because it absorbs heat from the sun – raising earth’s temperature.

According to a recent study, it appears that black carbon absorbs twice as much heat as previously thought. This makes it – molecule for molecule – the second strongest absorber of solar radiation in the atmosphere, soaking up two thirds the heat of a carbon dioxide molecule.

The findings have sparked intense discussion about soot’s role in recent warming and whether reducing black carbon emissions could help mitigate climate change.

But according to new research, published in the journal Science, models are a long way from representing black carbon fully – so how effective attempts to limit it would be is still uncertain.

Warming potential

Why is the new estimate of black carbon’s warming potential higher than previous ones? According to the research, it’s because older models didn’t take all the ways black carbon can influence climate into account.

For one thing, black carbon doesn’t just have the potential to warm the atmosphere –  it can cool it, too. Small airborne particles, or aerosols, containing black carbon can influence cloud formation. And clouds can either absorb or reflect sunlight, leading to warming or cooling respectively.

Models need to do a better job of accounting for black carbon’s effects on atmospheric temperature, say the researchers. And this means better observations at different scales, from detailed chemical analysis to satellite studies of cloud properties.

It’s a particularly tricky issue, as the climate impacts of black carbon may vary regionally because it accumulates close to its source.

Cutting black carbon: Pros and cons

Unlike carbon dioxide, black carbon is short-lived in the atmosphere, so any benefits of cutting emissions could be seen quickly.

But given the uncertainty over the size of the warming effect, a big question is whether efforts to mitigate black carbon would be worthwhile.

The authors suggest targeting diesel emissions could be effective because it’s pretty clear they result in a net warming effect. But burning wood can emit other particles like sulphur dioxide that have a cooling effect – which complicates matters.

The biggest reason to cut black carbon, according to the research, is the expected health benefits. Currently, household air pollution from burning solid fuels – concentrated in developing regions like India and China – causes an estimated 3.5 million deaths worldwide each year.

But while the question of black carbon is important, the authors conclude that it should not detract from efforts to mitigate carbon dioxide, which is by far the bigger driver of climate change.

Andreae, M.O. & Ramanathan, V. (2013) Climate's dark forcings, Science, DOI:  10.1126/science.1235731 

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