BioEd Online

Cleaning the air could accelerate global warming, according to a new study.

The particles in the soot and haze from industrial and domestic fires, called aerosols, cause respiratory diseases and other health problems for people in polluted areas, including many Asian cities.

But aerosols also dim the sky over land and sea, and so cool the planet. By scattering and absorbing sunlight - how much depends on the particles' size and optical properties - they prevent the Earth from heating up more than it already has owing to the greenhouse effect.

The balance between the two effects has been a wildcard in climate predictions. In its last report, published in 2001, the Intergovernmental Panel on Climate Change (IPCC) could put only a broad range of possible values on aerosol cooling.

Now researchers at the UK Met Office in Exeter and the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, have provided the first calculation based on observations, rather than models.

The team concludes that the cooling effect of aerosols is probably at the high end of IPCC estimates. So cleaning the air will lead to substantial warming. Without aerosols for example, since 1900 the planet might have warmed at least an additional 0.3 °C above the 0.7 °C rise that actually happened.

Dark skies

The team, led by Met Office researcher Nicolas Bellouin, calculated the amount of aerosols in the air using data from NASA's Terra and Aqua satellites and observations made from aircraft. The researchers then calculated how much light that material would absorb. They publish the results in Nature this week¹.

The result is the most comprehensive measurement of soot so far. "This is a very timely addition to modellers' work," says Meinrat Andreae, an atmospheric scientist at the Max Planck Institute of Chemistry in Mainz, Germany.

Recent models have pointed to a huge range of effects from aerosols, he says. Observations will help sort out the confusion.

In a recent model of his own, Andreae concluded that without aerosol cooling, global temperatures could rise between 6 and 10 °C by 2100 - well beyond current IPCC predictions². Aerosols partly disguise the sensitivity of our climate to rising levels of greenhouse gases, he says.

Heating up

Soot and haze could be even more cooling than Bellouin's team calculates, says Andreae. Aerosols affect other climate properties, such as cloud formation, the effects of which have yet to be measured.

Industrial aerosol emissions have dropped in the United States and Europe since 1990. Worldwide, however, Asia's economic growth is feeding a modest upward trend in their output. But Asian emissions are growing less sharply than was expected, thanks to policies such as a switch to clean energy in the industrial areas around Beijing and Shanghai.