Credit: © ISTOCKPHOTO / PATRICK HERRERA

J. Geophys. Res. 114, D17107 (2009)

Scientists have devised a novel, more exact way of gauging the effect of aerosols on the Earth's climate. Their study confirms that anthropogenic aerosols in the atmosphere cool the planet but rules out some of the upper estimates of the extent to which the tiny airborne particles can temper warming.

Daniel Murphy of the US National Oceanic and Atmospheric Administration and colleagues took a unique, observation-based approach to calculating the aerosol effect: they treated the Earth's energy budget like a bank account, albeit a complex one. Using the simple physical principle of conservation of energy, they calculated all the known drivers of climate change since 1950, such as greenhouse gases, volcanic eruptions and solar variations. This 'credit' was then compared to the sum of the 'debits' — that is, the heat content of the Earth and the amount of energy released back into space. Once everything had been totalled, they found that there was a missing cooling factor. This missing debit — about 1.1 watts per square metre between 1970 and 2000 — can primarily be attributed to the direct and indirect forcing of aerosols from human sources such as pollution and biomass burning.

The team's new calculation increases confidence in our understanding of this aspect of the climate system.