The effect of aerosols on modulating the sun’s radiation has been one of the biggest uncertainties in understanding climate change — with satellite data showing more aerosol cooling than computer models. New research reconciles the two different approaches and shows that official estimates of aerosol cooling have been too large, suggesting that any masking of overall warming will be smaller than previously thought.

Aerosols are small particles suspended in the atmosphere that either scatter or absorb solar radiation, a combined phenomenon known as the direct aerosol effect. Aerosols that scatter — such as sulphates, nitrates and organic carbon — tend to cool the Earth by sending some incoming radiation back into space, while absorbing aerosols, such as black carbon (formed from the incomplete burning of fossil fuels), heat up the Earth’s atmosphere.

Scientists know that scattering outweighs absorption, and therefore the direct aerosol effect leads to an overall cooling of the climate. Indeed, it may have contributed to a drop in global temperature around the middle of the 20th century. It may also have masked some of the current warming caused by increased greenhouse gas emissions, which could amplify future warming as strict controls on aerosol emissions come into effect.

Large margin of error

In its report of 2007, the Intergovernmental Panel on Climate Change (IPCC) estimated that the direct aerosol effect has a radiative forcing, or net cooling, of -0.5  Wm-2, which would offset warming due to anthropogenic carbon dioxide by almost a third. However, the margin of error was large – from -0.9 to -0.1  Wm-2.

This uncertainty was mainly caused by differences in the way that the direct aerosol effect is calculated. One option is to use computer modelling, which estimates emissions of the pollutants that produce aerosols and then models aerosol production and the absorption and scattering processes. The alternative is to use satellite measurements of the quantity of aerosols in the atmosphere combined with ground-based measurements of the relative strength of aerosol scattering to absorption. Satellite observations give larger estimates for the cooling.

Now, however, Gunnar Myhre of the Center for International Climate and Environmental Research in Oslo has used the Oslo CTM2 global aerosol model and measurements from the Moderate Resolution Imaging Spectroradiometer onboard NASA’s Terra and Aqua satellites together with data from the ground-based Aerosol Robotic Network of solar photometers to show that there are two main reasons for the discrepancy.

More black carbon

The first of these is the fact that calculations based on satellite measurements assume that the relative concentrations of different aerosols in the atmosphere have remained constant throughout the industrial age. This is a problem because calculating the cooling effect of anthropogenic aerosols involves subtracting the effect of aerosols naturally present in the atmosphere, in other words working out the relative strength of scattering and absorption before the industrial era. It turns out, in fact, that emissions of black carbon have increased by more than a factor of six whereas output of the various scattering aerosols has gone up by a factor of only three or four.

The second reason is that satellites have not been able to gather data on aerosol scattering above bright surfaces — such as polar ice caps — because light scattering from the surfaces themselves is so strong. This has tended to overstate global cooling because there are far lower densities of aerosols over the icecaps.

By bringing the two approaches into line, Myhre calculates a new best estimate of -0.3 Wm-2 for the cooling of the direct aerosol effect. He says that this will tend to reduce future projections of global warming. This is because the expected drop in aerosol production will not lead to as large a temperature rise as previously thought. Indeed, he estimates that the direct aerosol effect offsets only 10% of global warming. However, he points out that there is still some uncertainty in the vertical distribution of aerosols within the atmosphere, which is significant in so far as absorptive aerosols have a much greater effect when located above a cloud than when below.

Myhre also points out that the direct aerosol effect is smaller than another phenomenon known as the “indirect” effect, in which aerosols enhance scattering through cloud formation. The IPPC’s estimate for the indirect effect is -0.7  Wm-2, ranging from -1.8 Wm-2 to -0.3  Wm-2. Edwin Cartlidge