Author: Will Steffen

A new report published by the United Nations Environment Program this week caused some media stir and alarmism over the issue of aerosols in the atmosphere – now popularly referred to as ABCs (Atmospheric Brown Clouds).

The issues are well-known in the scientific community, and have been part of global change research for a couple of decades at least. The issue has only recently found its way into the popular press.

Aerosols are particles of various sizes emitted into the atmosphere by both natural and human processes. In some parts of the way – for example, around the Asian mega-cities – human-driven emissions of aerosols, primarily through the combustion of fossil fuels and of biomass for heating and cooking – now dominate the natural sources.

Aerosols have complex effects on climate, but in general tend to cool the climate at the Earth’s surface, thus counteracting the effects of greenhouse gases. This is still much uncertainty about the magnitude of this cooling effect at the global scale, but a good guess is that it’s masking effect is about 50% of the observed warming. That is, without the cooling effect of aerosols, we would be experience about 1.0 deg. C warming above pre-industrial levels compared to the 0.7 deg C that has been observed. This is bad news, because as aerosol emissions are reduced, the rate of warming will increase.

A second effect of aerosols is on the hydrological cycle – that is, on rainfall. Here the story is even more complicated. Depending on the size and the nature of the aerosol particles, they can either increase or decrease the amount of rainfall in the region. There are also indirect effects. Because aerosols are a regional phenomenon, they cause changes in the differential heating of the Earth’s surface. This, in turn, affects the patterns of atmospheric circulation, which influences the patterns of rainfall.

The bottom line is that we have much to learn about aerosols. For those who thought that science has largely done its job in terms of understanding climate change, you are seriously mistaken. There is much, much more to learn that is of relevance to policy and management.