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 50per cent 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.