Particulate petroleum pollutants in the upper atmosphere attract water condensation to form particulate droplets which act as mirrors reflecting light back away from the earth's surface. This has resulted in a measurable reduction in light intensity at ground level - global dimming - and an associated reduction in evaporation rates. Reduced evaporation rates effect cloud formation and rain patterns and so far this phenomenon is thought to have been responsible for at least the Ethiopian famines of the 1980's. The immediate solution seems to be to burn fossil fuels more cleanly and in Western Europe where scrubbers in power station flues and catalytic converters in cars have been used to clean the emissions there has been significantly less particulate pollution in the atmosphere over the past 15 years. So effective has this been that the rains have started to return to the Sahel.
But there is a sinister twist. Scientists are now beginning to see how Global Dimming which also cools the earth has been causing us to seriously underestimate the full effect of global warming.
As world nations bring fossil fuel burning under control which indeed we must to prevent runaway GW, the masking effect of Global Dimming will be lost and we will face the full impact of existing greenhouse emissions.
This is a time bomb hanging over us.