Firstly, it should be said that global dimming is not global in extent and it has not continued after 1990. Reported decreases in solar radiation at the Earth’s surface from 1970 to 1990 have an urban bias.
There has been continuing interest in the possibility of Global dimming. The effect was apparently first spotted by Gerry Stanhill, an English scientist working in Israel.
It is true that measurements of sunlight reaching the Earth’s surface gradually fell between 1970 and 1990. A reduction in downward solar radiation (‘dimming’) of about 1.3% per decade or about 7 W m–2 was observed from 1961 to 1990 at land stations around the world. However, over the entire period from 1984 to 2001, surface solar radiation has increased by about 0.16 W m–2 yr–1. The effect of the eruption of Mt. Pinatubo in 1991resulted in an increase in the outgoing shortwave radiation flux (and a corresponding dimming at the surface).
Dimming seems to be predominant in large urban areas where pollution plays a role. Burning coal, oil and wood, whether in cars, power stations or cooking fires, increases particulates in the air which redistribute cloud water over more and smaller droplets, brightening clouds, decreasing the potential for precipitation. An increasing aerosol load due to human activities decreases the regional air quality and the amount of solar radiation reaching the Earth’s surface. In some areas, such as Eastern Europe, recent observations of a reversal in the sign of this effect link changes in solar radiation to concurrent air quality improvements. Increases in aerosols also reduce direct radiation at the surface under clear skies, and this appears to be a key part of the explanation in China.
Some climatologists say that by allowing less sunlight to reach the Earth, this effect is cushioning us from the full impact of global warming. It is possible that as we burn coal and oil more cleanly, and dimming is reduced, the full effects of global warming will be unleashed.