An article in the Guardian on 18 December has created some interest, as many readers concluded it seems to have thrown the whole topic of global warming into confusion. As an indication of the article’s impact, the number of references to global dimming on Google went from 21 to 6000+ in the week after it appeared, though there have been no references in newspapers or magazines that I can find, before or since.
It publicised the fact — known since the late 1980s but supposedly ignored or disbelieved until recently — that over the past fifty years the average amount of sunlight reaching the ground (the name for which is insolation) has gone down by about 3% a decade. It doesn’t mean the sun is sending out less radiation, but that less of it is reaching the Earth’s surface because of pollution in the atmosphere. This effect seems to have been named global dimming in an article in Agricultural and Forest Meteorology in 2001.
There’s no conflict with observed global warming, whose likely severe impact on the world’s plant and animal species a report in this week’s Nature makes clear. The sun’s heat is still being absorbed, but at a higher level in the atmosphere, probably on particles of soot and the like. The Guardian article argues that the adverse impact on agriculture could be substantial, since even a 1% reduction on sunlight is enough to affect the ripening of some crops.