Measurements of the sunlight reaching the Earth's surface show that between about 1960 and 1990 the Earth experienced a decline of between 4 and 6 per cent in the amount of sunlight reaching the ground. It became known as 'global dimming'. Last week this was confirmed in a study showing that global dimming has indeed come to an end and that we have now entered a period of 'global brightening'. Over the past decade the days have brightened by about 4 per cent.
The most obvious explanation is that the Sun is going through a cycle of higher activity which is throwing more solar radiation our way. Others think that clouds could provide the answer, while some suggested that the answer lies in the amount of pollutant particles in the atmosphere caused by heavy industry.
"Air pollution has reduced as the atmosphere has got cleaner [, eliminating dimming]. However, in developing countries that are building an industry based on the burning of fossil fuels, sunlight dimming is significant," says Charles Long from the US Department of Energy's Pacific Northwest National Laboratory.