A series of independent studies around the world show a significant rise in the amount of sunshine penetrating the atmosphere to be absorbed by the earth’s surface and turned into heat.

The research will concern climate researchers who are already predicting a rapid rise in global temperatures due to man-made emissions of so-called greenhouse gases such as carbon dioxide.

“The enhanced warming we have seen since the 1990s along with phenomena such as the widespread melting of glaciers could well be due to this increased intensity of sunlight compounding the effect of greenhouse gases,” said Professor Martin Wild of the Institute of Atmospheric and Climate Science in Zurich, Switzerland.

Researchers will present their findings to the European Geophysical Union conference in Vienna next week.

They reverse a 30-year trend. Measurements of sunshine levels between 1960 and 1990 had shown a decrease in the amount of sunshine reaching the earth, a phenomenon known as global dimming.

This was thought to have been caused by dust, smog and other pollutants, mainly from industrialised western countries.The pollutants, known as aerosols, reduced sunshine levels by absorbing and scattering solar radiation and promoting the formation of clouds that reflected radiation back into space.

In the last two decades, however, there have been significant decreases in such pollutants, partly due to industry becoming cleaner but largely because of the collapse of the Soviet Union and much of its heavy industry.

Wild said: “Sunshine levels had been decreasing by 2% a decade between 1960 and 1980 — a total decline of about 6%. Now they are going up again. Perhaps this is why our Swiss glaciers are melting.”

A 6% increase in the amount of solar radiation reaching earth would have a powerful impact on climate, especially when added to the warming effect of greenhouse gases which have already raised global temperatures by about 0.6C. Researchers predict an additional rise of at least 1.5C by 2050.

Such rises could be disastrous for agriculture, wildlife and human settlements in many regions, especially the tropics. But scientists warn they may have to revise these calculations sharply upwards if the impact of “global brightening” has to be factored in.

Atsumu Ohmura, of the Swiss Federal Institute of Technology in Zurich, has collated measurements from 400 sites worldwide and found an increase in sunshine at 300 of them, sited mainly in Eurasia and the Polar regions.

Some showed a decline in sunshine since 1990, largely in fast-developing countries such as China and India.

“A widespread brightening has been observed since the 1980s. This may substantially affect surface climate, the water cycle, glaciers and ecosystems,” said Ohmura.

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