Solar variations not behind global warming: study

By Ben Hirschler

LONDON (Reuters) - The sun's changing energy levels are not to blame for recent global warming and, if anything, solar variations over the past 20 years should have had a cooling effect, scientists said on Wednesday.

Their findings add to a growing body of evidence that human activity, not natural causes, lies behind rising average world temperatures, which are expected to reach their second highest level this year since records began in the 1860s.

There is little doubt that solar variability has influenced the Earth's climate in the past and may well have been a factor in the first half of the last century, but British and Swiss researchers said it could not explain recent warming.

"Over the past 20 years, all the trends in the sun that could have had an influence on Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures," they wrote in the Proceedings of the Royal Society.

Most scientists say emissions of greenhouse gases, mainly from burning fossil fuels in power plants, factories and cars, are the prime cause of the current warming trend.

A dwindling group pins the blame on natural variations in the climate system, or a gradual rise in the sun's energy output.

In order to unpick that possible link, Mike Lockwood of Britain's Rutherford Appleton Laboratory and Claus Froehlich of the World Radiation Centre in Davos, Switzerland, studied factors that could have forced climate change in recent decades, including variations in total solar irradiance and cosmic rays.

The data was smoothed to take account of the 11-year sunspot cycle, which affects the amount of heat the sun emits but does not impact the Earth's surface air temperature, due to the way the oceans absorb and retain heat.