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.
They concluded that the rapid rise in global mean temperatures seen since the late 1980s could not be ascribed to solar variability, whatever mechanism was invoked.
Britain's Royal Society -- one of the world's oldest scientific academies, founded in 1660 -- said the new research was an important rebuff to climate change skeptics.
"At present there is a small minority which is seeking to deliberately confuse the public on the causes of climate change. They are often misrepresenting the science, when the reality is that the evidence is getting stronger every day," it said in a statement.
The 10 warmest years in the past 150 years have all been since 1990 and a United Nations climate panel, drawing on the work of 2,500 scientists, said this year it was "very likely" human activities were the main cause.
The panel gave a "best estimate" that temperatures would rise 1.8 to 4.0 degrees Celsius (3.2 to 7.8 Fahrenheit) this century.