PARIS (AFP) - Scientists on Wednesday said that the rise in global temperatures that has been detected over the past two decades cannot be blamed on the Sun, a theory espoused by climate-change sceptics.
British and Swiss researchers looked at data for radiation from the Sun, levels of which can cool or warm our planet's atmosphere.
They factored in a cycle which solar radiation goes through peaks and troughs of activity over a period of about 11 years.
Writing in Proceedings of the Royal Society A, a journal of Britain's de-facto academy of sciences, the team said that the Sun had been less active since 1985, even though global temperatures have continued to rise.
"Over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures," they write.
The study is co-authored by Mike Lockwood of Britain's Rutherford Appleton Laboratory and Claus Froehlich of the World Radiation Centre in Switzerland.
The overwhelming consensus among scientists is that human activity is to blame for the rise in global temperatures. In its latest report, issued this year, the UN's Intergovernmental Panel on Climate Change (IPCC) said that this warming is already affecting the climate system.
Since 1900, the mean global atmospheric temperature has risen by 0.8 C (1.44 F), and the sea level by 10-20 centimetres (four to eight inches).
Levels of carbon dioxide (CO2), the main greenhouse gas, have risen by around a third since the Industrial Revolution and are now at their highest in 650,000 years. Eleven of the past 12 years rank among the dozen warmest years on record.
In the past few years, glaciers and snow and ice cover have fallen back sharply in alpine regions, the edges of the Greenland icesheet and on the Antarctic peninsula have shrunk, Arctic summer sea ice has thinned and retreated and Siberian and Canadian permafrost have shown signs of thaw and fallback.