The sun's role in climate change may have been overplayed, according to a study indicating that the Earth could actually get slightly cooler, rather than warmer, as the activity of the 11-year solar cycle increases.
Until now it was assumed that as solar activity – indicated by the number of sunspots on the Sun's surface – increases, then so does the amount of solar radiation coming to the Earth to heat the planet.
However, a study based on satellite data of the Earth's atmosphere has shown there is a complicated interaction between the varying amounts of radiation from the Sun and the amount of ozone in the atmosphere.
The investigation, which ran from 2004 to 2007 when the solar activity cycle was decreasing, found there was a rise in ozone that may have resulted in a corresponding increase in temperatures, which would have been indirectly due to the Sun rather than to increases in man-made greenhouse gases, the scientists said.
The researchers emphasised the findings do not undermine the idea that carbon dioxide in the atmosphere is the main cause of global warming – but they do suggest the Sun's direct role in warming the planet has been overplayed in computer models.
"These results are changing what we thought we knew about the Sun's effect on our climate. However, they only show us a snapshot of the Sun's activity and its behaviour over the three years of our study could be an anomaly," said Joanna Haigh, of the Grantham Institute for Climate Change, at Imperial College London.
"We cannot jump to conclusions based on what we have found during this comparatively short period," Dr Haugh said."However, if further studies find the same pattern over a longer period of time, this could suggest that we may have overestimated the Sun's role in warming the planet."
Although there is an 11-year solar cycle, the overall activity of the Sun has risen slightly over the past century, leading some climate "sceptics" to suggest the Sun rather then carbon dioxide is causing global warming.
However, the authors of the latest study, published in Nature, said solar activity could account for at most about 10 per cent of the extra warming this century. But if the new findings can be supported, it would mean this greater solar activity may have kept global warming in check by lowering temperatures slightly and counteracting the influence of greenhouse gases.