Scientists found that a decline in the Sun's activity did not lead as expected to a cooling of the Earth, a surprise finding that could have repercussions for computer models on climate change.
The Sun's activity is known to wax and wane over 11-year cycles, which means that in theory the amount of radiation reaching Earth declines during the "waning" phase.
The new study, which appears in the journal Nature, was carried out between 2004 and 2007 during a solar waning phase.
The amount of energy in the ultraviolet part of the energy spectrum fell, the researchers found.
But, contrary to expectation, radiation in the visible part of the energy spectrum increased, rather than declined, which caused a warming effect.
The investigation, based mainly on satellite data, is important because of a debate over how far global warming is attributable to humans or to natural causes.
Climatologists say that warming is overwhelmingly due to human-produced greenhouse gases - invisible carbon emissions from coal, gas and coal that linger in the atmosphere and trap solar heat.
But a vocal lobby of sceptics say that this is flawed or alarmist, and point out that Earth has known periods of cooling and warming that are due to variations in the Sun's output.
"These results are challenging what we thought we knew about the Sun's effect on our climate," says lead author Professor Joanna Haigh of Imperial College London where she is also a member of the Grantham Institute for Climate change.
"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."
Insisting on caution, Haigh says that if the Sun turned out to have a warming effect during the "waning" part of the cycle, it might also turn out to have a cooling effect during the "waxing" part of the cycle.
In that case, greenhouse gases would be more to blame than thought for the perceptible rise in global temperatures over the past century.
"We cannot jump to any conclusions based on what we have found during this comparatively short period," Haigh says. "We need to carry out further studies to explore the Sun's activity, and the patterns that we have uncovered, on longer timescales."