Obviously some things about the Sun's relationship to Earth are known - if we get too close or too far away all life disappears.  But other aspects, like the activity of the Sun related to heating and cooling, are less clear.

It has long been known that the Sun's activity waxes and wanes over an 11-year cycle and that as its activity wanes, the overall amount of radiation reaching the Earth decreases. A new study looked at the Sun's activity over the years 2004-2007, when it was in a declining part of its 11-year activity cycle.

Although the Sun's activity declined over this period, a new speculates that it may have actually caused the Earth to become warmer. Contrary to expectations, the amount of energy reaching the Earth at visible wavelengths increased rather than decreased as the Sun's activity declined, causing this warming effect.

But that would also mean it is possible that the inverse is true and that in periods when the Sun's activity increases it tends to cool rather than warm, the Earth, like during the last century when solar activity has been increasing.    This leads the researchers to speculate that during this period, the Sun has been contributing a small cooling effect, rather than a small warming effect as had previously been thought.

The researchers used satellite data and computer modeling to analyze how the spectrum of radiation and the amount of energy from the Sun has been changing since 2004. Instruments on the SORCE satellite have been measuring the Sun's energy output at many different wavelengths. The researchers fed the data from SORCE into an existing computer model of the Earth's atmosphere and compared their results with the results obtained using earlier, less comprehensive, data on the solar spectrum.

Professor Joanna Haigh, the lead author of the study who is Head of the Department of Physics and member of the Grantham Institute for Climate Change at Imperial College London, said, "These results are challenging 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. 

"We cannot jump to any conclusions based on what we have found during this comparatively short period and we need to carry out further studies to explore the Sun's activity, and the patterns that we have uncovered, on longer timescales. 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, rather than underestimating it."

Professor Sir Brian Hoskins, the Director of the Grantham Institute for Climate Change at Imperial College London, added: "We know that the Earth's climate is affected both by human activity and by natural forces and today's study improves our understanding of how the Sun influences our climate. Studies like this are vital for helping us to create a clear picture of how our climate is changing and through this, to work out how we can best protect our planet."

Citation: Joanna D. Haigh, Ann R. Winning, Ralf Toumi, Jerald W. Harder, 'An influence of solar spectral variations on radiative forcing of climate', Nature 467, 696-699 (6 October 2010) doi:10.1038/nature09426