Researchers have found that the waxing and waning of the sun affects our planet's temperature in exactly the opposite way scientists had thought. The work suggests, counterintuitively, that when the sun is at the dimmest point of its 11-year solar cycle, as it was in December 2009, it warms the Earth most, and vice versa.
"When I first saw the results I thought we had done the calculations wrong," said the physicist Prof Joanna Haigh, at Imperial College London, who led the research published today in Nature. While they only have three years of satellite data so far, Haigh said the discovery could have far-reaching consequences. "If further studies find the same pattern over a longer period of time, [then] we may have overestimated the sun's role in warming the planet," she said. The re-think comes from a better understanding of how the mixture of light emitted by the sun changes as its intensity shifts.
The revelation also helps explain some seemingly strange regional climate phenomena, such as how Europe can have very cold winters at a time when the world as a whole is warming.
Some climate change sceptics have suggested the changes in the sun's brightness can explain the global warming seen over the past century. But Haigh said: "It does not give comfort to climate sceptics at all." If the sun warmed the Earth less when it was at the solar maximum, then the reverse was also true, she said: "You can't have it one way and not the other."
In addition, she said, the warming influence of rising greenhouse gases in the atmosphere, emitted by human activities, was at least 10 times greater than changes in the strength of the sun.
Prof Mike Lockwood, a solar physicist at the University of Reading said: "We don't have any reason at the moment to change our overall view of the contributions of changing solar radiation to climate change, not on a global scale, but there is quite a lot of evidence coming forward that these changes do matter on a regional scale and particularly to us here in Europe."
That is because the sun's intensity plays a crucial role at mid-latitudes, where the UK sits, by controlling the jet stream winds, which in turn govern weather, he said. Changes to the jet stream are responsible for extremely cold European winters, such as the last one, and also the conditions which caused the volcanic ash cloud from the Eyjafjallajökull volcano to blow southwards and ground flights in April and May.
As well as the 11-year solar cycles, there are "grand" cycles lasting 200-300 years and, looking forwards, Lockwood said the end of the current grand solar maximum was overdue. "That does imply under these regional effects that we may go back to the sort of temperatures seen in what is called the little ice age, [a period of colder European temperatures between 1600 and 1800]. So we would have the ultimate paradox that in a globally warming world, we would have colder winters here in Europe - while it would be an awful lot warmer in Greenland."
The new work, Lockwood said, helps resolve why solar activity can affect regional climate in this way while not affecting the overall warming of the globe.
The research is based on the first ever measurements of solar radiation across the entire spectrum from X-rays to infrared light which showed that the mix of different wavelengths of light – for example infra-red, ultraviolet – was very different to what had been expected. The data, collected by the Sorce satellite between 2004 and 2007, revealed that the intensity of the ultraviolet light in the sun's rays fell by six times more than predicted over that period, while the amount of visible light exceeded expectations. Less intense ultraviolet light means less ozone is formed in the upper atmosphere, which in turn means the Earth warms, as does an increase in visible light.
Haigh said that future measurements would enable scientists to determine if the reversal of the link between solar intensity and warming on the Earth seen between 2004 and 2007 is normal solar behaviour, or an anomaly. "I think it is a case of watch this space," she said.
