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Much of recent global warming actually caused by Sun

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New data indicates that changes in the Sun's output of energy were a major factor in the global temperature increases seen in recent years. The research will be unwelcome among hardcore green activists, as it downplays the influence of human-driven carbon emissions.

As the Sun has shown decreased levels of activity during the past decade, it had been generally thought that it was warming the Earth less, not more. Thus, scientists considered that temperature rises seen in global databases must mean that human-caused greenhouse gas emissions - in particular of CO2 - must be exerting a powerful warming effect.

Now, however, boffins working at Imperial College in London (and one in Boulder, Colorado) have analysed detailed sunlight readings taken from 2004 to 2007 by NASA's Solar Radiation and Climate Experiment (SORCE) satellite. They found that although the Sun was putting out less energy overall than usual, in line with observations showing decreased sunspot activity, it actually emitted more in the key visible-light and near-infrared wavelengths.

These shorter wavelength forms of radiated heat penetrate the atmosphere particularly well to heat up the Earth's surface - just as the same frequencies get in through car windows to heat up its interior. The hot seats and dashboard - in this case the seas, landmasses etc - then radiate their own increased warmth via conduction, convection and longer-wave infrared, which can't escape the way the shortwave energy came in. This is why the car, and the planet, become so hot.

Thus the Sun, though it was unusually calm in the back half of the last decade, was actually warming the planet much more strongly than before.

According to a statement released by Imperial College:

Although the Sun's activity declined over this period, the new research shows 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.

"These results are challenging what we thought we knew about the Sun's effect on our climate," says Professor Joanna Haigh of Imperial, lead author of the study.

"It does require verification, but our findings could be too important to not publish them now," she told hefty boffinry mag Nature, which published the new research. The prof considers that increased sun-powered warming probably had as much effect on global temperature as carbon during the period of her study.

Haigh thinks, however, that while recent temperature rises may well have been down to the Sun as much as anything humanity has done, over long periods of time solar warming probably has little effect on the Earth's temperature one way or the other, as solar activity cycles up and down regularly.

"If the climate were affected in the long term, the Sun should have produced a notable cooling in the first half of the twentieth century, which we know it didn't," she says.

Nonetheless, the research indicates that the Sun's influence on the climate is poorly understood, and that current climate models will probably have to be amended in some way. Other scientists have lately said that solar influences are stronger than established climate theory had originally estimated.

It has also been more and more widely admitted among climate scientists in recent years that among human-caused emissions, other factors - in particular black carbon (soot) and sulphate aerosols - may exercise an influence as powerful as that of greenhouse gases.

For now the long-term implications of the SORCE data are unknown. All that can be said with any certainty is that through 2004-2007, the Sun warmed the planet much more powerfully than had been thought.

"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," says Haigh.

Subscribers to Nature can read Haigh and her colleagues' paper, An influence of solar spectral variations on radiative forcing of climate, here. ®

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