Scientists who argue that global warming may be caused by cosmic rays rather than man-made greenhouse gases have been dealt a setback.
A prominent alternative scenario to that of human-induced global warming postulates that changes in the intensity of cosmic rays, affected by variations in solar activity, in turn affect the cloud cover of the Earth.
Fewer clouds would allow heat from the sun to build up and account for the perceived rise of global temperatures in recent decades.
But a new study detailed in the Institute of Physics' journal, Environmental Research Letters, fails to find a link between incoming cosmic rays and global cloud cover.
The research was led by Terry Sloan of Lancaster University and Arnold Wolfendale of Durham University, both in northern England.
The sun's rays are the key drivers of Earth's climate, and the main source of our planet's energy.
As such, some skeptics, led by Danish physicist Henrik Svensmark and British journalist Nigel Calder, argue that changes in solar activity are indirectly responsible for global warming.
They contend that variations in the strength of the sun's solar wind in turn causes fluctuations in the intensity of cosmic rays, which have been shown in experiments to create aerosol particles similar to those needed for cloud formation.
An increase in cosmic rays would hypothetically then lead to an increase in cloud cover, relfecting more sunlight back into space and cooling the Earth; a decrease would similarly lead to lesser clouds and a corresponding rise in temperature.
A 2006 British television special, "The Great Global Warming Swindle," named the latter effect as the main cause of Earth's rising temperatures.
But the new study found no significant link between the intensity of cosmic rays hitting the Earth and low-level cloudiness.
Its findings "suggest that it is fairly unlikely that [cosmic rays] have any discernable effect on the cloudiness," said Rasmus Benestad of the Norwegian Meteorological Institute, who was not involved with the study.
"If the effect were important, then we would have seen it," Benestad told LiveScience. "[Cosmic rays] cannot account for the present warming trend."
Benestad also pointed out that no long-term trend in cosmic-ray intensity that corresponds with the decades-long rise in global temperatures has been detected.
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