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Tuesday, 6 August, 2002, 14:35 GMT 15:35 UK
Cosmic rays 'explain climate conundrum'
Cloudy sky   PhotoDisk/Eyewire
Clouds are key to climate: Cosmic rays "may alter cloud cover"

A puzzling feature of climate change may have an extra-terrestrial explanation, according to a researcher in the US.

Many scientists agree that the Earth's surface appears to be warming, while low atmosphere temperatures remain unchanged.

Fangqun Yu, of the State University of New York-Albany, suggests the answer may lie in cosmic rays.

He argues the rays may cause changes in cloud cover which could explain the temperature conundrum.

Writing in the Journal of Geophysical Research - Space Physics, published by the American Geophysical Union, Yu suggests the rays may have height-dependent effects on the Earth's cloudiness.

The discrepancy in temperatures has led some scientists to argue that the case for human-induced climate change is weak, because our forcing should presumably show a uniform temperature rise from the surface up through the atmosphere.

Altitude the key

Although researchers have proposed that changes in cloud cover could help to explain the discrepancy, none has been able to account for the varying heat profiles.

But Yu's study suggests that cosmic rays, tiny charged particles which bombard all planets with varying frequency depending on solar wind intensity, could be the missing link.

Earlier research suggested a link between cosmic rays and cloud cover, but without positing the altitude dependence Yu believes exists.

Soldiers and weather balloon   BBC
Atmospheric measurements found the temperature gap
He said: "A systematic change in global cloud cover will change the atmospheric heating profile.

"In other words, the cosmic ray-induced global cloud changes could be the long-sought mechanism connecting solar and climate variability."

The amount of cosmic rays reaching the Earth depends on the strength of the solar winds, and Yu says indications of terrestrial warming coincided with decreased cosmic ray intensity during the 20th Century.

This underlines the importance of natural as well as human influences in climate change.

Recent satellite data have found a correlation between cosmic ray intensity and the fraction of the Earth covered by low clouds.

Chemical influence

Yu believes the amount and charge of cosmic ray-generated ions can contribute to the formation of dense clouds by stimulating the production rate of low atmosphere particles which make the clouds more opaque.

Sun and silhouetted figures   PA
Human and solar influences help shape the climate
He also believes that differences in atmospheric chemistry, both natural and of human origin, can affect the rays' influence on clouds.

These height-dependent atmospheric differences, he argues, can increase the quantity of ambient particles in the lower troposphere and decrease the number in the upper air, affecting the type of cloud cover.

High clouds normally reflect sunlight back into space, while lower clouds tend to retain energy at the surface.

Theory 'unnecessary'

Dr David Viner, of the Climatic Research Unit, University of East Anglia, UK, is not convinced by Yu's work.

Dr Viner told BBC News Online: "He puts forward a useful hypothesis, but he doesn't define any mechanism by which cosmic rays could affect any component of the atmosphere.

"The warming we've seen over the last 50 or 100 years is not all down to human activities. Solar activity does play a part.

"But we can explain the temperature discrepancy between the surface and the low atmosphere without recourse to this proposal."

See also:

31 Jul 02 | Science/Nature
23 Apr 02 | Science/Nature
25 May 01 | Asia-Pacific
28 Nov 00 | Science/Nature
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