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Tuesday, 3 October, 2000, 16:49 GMT 17:49 UK
Climate feels the Sun's effects
Lake AP
What part has the Sun played in recent climate changes?
By environment correspondent Alex Kirby

Recent reports that global warming is caused "mainly by the Sun" have been dismissed by leading scientists.

The reports claimed that research by the European Space Agency (Esa) and others showed that computer models had severely underestimated the Sun's impact on the climate.

But a conference sponsored by the Esa and the European Union has heard that the evidence is far more complex.

And some participants say solar influences have diminished, while the human effects are intensifying.

The conference, entitled The Solar Cycle and Terrestrial Climate, took place in Tenerife, and was held to review the mechanisms of the Sun's influence on climate and its importance compared with human influences.

Possible mechanisms of solar influence on the Earth's climate include variations in ultraviolet (UV), visible and infrared (IR) radiation, and also in cosmic rays.

Professor Mike Lockwood, of the United Kingdom's Rutherford Appleton Laboratory, told BBC News Online he believed that climate change was solar-induced to begin with, but that it was now caused increasingly by anthropogenic factors.

Increasing human influence

"I have doubts about how low some people want to keep the solar contribution," he said. "Over the whole of the last century, I'd say it was perhaps about 40-50% of the total.

Clouds AP
Clouds are one of the biggest climate puzzles
"But the important point is that most of that was in the first 50 years. From 1970 to now the main influence has been human activity, and that's rather scary.

"The anthropogenic effects are now kicking in for real. And after all, the amount of carbon dioxide and methane in the atmosphere should have had some effect, though there are still a lot of unknowns."

Paal Brekke is the deputy project scientist for the Esa's Solar and Heliospheric Observatory satellite, Soho.

He told BBC News Online: "The Sun may explain up to 20% of global warming over the last 30 years, if you look only at irradiance.

"But if you include other, indirect effects, including cosmic rays and their influence on cloud cover, that percentage could rise.

"The pattern of systematic change in the global climate over recorded history seems to follow the observed changes in cosmic ray flux.

"It is consistent with the explanation that a low flux corresponds to fewer clouds and a warmer climate, and vice versa."

Dr Joanna Haigh, of Imperial College, London, said Soho's measurements have shown that changes in solar UV radiation are larger than once thought.

Ozone's equivocal role

She believes that ozone responds to changes in solar UV, but said the picture was complex.

Dr Haigh told BBC News Online: "How much the ozone responds, and where it changes, is crucial.

"In the upper stratosphere, about 50 km up, an increase in ozone will have a cooling effect.

"But about 20 km above the Earth, more ozone will act like other greenhouse gases, trapping IR radiation and enhancing warming.

"I think it's very unlikely anyway that the response of ozone to solar UV will be as dramatic as some reports have claimed."

Camel AP
Debate continues over the Sun's role
Dr Mike Hulme, executive director of the Tyndall Centre for Climate Change Research at the University of East Anglia, UK, said the so far unquantifiable contribution of the Sun is consistent with climatologists' understanding of what is happening.

He told BBC News Online: "The case argued by the Intergovernmental Panel on Climate Change (IPCC) is a carefully-worded judgement.

"Most scientists say it is fairly guarded, and is supportable.

"It allows both a substantial role for the Sun, and an inconsequential one.

"All the evidence suggests that it's greenhouse rather than solar forcing that's the problem, but the IPCC leaves the door open.

"It is this range of uncertainties that makes future predictions so difficult."

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See also:

08 Sep 00 | Sci/Tech
Ice records reveal warming trend
06 Sep 00 | Sci/Tech
Global warming 'a reality'
17 Aug 00 | Sci/Tech
Carbon at 20 million year high
13 Jun 00 | Sci/Tech
US warned on warming world
21 Jul 00 | Sci/Tech
Greenland's coastal ice thins fast
28 Jun 00 | Sci/Tech
Red Cross warns on climate
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