Leading scientists, including a Nobel Prize-winner, have rounded on studies used by climate sceptics to show that global warming is a natural phenomenon connected with sunspots, rather than the result of the man-made emissions of carbon dioxide.
The researchers – all experts in climate or solar science – have told The Independent that the scientific evidence continually cited by sceptics to promote the idea of sunspots being the cause of global warming is deeply flawed.
Studies published in 1991 and 1998 claimed to establish a link between global temperatures and solar activity – sunspots – and continue to be cited by climate sceptics, including those who attended an "alternative" climate conference in Copenhagen last week.
However, problems with the data used to establish the correlation have been identified by other experts and the flaws are now widely accepted by the scientific community, even though the studies continue to be used to support the idea that global warming is "natural".
The issue has gained new importance in the light of opinion polls showing that nearly one in two people now believe global warming is a natural phenomenon unconnected with CO2 emissions. Public distrust of the accepted explanation of global warming has been exacerbated by emails leaked from the Climatic Research Unit at the University of East Anglia, which appeared to suggest that scientists were engaged in a conspiracy to suppress contrarian views.
Many sceptics who accept that global temperatures have risen in recent decades suggest it is part of the climate's natural variability and could be accounted for by normal variations in the activity of the Sun. Powerful support for this idea came in 1991 when Eigil Friis-Christensen, director of the Danish National Space Centre, published a study showing a remarkable correlation between global warming and the length of sunspot cycles.
A further study published in 1998 by Mr Friis-Christensen and his colleague Henrik Svensmark suggested a possible explanation for the warming trend with a link between solar activity, cosmic rays and the formation of clouds.
However, many scientists now believe both of these studies are seriously flawed, and that when errors introduced into the analysis are removed, the correlations disappear, with no link between sunspots and global warming. Peter Laut, a former adviser to the Danish Energy Agency who first identified the flaws, said there were practically no observations to support the idea that variations in sunspots played more than a minor role in global warming.
Mr Laut's analysis of the flaws is accepted by most scientists familiar with the research, including Paul Crutzen, an atmospheric chemist at the Max Planck Institute in Germany, who won a Nobel Prize for his work on understanding the hole in the ozone layer. "There is definitely a problem [with these studies]. Laut has really pinned it down but the [sunspot] argument keeps reappearing and its quite irritating," Professor Crutzen said.
Professor Stefan Rahsmstorf, of Potsdam University, agreed: "I've looked into this quite closely and I'm on Laut's side in terms of his analysis of the data."
Some scientists believe the flaws are so serious that the papers should be retracted or at least the authors should acknowledge that their work contains problems that question the correlations they have apparently established.
"Their controversial papers must be retracted or at least that there will be an official statement by them acknowledging their mistake," said Andre Berger, honorary president of the European Geosciences Union.
Messrs Svensmark and Friis-Christensen stand by their studies and continue to believe there is evidence to support their sunspot theory of global warming, despite the doubts first raised by Laut.
"It's not a critique of the science or the correlations, it's a critique of person," Mr Friis-Christensen said. "It's a character assassination. [Laut] is not interested in the science, he's interested in promoting the idea Henrik did something unethical."
What are sunspots and cosmic rays?
Differences in the rotation between the Sun's equator and poles amplify the solar magnetic field until it bursts through to the surface as "sunspots". The activity follows an 11-year cycle, which may vary by a few years. High sunspot activity is associated with a strong solar winds and slightly higher radiation intensity – about 0.1 per cent higher than a sunspot minimum.
Stellar explosions in deep space give off cosmic radiation that continually bombards Earth. The energy, however, is very weak (equivalent to starlight). Some scientists believe that cosmic rays may influence cloud formation by aerosol "seeding".
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