Solar activity 'not the cause of global warming'
By Steve Connor, Science Editor
Published: 11 July 2007
Claims that increased solar activity is the cause of global warming - rather than man-made greenhouse gases - have been comprehensively disproved by a detailed study of the Sun.
Scientists have delivered the final blow to the theory that recent global warming can be explained by variations in the natural cycles of the Sun - a favourite refuge for climate sceptics who dismiss the influence of greenhouse-gas emissions.
An analysis of the records of all of the Sun's activities over the past few decades - such as sunspot cycles and magnetic fields - shows that since 1985 solar activity has decreased significantly, while global warming has continued to increase.
Mike Lockwood, of the Rutherford Appleton Laboratory in Chilton, Oxfordshire, said: "In 1985, the Sun did a U-turn in every respect. It no longer went in the right direction to contribute to global warming. We think it's almost completely conclusive proof that the Sun does not account for the recent increases in global warming."
The study, published today in the journal Proceedings of the Royal Society A, shows there is no doubt that solar activity over the past 20 years has run in the opposite direction to global warming, and therefore cannot explain rises in average global temperatures.
Dr Lockwood and his colleague Claus Fröhlich, of the World Radiation Centre in Davos Dorf, Switzerland, have produced the most powerful counter argument to suggestions that current warming is part of the natural cycle of solar activities. "There is considerable evidence for solar influence on Earth's pre-industrial climate, and the Sun may well have been a factor in post-industrial change in the first half of the last century," they write.
However, since about 1940 there has been no evidence to suggest that increases in global average temperatures were caused by solar activity. "Our results show that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified," the two scientists said.
The theory that past changes in solar activity may have explained some changes in the climate before the industrial revolution is not in dispute. In previous centuries, for instance, notably between about 1420 and 1570, when the Vikings had to abandon their Greenland settlements, solar minima corresponded with unusually cool weather, such as the "little ice age" of the 17th century.
But climate sceptics have exploited this to dispute the idea that man-made emissions are responsible for global warming. In the recent Channel 4 programme The Great Global Warming Swindle, the rise in solar activity over the latter half of the 20th century was erroneously presented as perfectly matching the rise in global average temperatures.
Dr Lockwood said he was outraged when he saw the documentary, because of the way the programme-makers used graphs of temperature rises and sunspot cycles that were cut off in the 1980s, when the two trends went in the opposite direction.
"The trouble is that the theory of solar activity and climate was being misappropriated to apply to modern-day warming. The sceptics were taking perfectly good science and bringing it into disrespect," Dr Lockwood said.
The Royal Society said yesterday: "There is a small minority which is seeking to confuse the public on the causes of climate change. They are often misrepresenting the science, when the reality is that the evidence is getting stronger every day."
| By Richard Black BBC Environment Correspondent |
 Scientists have been measuring the frequency of solar flares |
It shows that for the last 20 years, the Sun's output has declined, yet temperatures on Earth have risen.
It also shows that modern temperatures are not determined by the Sun's effect on cosmic rays, as has been claimed.
Writing in the Royal Society's journal Proceedings A, the researchers say cosmic rays may have affected climate in the past, but not the present.
"This should settle the debate," said Mike Lockwood from the UK's Rutherford-Appleton Laboratory, who carried out the new analysis together with Claus Froehlich from the World Radiation Center in Switzerland.
Dr Lockwood initiated the study partially in response to the TV documentary The Great Global Warming Swindle, broadcast on Britain's Channel Four earlier this year, which featured the cosmic ray hypothesis.
"All the graphs they showed stopped in about 1980, and I knew why, because things diverged after that," he told the BBC News website.
"You can't just ignore bits of data that you don't like," he said.
Warming trend
The scientists' main approach on this new analysis was simple; to look at solar output and cosmic ray intensity over the last 30-40 years, and compare those trends with the graph for global average surface temperature, which has risen by about 0.4C over the period.
The Sun varies on a cycle of about 11 years between periods of high and low activity.
But that cycle comes on top of longer-term trends; and most of the 20th Century saw a slight but steady increase in solar output.
But in about 1985, that trend appears to have reversed, with solar output declining.
Yet this period has seen temperatures rise as fast as, if not faster than, at any time during the previous 100 years.
"This paper re-enforces the fact that the warming in the last 20 to 40 years can't have been caused by solar activity," said Dr Piers Forster from Leeds University, a leading contributor to this year's Intergovernmental Panel on Climate Change (IPCC) assessment of climate science.
Cosmic relief
The IPCC's February summary report concluded that greenhouse gases were about 13 times more responsible than solar changes for rising global temperatures.
But the organisation was criticised in some quarters for not taking into account the cosmic ray hypothesis, developed among others by Henrik Svensmark and Eigil Friis-Christensen of the Danish National Space Center.
Their theory holds that cosmic rays help clouds to form by providing tiny particles around which water vapour can condense. Overall, clouds cool the Earth.
During periods of active solar activity, cosmic rays are partially blocked by the Sun's more intense magnetic field. Cloud formation diminishes, and the Earth warms.
Mike Lockwood's analysis appears to have put a large, probably fatal nail in this intriguing and elegant hypothesis.
He said: "I do think there is a cosmic ray effect on cloud cover. It works in clean maritime air where there isn't much else for water vapour to condense around.
"It might even have had a significant effect on pre-industrial climate. But you cannot apply it to what we're seeing now, because we're in a completely different ball game."
Drs Svensmark and Friis-Christensen could not be reached for comment.
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