LONDON The sun may warm the Earth more during waning solar cycles, new satellite data has shown, turning scientific understanding on its head and helping to explain extreme local weather patterns, scientists said on Wednesday.
But scientists said the findings did not undermine the case for man-made global warming through greenhouse gas emissions.
The heat and light of the sun waxes and wanes over a roughly 11-year period. Scientists had thought it warmed the Earth more during peaks of activity, for example as measured by the number of spots visible in the sun's atmosphere.
But new satellite data showed that, in fact, from 2004-2007 as the cycle waned, more light reached the Earth's surface. It was only high frequency, ultraviolet light -- which hardly reaches the Earth's surface -- that faded.
That means the sun warmed the Earth more during that declining cycle, a new and counter-intuitive finding.
"It's quite intriguing, it's suggesting the solar influence is completely opposite to expectations," said Imperial College London's Joanna Haigh, lead author of the paper titled "An influence of solar spectral variations on radiative forcing of climate," to be published in the journal Nature Thursday.
The authors underlined that the findings were only for a three-year period, more proof was needed and could be provided if light at the Earth's surface fell when the sun becomes more active again, as expected in the next year or so.
Skeptics of the theory of man-made climate change have long argued that variation in solar activity could explain a warming Earth, rather than rising greenhouse gas emissions. The average amount of heat and light the sun pumps out has increased slightly over the past 150 years.
But climate scientists say that increase is only about one tenth of the warming effect of rising greenhouse gas emissions, which have helped cause global average temperatures to rise by more than 0.7 degrees Celsius since pre-industrial times.
"It doesn't give comfort to the climate skeptics at all," said Haigh. "It may suggest that we don't know that much about the sun. The climate models would still be producing much the same results with or without these solar effects."
However the study may help explain regional, extreme weather, said Reading University's Mike Lockwood, because it had revealed that the amount of ultraviolet light fell far more than expected during the declining solar cycle.
That large dip would cause falling temperatures in the upper atmosphere, which could explain a twisting of the jet stream -- a powerful wind in the high atmosphere which can drag cold air from the polar regions.
The sun in January this year reached its deepest solar minimum since about 1900. That could help explain a change in the jet stream and cold winters in Europe and North America against a backdrop of hotter average temperatures globally, said Lockwood.
The first half of this year has been the hottest since records began in 1850, say U.S. scientists [ID:nN16141783].
Lockwood added that the sun had been rather more active over the last 70 years in a longer-term cycle which he expected to turn over the course of this century, potentially bringing colder winters to Europe, because of similar jet stream effects.
"We would have the paradox of a globally warming world where we might have more cold winters here in Europe," Lockwood said.
(Reporting by Gerard Wynn, Editing by Janet Lawrence)