The Sun's role in climate change has been called into question again.
By Helen Briggs
BBC News Online
A US scientist has cast doubt on the controversial idea that the Sun has been the main contributor to climate change over the past 20 years.
Climate science wrestles with uncertainty
The Sun is known to brighten and fade from time to time, influencing temperatures on Earth.
Scientists are divided over how significant a factor this is compared with what people are doing to the planet.
A recent study suggests the Sun has brightened steadily over the past 20 years, accounting for half or more of the 0.3 degrees Celsius warming blamed on greenhouse gas emissions since 1980.
"The study is very controversial," Dr Judith Lean of the Naval Research Laboratory in Washington DC told the UK/Ireland National Astronomy Meeting in Dublin. "Other claims in recent years have also exaggerated the role of the Sun in climate change."
Dr Lean is critical of how the researchers have reached their conclusion. Much of the debate is over how measurements of the Sun's activity made by different spacecraft are interpreted.
"Because the datasets do not have the same absolute scale, they must be cross-calibrated to construct the long-term record needed for studying climate change," she said.
"Drifts in the instrument sensitivities must be properly clarified as well, to avoid mistaking spurious trends for real solar brightness changes.
"For this purpose, the recent study used observations previously reported to suffer from known instrumental effects but did not take these effects into account."
The verdict is unlikely to satisfy the rival camp. What is clear, however, is that more work needs to be done to iron out uncertainties.
"The only way to improve our understanding of Sun-climate links is long-term measurements of sunlight from space and continued monitoring of the Earth's climate," Dr Simon Tett, of the Met Office's Hadley Centre in Bracknell, Berkshire, told BBC News Online.
"Even so I think it unlikely that the Sun has played a major role in climate change over the last 20 to 30 years."
Professor Joanna Haigh says long-term measurements are crucial.
"Recent work at Imperial College London has shown that even small changes in solar ultraviolet can produce a significant impact on climate in preferred locations, including shifts in the jet streams and mid-latitude storm tracks," she said.