IT IS an ''increasingly remote possibility'' that human activity is not the main cause of climate change, concludes a review of more than 100 scientific studies that have tracked observed changes in the Earth's climate system.
The research will strengthen the case for human-induced climate change against the viewpoints of sceptics who argue the observed changes in the Earth's climate can largely be explained by natural variability.
Climate scientists and the UN's climate body, the Intergovernmental Panel on Climate Change, have come under intense pressure in recent months after the panel was forced to admit it had made two errors in its fourth assessment report, published in 2007. Asked whether his study was specifically scheduled as a fightback, Peter Stott, who led the review for Britain's Meteorological Office, said the paper was drafted a year ago.
But he added: "I hope people will look at that evidence and make up their minds." Scientists matched computer models of possible causes of climate change, both human-led and otherwise, to measured changes in factors such as air and sea temperature, Arctic sea ice cover and global rainfall patterns. This technique, called optimal detection, showed clear fingerprints of human-induced global warming, Dr Stott said. The review, published in Wiley Interdisciplinary Reviews: Climate Change, finds the natural causes of climate variation, including changing energy output from the sun and volcanic eruptions, cannot alone explain the observed changes.
''There hasn't been an increase in solar output for the last 50 years and solar output would not have caused the cooling of the higher atmosphere and the warming of the lower atmosphere we have seen,'' the review said.
Evidence that the climate system is changing goes beyond measured air temperatures, with much in the fresh body of facts relating to the oceans. ''Over 80 per cent of the heat that's trapped in the climate system as a result of the greenhouse gases is exported into the ocean and we can see that happening,'' Dr Stott said.
Arctic sea ice is also retreating: the summer minimum of sea ice is declining at a rate of 600,000 square kilometres a decade. Rainfall is increasing in the higher latitudes of the northern hemisphere and in large parts of the southern hemisphere, while in the tropics and sub-tropics there are decreases. ''The already wet regions are getting wetter and the dry regions are getting drier,'' Dr Stott said.
If the observed climate change were entirely due to solar activity, the Earth's atmosphere would have warmed more evenly, and the troposphere and stratosphere would have been affected.
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