It is an "increasingly remote possibility" that human activity is not the main cause of climate change, according to a major Met Office review of more than 100 scientific studies that track the observed changes in the Earth's climate system.
The research will strengthen the case for human-induced climate change against sceptics who argue that the observed changes in the Earth's climate can largely be explained by natural variability.
Asked whether his study was specifically scheduled as a fightback, Peter Stott, who led the review, said that the paper was originally drafted a year ago. But he added: "I hope people will look at that evidence and make up their minds informed by the scientific evidence."
Scientists matched computer models of different possible causes of climate change - both human and natural - 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, according to Stott. "This wealth of evidence shows that there is an increasingly remote possibility that climate change is being dominated by natural factors rather than human factors." The paper reviewed numerous studies that were published since the last IPCC report.
Optimal detection considers to what extent an observation can be explained by natural variability, such as changing output from the sun, volcanic eruptions or El Niño, and how much can be explained by the well-established increases in carbon dioxide and other greenhouse gases in the atmosphere.
According to Nasa, the last decade was the warmest on record and 2009 the second warmest year. Temperatures have risen by 0.2C per decade, over the past 30 years and average global temperatures have increased by 0.8C since 1880.
The evidence that the climate system is changing goes beyond measured air temperatures, with much of the newest evidence coming from the oceans. "Over 80% 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," said Stott. "Another feature is that salinity is changing - as the atmosphere is warming up, there is more evaporation from the surface of the ocean [so making it more salty], which is most noticeable in the sub-tropical Atlantic."
This also links into changes in the global water cycle and rainfall patterns. As the atmosphere warms, it has been getting more humid, exactly as climate modellers had predicted. "This clear fingerprint has been seen in two independent datasets. One developed in the Met Office Hadley Centre, corroborated with data from satellites."
Arctic sea ice is also retreating - the summer minimum of sea ice is declining at a rate of 600,000 km² per decade, an area approximately the size of Madagascar. Again, decreasing sea ice is predicted by climate models.
Rainfall is also on the rise in the higher latitudes of the northern hemisphere and large swaths 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," said Stott. "We now have studies that can identify this fingerprint in the observational data."
The review, published in Wiley Interdisciplinary Reviews: Climate Change, found that the natural causes of climate variation, including changing energy output from the sun and volcanic eruptions, could not explain the observed changes by themselves. "There hasn't been an increase in solar output for the last 50 years and solar output would not have caused cooling of the higher atmosphere and the warming of the lower atmosphere that we have seen," said Stott.
If the observed climate change was entirely due to solar activity, the Earth's atmosphere would have warmed more evenly - both the troposphere and stratosphere would have been affected. Warming due to the Sun would also have meant temperatures should have increases more quickly early than late in the 20th century, which is the reverse of what was actually measured.
The review is published as scientists also report a rise in methane emissions from a section of the Arctic Ocean sea floor. That study, published today in the journal Science, shows that the permafrost under the East Siberian Arctic shelf, once considered an safe store of methane, is leaking large amounts of the gas into the atmosphere. Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming as this is a greenhouse gase around 30 times more potent than CO2.
"The amount of methane currently coming out of the East Siberian Arctic shelf is comparable to the amount coming out of the entire world's oceans. Sub-sea permafrost is losing its ability to be an impermeable cap," said Natalia Shakhova, a researcher at the University of Alaska Fairbanks's International Arctic Research Centre. "The release to the atmosphere of only one percent of the methane assumed to be stored in shallow hydrate deposits might alter the current atmospheric burden of methane up to three to four times. The climatic consequences of this are hard to predict."