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Scientists have been able to say with virtual certainty for the first time that the climate change observed over the past four decades is man made and not the result of natural phenomena.
The research compounds the conclusion of the biggest scientific report on global warming to date, the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) last year, which asserted a strong likelihood that human action was changing the climate.

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The new study raises the likelihood of "unnatural" causes of global warming to near certainty.

Authors of the study, published on Thursday in the peer-review journal Nature, examined a greater range of data than any other study so far. "Changes in natural systems since at least 1970 are occurring in regions of observed temperature increases, and these temperature increases at continental scales cannot be explained by natural climate variations alone," they say.

The authors, including scientists from Australia, China and the US, warn that man-made climate change is having "a significant impact on physical and biological systems globally". They found that more than 90 per cent of the data examined showed evidence that natural systems were responding to warming.

Spring is coming earlier, permafrost is melting and coastal erosion is increasing under the influence of rising sea levels, while animals and birds are changing migration and reproductive patterns.

Barry Brook, director of climate change research at the University of Adelaide, said: "[We should] consider that there has been only 0.75ºC of temperature change so far, yet the expectation for this century is four to nine times that amount.

"So these changes are only a minor portent of what is likely to come, especially if we continue on our carbon-profligate pathway."

Scientists know they may face difficulties ahead in persuading the public and politicians of the importance of tackling global warming – research published recently in Nature suggested that global temperatures were unlikely to increase in the next decade, and could even decline.

Experts at Germany's Leibniz Institute of Marine Sciences and the UK Met Office's Hadley Centre say natural climate variations linked to the Pacific cooling system known as La Niña, as well as a cooling phase of a system of Atlantic currents, may push temperatures down despite the effects of greenhouse gases.

However, after those effects wear off in about a decade, temperatures are likely to rise much more strongly as the warming effect of carbon emissions regains the upper hand in altering the climate.

Scientists fear that the expected lull might dispel any sense of urgency in tackling global warming and provide ammunition for climate change sceptics.

Copyright The Financial Times Limited 2008