The earth is warmer now than it has been at any time in the past 2,000 years, the most comprehensive study of climatic history has revealed.
Confirming the worst fears of environmental scientists, the newly published findings are a blow to sceptics who maintain that global warming is part of the natural climatic cycle rather than a consequence of human industrial activity.
Prof Philip Jones, a director of the University of East Anglia's climatic research unit and one of the authors of the research, said: "You can't explain this rapid warming of the late 20th century in any other way. It's a response to a build-up of greenhouse gases in the atmosphere."
The study reinforces recent conclusions published by the UN's intergovernmental panel on climate change (IPCC). Scientists on the panel looked at temperature data from up to 1,000 years ago and found that the late 20th century was the warmest period on record.
But the IPCC's report was dismissed by some quarters in the scientific community who claimed that while the planet is undoubtedly warming, it was warmer still more than a thousand years ago. So warm, in fact, that it had spurred the Vikings to set up base in Greenland and led to northern Britain being filled with productive vineyards.
To discover whether there was any truth in the claims, Prof Jones teamed up with Prof Michael Mann, a climate expert at the University of Virginia, and set about reconstructing the world's climate over the past 2,000 years.
Direct measurements of the earth's temperature do not exist from such a long time ago, so the scientists had to rely on other indicators of how warm - or not - the planet was throughout the past two millennia.
To find the answer, the scientists looked at tree trunks, which keep a record of the local climate: the rings spreading out from the centre grow to different thicknesses according to the climate a tree grows in. The scientists looked at sections taken from trees that had lived for hundreds and even thousands of years from different regions and used them to piece together a picture of the planet's climatic history.
The scientists also studied cores of ice drilled from the icy stretches of Greenland and Antarctica. As the ice forms, sometimes over hundreds of thousands of years, it traps air, which holds vital clues to the local climate at the time.
"Drill down far enough and you could use the ice to look at the climate hundreds of thousands of years ago, but we just used the first thousand metres," said Prof Jones.
The scientists found that while there was not enough good data to work out what the climate had been like in the southern hemisphere over that period, they could get a good idea of how warm the northern hemisphere had been.
"What we found was that at no point during those two millennia had it been any warmer than it is now. From 1980 onwards is clearly the warmest period of the last 2,000 years," said Prof Jones.