The planet could warm by seven degrees Celsius this century, a figure that lies at the farthest range of expert predictions made only two years ago, scientists said Tuesday.
The planet could warm by seven degrees Celsius this century, a figure that lies at the farthest range of expert predictions made only two years ago, scientists said Tuesday.

The study is the biggest overview on global warming since the United Nations Intergovernmental Panel on Climate Change released a landmark report in 2007. Several authors of the new paper were part of that Nobel-winning group.

Entitled the "Copenhagen Diagnosis," the 64-page summary is pitched at the Dec. 7-18 U.N. conference in Denmark tasked with forging a planet-wide deal to curb greenhouse-gas emissions.

"This is a final scientific call for climate negotiators from 192 countries who must embark on the climate protection train in Copenhagen ," said Hans Schellnhuber, director of Germany 's Potsdam Institute for Climate Impact Research, which oversaw the paper.

"They need to know the stark truth about global warming and the unprecedented risks involved," he said in a statement.

The authors say the document "serves as an interim scientific evaluation" of climate change, between the IPCC's Fourth Assessment Report published in 2007 and its next big handbook, due in 2013.

New evidence published in peer-reviewed scientific literature since 2005--the cutoff date for data in the 2007 report--suggest many of these earlier estimates are too low.

Among the findings is that emissions of heat-trapping carbon dioxide from fossil fuels, the main engine of global warming, "are tracking near the highest scenarios considered so far" by the IPCC. They were nearly 40% higher in 2008 than in 1990, the benchmark year for the Kyoto Protocol, whose provisions expire in 2012.

Over the past 25 years temperatures have gone up at a rate of 0.19C per decade, placing Earth on track for global mean warming as high as 7C.

In 2007, the IPCC predicted warming of between 1.1C and 6.4C compared to 1980-99 levels, with the likeliest rise being 1.8-4.0C. Added to this is warming of around 0.74C during the 20th century.

If global warming is to be held below 2C compared with pre-industrial times, emissions must peak before 2020 and reach a "zero"-level "well within this century," the researchers conclude.

The report says that summertime melting of Arctic sea ice has outstripped IPCC climate models by about 40% for the period 2007 to 2009.

A wide array of satellite and ice measurements leave no doubt that both Greenland and West Antarctic icesheets--with enough frozen water between them to raise sea levels by 12 meters--are losing mass at an increasing rate.

The IPCC says major ecosystems are nearing individual "tipping points," the threshold beyond which they may spiral into irretrievable decline and will no longer mitigate global warming but, instead, amplify it.

Some of these ecosystems--Amazon forest, the West African monsoon, coral reefs--directly support populations in the hundreds of millions, and could create huge numbers of climate refugees were they to collapse.

Once this happens, tropical forests, ice sheets and the vanishing Arctic ice cap will become not just victims of global warming but additional drivers as well.

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