In the widest overview on global warming since a landmark report by the UN Intergovernmental Panel on Climate Change (IPCC) report in 2007, the authors said manoeuvering room for tackling the carbon crisis was now almost exhausted.

The 64-page "Copenhagen Diagnosis" aims at the December 7-18 UN conference in Denmark, tasked with forging a planet-wide deal on 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 (PIK), which oversaw the paper.

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

The scientists billed the summary as "an interim scientific evaluation" between the IPCC's Fourth Assessment Report, known as AR4, and its next big handbook, due in 2013. Several authors were part of that Nobel-winning group.

New evidence published in peer-reviewed scientific literature suggests many of the estimates published in 2007 are too low, the "Copenhagen Diagnosis" suggested.

Among the findings:

-- Emissions of heat-trapping carbon dioxide from fossil fuels "are tracking near the highest scenarios considered so far" by the IPCC.

They were nearly 40 percent higher in 2008 than in 1990, the benchmark year for the Kyoto Protocol, whose pledges expire in 2012.

Over the past 25 years temperatures have rise by 0.19 C (0.34 F) per decade, and current emissions place Earth on track for global mean warming as high as 7.0 C (10.8 F).

In 2007, the IPCC predicted warming of between 1.1 C (1.98 F) and 6.4 C (11.52 F) compared to 1980-99 levels, with the likeliest rise being 1.8-4.0 C (3.24-7.2 F). Added to this is warming of around 0.74 (1.33 F) during the 20th century.

-- To limit global warming to 2.0 C (3.6 F) compared to pre-industrial times, emissions must peak before 2020 and reach a "zero"-level "well within this century," the researchers conclude.

"Our available emissions to ensure a reasonably secure climate future are just about used up," said Matthew England, co-director of the Climate Change Research Centre of the University of New South Wales in Australia.

In July, leaders from the world's major developed and developing economies agreed on the need to prevent average global temperatures from climbing more than 2.0 C (3.6 F) over pre-industrial times.

-- Summer-time melting of Arctic sea ice has outstripped IPCC climate models by about 40 percent 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 metres (40 feet) -- are losing mass at an increasing rate.

The IPCC estimated sea levels would rise 18-59 centimetres (7.2-23.2 inches) by 2100, but this was from thermal expansion (water expands when it warms) and did not factor in runoff from melting land ice.

"By 2100, global sea-level is likely to rise at least twice as much as projected (in) AR4; for unmitigated emissions it may well exceed one metre (3.25 feet)," said the new report.

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

Meanwhile, a study published by the World Wildlife Fund (WWF) and the German insurer Allianz calculated the risk to port cities from a global sea level rise of 50 cms (20 inches) by 2050.

Worldwide, property worth up to 28.21 trillion dollars in 136 coastal megacities would be exposed.

On the US Eastern Seaboard alone, a 50cm (20-inch) rise, when coupled with a local storm surge of 15 cms (six inches), would imperil assets potentially worth 7.4 trillion dollars.

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