The planet could warm by seven degrees Celsius (10.
8 degrees Fahrenheit) this century, a figure that lies at the farthest range of expert predictions made only two years ago, scientists said on Tuesday.
The study is the biggest overview on global warming since the UN Intergovernmental Panel on Climate Change (IPCC) 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 December 7-18 UN 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 (PIK), 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 cut-off date for data in the 2007 report - suggest many of these earlier estimates are too low.
Among the findings:
- 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 percent 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 0.19 C (0.34 F) per decade, placing 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.
- If global warming is to be held below 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.
A study published in Nature Sunday said the West Antarctic ice sheet alone shed about 132 billion tonnes of ice per year from March 2002 to January 2009.
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.
- 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.Reuse content