UXBRIDGE, Canada - The prospect of a four-degree Celsius rise in global average temperatures in 50 years is alarming - but not alarmist, climate scientists now believe.
Eighteen months ago, no one dared imagine humanity pushing the climate beyond an additional two degrees C of heating, but rising carbon emissions and inability to agree on cuts has meant science must now consider the previously unthinkable.
"Two degrees C is already gone as a target," said Chris West of the University of Oxford's UK Climate Impacts Programme.
"Four degrees C is definitely possible...This is the biggest challenge in our history," West told participants at the "4 Degrees and Beyond, International Climate Science Conference" at the University of Oxford last week.
A four-degree C overall increase means a world where temperatures will be two degrees warmer in some places, 12 degrees and more in others, making them uninhabitable.
It is a world with a one- to two-metre sea level rise by 2100, leaving hundreds of millions homeless. This will head to 12 metres in the coming centuries as the Greenland and Western Antarctic ice sheets melt, according to papers presented at the conference in Oxford.
Four degrees of warming would be hotter than any time in the last 30 million years, and it could happen as soon as 2060 to 2070.
"Political reality must be grounded in physical reality or it's completely useless," John Schellnhuber, director of the Potsdam Institute for Climate Impact Research, told the conference.
Schellnhuber recently briefed U.S. officials from the Barack Obama administration, but he says they chided him that his findings were "not grounded in political reality" and that "the [U.S.] Senate will never agree to this".
He had told them that the U.S. must reduce its emissions from its current 20 tonnes of carbon per person average to zero tonnes per person by 2020 to have an even chance of stabilising the climate around two degrees C.
China's emissions must peak by 2020 and then go to zero by 2035 based on the current science, he added.
"Policymakers who agreed to a two-degree C goal at the G20 summit easily fool themselves about what emission cuts are needed," Schellnhuber said.
Even with a two-degree rise, most of the world's coral reefs will be lost, large portions of the ocean will become dead zones, mountain glaciers will largely vanish and many other ecosystems will be at risk, Schellnhuber warned. And there is the risk of reaching a tipping point where the warming rapidly accelerates.
The planet has already warmed 0.74 C over the past century and the warming is now increasing at a rate of 0.16 C per decade, according the Intergovernmental Panel on Climate Change (IPCC) 2007 report.
With 2008 emissions at the very top end of the IPCC's worst case estimates, it is time to look at what that may mean for the planet, said Richard Betts of the Climate Impacts research team at the Met Office Hadley Centre in London.
Continuing on the current high emissions path means average global temperatures would increase by 4.0 to 5.6 degrees by 2090. Brazil, much of Canada, parts of the U.S., Siberia and Central Europe would be eight degrees warmer than in the past 50 years, computer models show. Rainfall in the north will increase but wet tropics will become 20 percent drier.
The models are based on human emissions alone, and do not include heat-amplifying feedbacks from melting ice or changes in carbon sinks. When those are factored in, it moves the timetable forward so that "reaching four degrees by 2060 is a plausible, worst-case scenario" with the median being 2070. By 2100, 5.5 degrees is possible, he said.
Few places would experience the global average temperature, Betts cautioned, noting that the computer models show the Arctic warming 15 degrees while many other regions of the world would experience 10 degrees of additional warming.
These scenarios do not include potential tipping points like the release of the 1.5 trillion tonnes of carbon in northern permafrost or the melting of undersea methane hydrates.
What would the world look like when it is four degrees warmer? It will likely mean one to two billion people will not have access to adequate fresh water because of the major shift in rainfall patterns, said Nigel Arnell, director of the Walker Institute for Climate Systems Research at the University of Reading in Britain.
Up to 15 percent of existing or potential cropland - and 40 percent in Africa - will become too dry and too hot for food production. While there might be some gains in northern areas like Canada and Russia, generally the soils there are not suitable for crops, he said.
Flooding will affect at least 500 million people because sea levels will rise more than one metre by 2100. The somewhat contentious issue of future sea level rise has been resolved with a new computer model that almost perfectly matches the historical changes in sea level since 1880, reported oceanographer Stefan Rahmstorf at Germany's Potsdam Institute for Climate Impact Research.
The new model projects sea level rise by 1.2 to 1.9 metres from 1990 levels by 2100, said Rahmstorf.
"We're expecting a really big sea level rise in the longer run," he said.
Even at two or three degrees of warming, sea level will inevitably rise many metres higher in the centuries to come. The main questions are how fast levels will increase, and whether vulnerable countries like Holland can build seawalls fast enough to keep up with the rising water levels and the extraordinary costs involved, he said.
In a four-degree warmer world, adaptation means "put your feet up and die" for many people in the world, Oxford's Chris West said bluntly. "In accepting the many alarming impacts, we see that it (a four-degree C increase) is not acceptable."
The climate negotiators heading to Copenhagen in December must accept the fact that the world's carbon emissions must eventually stop - and stop completely. There is no sustainable per capita carbon emission level because it is the total amount of carbon emitted that counts, explains Myles Allen of the Climate Dynamics group at University of Oxford's Atmospheric, Oceanic and Planetary Physics Department.
Carbon dioxide remains in the atmosphere for many centuries, which makes it the most important greenhouse gas to reduce and eliminate. The current focus on CO2 concentrations like 450 ppm or 350 ppm is the not the right approach since it is the total cumulative emissions that determine how warm the planet will get, Allen told the conference.
If climate negotiators only look at slowing rates of carbon emissions, then natural gas will be substituted for coal because it has half of the carbon - but the total amount of carbon in the atmosphere will continue to increase.
"We didn't save the ozone layer by rationing deodorants," said Allen.