杏吧原创

0.13 掳C. The amount the atmosphere is warming each decade

1.3 times as much CO₂ is entering the atmosphere compared with just 20 years ago

3 kilometres. The depth to which the oceans have warmed

3.1 centimetres. The rise in sea level each decade

90 per cent certainty that we are to blame

The word they were most pleased with was 鈥渦nequivocal鈥�. Three hundred government-appointed delegates from 113 countries were last week unanimous in agreeing what most climate scientists have believed for years: that the world is warming fast and that humans are almost certainly to blame.

Some 600 scientists wrote the summary of the fourth assessment by the Intergovernmental Panel on Climate Change, published this week. Virtually everything they wanted to say in it survived the politicians, but the IPCC鈥檚 review process was so rigorous that research deemed controversial, not fully quantified or not yet incorporated into climate models was excluded. The benefit 鈥� that there is now little room left for sceptics 鈥� comes at what many see as a dangerous cost: many legitimate findings have been frozen out.

This is the untold story of the report, uncovered in interviews with many of the scientists involved, the story of how a complex mixture of scientific rigour and political expediency resulted in many of the scientists鈥� more scary scenarios for climate change 鈥� those they constantly discuss among themselves 鈥� being left on the cutting room floor.

Dozens of climate scientists, including many of the leading lights of the IPCC study, came together two years ago this month to discuss 鈥渄angerous鈥� climate change at a conference organised by the UK government in Exeter. They identified a series of potential positive feedbacks and 鈥渢ipping points鈥� not included in current models of the Earth鈥檚 climate system that could accelerate global warming or sea-level rise. These included the physical collapse of the Greenland ice sheet, rapid melting in Antarctica, a shut-down of the Gulf Stream in the Atlantic, and the release of carbon dioxide and methane from soil, the ocean bed and melting permafrost.

鈥淐urrent models assume the ice sheets will melt only slowly, but many glaciologists no longer believe this will happen鈥�

Yet last week鈥檚 summary report virtually ignored most of the Exeter findings. One concern is that the huge ice sheets of Greenland and Antarctica could be close to disintegration. This would cause rises in sea levels that would be measured in metres, but the report restricts itself to noting that sea levels are rising by 3.1 centimetres a decade 鈥� still almost twice the rate of the early 1990s. Current climate models assume that the ice sheets will melt only slowly, as heat works its way down through ice more than 2 kilometres thick. But many glaciologists no longer believe this is what will happen.

In reality, they say, ice sheets fracture as they melt, so water can penetrate to the bottom of the ice within seconds, warming its full depth and lubricating the frozen join between ice and the bedrock. Physical break-up of the ice sheets will happen long before thermal melting, they say.

Richard Alley, a US glaciologist who has published widely on the dangers, says climatologists have yet to be convinced that they need to rewrite their models, even though the rate of ice loss in Greenland has unexpectedly doubled in the past decade. The report does note that permanent Arctic sea ice is contracting by 7 per cent every decade.

鈥淥ur chapter of the report will say that Greenland is doing things that could make it disintegrate much faster than people think,鈥� Alley says. 鈥淏ut we don鈥檛 have a strong basis yet for projecting exactly what the ice sheets will do,鈥� So, he says, the summary excluded the new thinking.

Last week another IPCC author, Stefan Rahmstorf of Germany鈥檚 Potsdam Institute for Climate Impact Research, published a paper showing that world sea levels are rising 50 per cent faster today than predicted in the last IPCC report in 2001 (Science, DOI: 10.1126/science.1136843). Co-author Jim Hansen of NASA鈥檚 Goddard Institute for Space Studies believes this is the first sign of a dramatic acceleration of sea level rise likely in the coming decades, as ice sheets start to disintegrate.

Both acknowledge in the paper that there may not yet be enough data to extrapolate a trend, but the IPCC last week reduced its estimate of worst-case sea level rise in the coming century from 88 to 59 centimetres. Real-world evidence was specifically excluded, the IPCC said, because it is not yet included in the models.

鈥淩eal-world evidence was specifically excluded because it is not yet included in the models鈥�

Researchers outside the IPCC process have been outspoken in condemning this approach. Bob Corell, a leading US meteorologist and chairman of the Arctic Climate Impact Assessment, warned before the report鈥檚 publication that any prediction of sea level rise of less than 1 metre would 鈥渘ot be a fair reflection of what we know鈥�.

The IPCC team also sidelined findings from the British Antarctic Survey. BAS researchers say that the Antarctic Peninsula is warming faster than almost anywhere on the planet. They have documented a sharp decline in sea ice around the peninsula, and warn that the giant West Antarctic ice sheet is 鈥渦nstable and contributing significantly to sea level rise鈥�.

In contrast, the IPCC summary claims there are 鈥渘o statistically significant average trends [in sea ice],鈥� and that this is 鈥渃onsistent with a lack of warming, reflected in atmospheric temperatures averaged across the region鈥�. It asserts that overall 鈥渢he Antarctic ice sheet鈥� is expected to gain in mass due to increased snowfall鈥�.

Researchers at the UK鈥檚 National Oceanography Centre, Southampton, will also feel overlooked. In 2005, they reported that the Gulf Stream slowed by about 30 per cent between 1957 and 2004. The Gulf Stream is a key feature of the world ocean circulation system, and any failure could have huge and unpredictable repercussions for world climate. But the IPCC summary insists that 鈥渢here is insufficient evidence to determine whether trends exist鈥�.

Water vapour is increasing in the atmosphere, the summary says, thanks to more evaporation from the oceans. Weather systems are changing, with more intense droughts and tropical cyclones at low latitudes. Rainfall, when it occurs, is measurably heavier because the warmer air holds more moisture.

However, the summary fails to take up warnings made at the Exeter meeting about 鈥渃arbon-cycle feedbacks鈥� 鈥� the release of greenhouse gases from warmed soils, forests, permafrost and sea beds. It does note that carbon dioxide is accumulating in the atmosphere at a record rate, with annual increases now a third greater than even 20 years ago.

Another IPCC author, Venkatchalam Ramaswamy of the National Oceanic and Atmospheric Administration, told New 杏吧原创 that the IPCC鈥檚 predictions of significant warming in northern latitudes should give urgency to assessing potential methane releases from Siberia and the Arctic. But, he said, his fears had failed to make it into the summary.

鈥淭he chapters went through three sets of reviews,鈥� said Ramaswamy. 鈥淎nything qualitative rather than quantitative was knocked out. And if it got into the chapter, then the question was whether it would get into the summary. By and large where there was ambiguity or controversy, it didn鈥檛 make it.鈥�