0.13 °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
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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 “unequivocal”. 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’s review process was so rigorous that research deemed controversial, not fully quantified or not yet incorporated into climate models was excluded. The benefit – that there is now little room left for sceptics – 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’ more scary scenarios for climate change – those they constantly discuss among themselves – 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 “dangerous” climate change at a conference organised by the UK government in Exeter. They identified a series of potential positive feedbacks and “tipping points” not included in current models of the Earth’s 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.
“Current models assume the ice sheets will melt only slowly, but many glaciologists no longer believe this will happen”
Yet last week’s 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 – 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.
“Our chapter of the report will say that Greenland is doing things that could make it disintegrate much faster than people think,” Alley says. “But we don’t have a strong basis yet for projecting exactly what the ice sheets will do,” So, he says, the summary excluded the new thinking.
Last week another IPCC author, Stefan Rahmstorf of Germany’s 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’s 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.
“Real-world evidence was specifically excluded because it is not yet included in the models”
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’s publication that any prediction of sea level rise of less than 1 metre would “not be a fair reflection of what we know”.
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 “unstable and contributing significantly to sea level rise”.
In contrast, the IPCC summary claims there are “no statistically significant average trends [in sea ice],” and that this is “consistent with a lack of warming, reflected in atmospheric temperatures averaged across the region”. It asserts that overall “the Antarctic ice sheet… is expected to gain in mass due to increased snowfall”.
Researchers at the UK’s 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 “there is insufficient evidence to determine whether trends exist”.
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 “carbon-cycle feedbacks” – 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 Scientist that the IPCC’s 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.
“The chapters went through three sets of reviews,” said Ramaswamy. “Anything 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’t make it.”
Reasons to be cautious
Any committee that requires politicians to agree is going to take time arriving at a consensus. Last week climate scientists had to run the gauntlet of government delegations, who had to approve every word of the summary prior to publication.
Delegates spent five hours debating whether it was “extremely likely” or only “likely” that humans were responsible for global warming since the mid-20th century (see Graphs below for changes in man-made emissions). In the language of the IPCC “extremely” means a greater than 95 per cent certainty and “likely” a certainty greater than 66 per cent. A hawkish British government delegation wanted the summary to say “extremely likely”; the Chinese and Saudi Arabians wanted “likely”; in the end exhausted delegates settled for “very likely”, meaning a certainty of at least 90 per cent.FIG-mg25903801.jpg
Old IPCC hands say that Saudi delegations have a track record of vocal intransigence in the face of scientists’ findings; this time they were more constructive. The main problem came from the large Chinese delegation, which was asking for the removal of five key passages from the summary.
They got their way only once when, after a 10-hour debate on the relative influences of solar and human activity, an exasperated meeting agreed to remove a sentence saying that the change in radiative forcing – the heat entering the system – that is attributable to human activities was “likely” to have been at least five times greater than that due to changes in solar activity. The Chinese argued that the influence of the sun could be greater.
“We let it go in the end, because the figure was in a graph anyway,” says Kenneth Denman of the Institute of Ocean Sciences in Sidney, British Columbia, Canada (see “The solar effect”).
Fears that the US delegation might try to veto the scientists’ findings proved unfounded. “We could all sense a change with the American delegation, which behaved with great scientific integrity,” Denman says. Others put this down to the Democratic victory in November’s congressional elections. “The timing could not have been better,” says one scientist. “If we had been writing this report any time in the last five years, we would have expected a lot more trouble.”
Other insiders complain that the US group in charge of the current assessment, headed by Susan Solomon of the National Oceanographic and Atmospheric Administration, has been inherently more cautious than the British team from the Met Office’s Hadley Centre for Climate Change, who ran the previous three assessments. Fear of the wrath of sceptics back home may have contributed to their caution.
By and large, the scientists insist they faced down political interference. The prize of having governments formally sign off on the report will, they hope, make any compromises worthwhile.
