Issued less than two years ago, the report from the Intergovernmental Panel on Climate Change was a voluminous and impressive document. Yet key portions of the report are already out of date, as evidence shows the impacts of warming intensifying from the Arctic to Antarctica.
In a sense, it might be better if the major reports that come from the UN-sponsored Intergovernmental Panel on Climate Change every half-decade or so were never released to the public. It’s not that the documents themselves are scientifically dubious; on the contrary, they’re quite remarkable in their scope and thoroughness.
Like its three predecessors, the IPCC Fourth Assessment Report, issued in 2007, took more than two years to compile. It synthesised literally thousands of individual peer-reviewed papers, written by hundreds of experts in a dozen different climate-related fields, based on innumerable ground-based and satellite observations, and scores of runs of the most sophisticated climate models available.
As a result, the Fourth Assessment Report (or FAR, to insiders) amounted to a high-resolution snapshot of the state of the planet’s climate, and the best possible set of projections for where climate is headed in the future. But because it was a snapshot, and because both the climate and the human activities that contribute to climate change have continued to evolve, the report was largely out of date the moment it was issued.
Since then, new reports have continued to pour in from all over the world, and climate modelers have continued to feed them into their supercomputers. And while a full accounting will have to wait for the next IPCC report, which is already being assembled, the news is not encouraging.
Unexpectedly rapid melting of the vast ice sheet in Greenland, for example, suggests that sea level could rise between 1 and 2 metres by the end of the century - nearly triple what scientists projected just two years ago. A surprisingly rapid round of melting around the North Pole suggest that the Arctic Ocean could be essentially ice-free in summer within two decades or even less - at least 20 years ahead of the most pessimistic FAR predictions. West Antarctica, whose ice cap is bigger than Greenland’s, is warming up faster than anyone thought, and a major ice stream in West Antarctica - the Pine Island Glacier - is sliding into the sea at the astonishing rate of two miles a year, adding its mass to steadily rising global sea levels.
Meanwhile, carbon dioxide is spewing into the atmosphere faster than any model anticipated, pointing to atmospheric levels of that most important of greenhouse gases of 600 parts per million (ppm) - more than twice the pre-industrial level - by the end of the century. That could boost worldwide temperatures by an average of more than 4C.
In fairness to the scientists who assembled the FAR, it’s no surprise that there should be surprises. Take sea level: The now-outdated projection in the report pointed to a rise of between 0.18 and 0.59 metres by the last decade of the century. But if you read the footnote, notes James McCarthy, the Harvard oceanographer who co-chaired one of three major working groups for the previous report, “it makes clear that the projections don’t take into account changes in ice flow”.
It wasn’t that modelers didn’t think these changes would be important; it was that nobody knew how to incorporate them in a reliable way. So they just left them out. But even as the final version of the FAR was being put together, new observations were beginning to show that ice flow is indeed changing, especially in Greenland. The 650,000-square-mile icecap that covers most of this enormous island drains to the sea through hundreds of glaciers around its perimeter. And the flow in many of them had sped up - doubled, in fact. At the same time, the gravity-sensing GRACE satellites were detecting a significant loss of mass in the icecap.
Initially, glaciologists thought they knew why. Warmer temperatures are melting ice on the glaciers’ surfaces, and when that water finds a crack to flow into, it does. Eventually, all of that water reaches the rock below, where it serves as a lubricant, allowing the ice to slide more efficiently towards the sea.
More recently, scientists have come up with a second, perhaps more important effect: glaciers that reach all the way into the sea have become thinner at their outlets. That makes them float free of the ground beneath, and the resulting loss of friction lets the upstream part of the glacier speed up. It is, wrote Mauri Pelto, a glaciologist at Nichols College, in Massachusetts, on the blog RealClimate last April, “akin to letting off the emergency brake a bit”. The same phenomenon is also accelerating the slide of the Pine Island Glacier and its neighbour, the Thwaites Glacier, into the Southern Ocean.
Armed with this new information, Tad Pfeffer, of the University of Colorado, along with several colleagues, recalculated the projections of sea-level rise and came up with a range of 0.8 to 2 metres; their paper was published last fall in Science. “Even one metre,” says Gavin Schmidt, of NASA’s Goddard Institute of Space Sciences, “is a disaster. It would directly threaten millions of people, and trillions of dollars of infrastructure.” The original figure wasn’t misguided; it just means that it wasn’t meant to be taken at face value. “We know,” says McCarthy, “that we have a lot to learn”.