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New UN Report Is Cautious On Making Climate Predictions

A new study warns that the world faces serious risks from warming and that the poor are most vulnerable, but it avoids the kinds of specific forecasts that have sparked controversy in the past.
 
 
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Batten down the hatches; fill the grain stores; raise the flood defenses. We cannot know exactly what is coming, but it will probably be nasty, the latest report from the UN’s Intergovernmental Panel on Climate Change (IPCC) will warn next week. Global warming will cause wars, displace millions of people, and do trillion-dollar damage to the global economy.

But careful readers will note a new tone to its discussion of these issues that is markedly different from past efforts. It is more humble about what scientists can predict in advance, and far more interested in how societies can make themselves resilient. It also places climate risks much more firmly than before among a host of other problems faced by society, especially by the poor. That tone will annoy some for taking the edge off past warnings, but gratify others for providing a healthy dose of realism.

The study, the result of a five-year review of published papers, is from the IPCC’s scientists working on the impacts of climate change. It complements an IPCC study late last year on the planetary science and will be followed next month by another that will focus on what we should do about it.

A leak of the final draft prepared by scientists at the end of October 2013 is circulating. It is not the final version, which will be a summary for policymakers that will be released on March 31, though there is unlikely to be much change. And, since government delegates at international talks in Japan this week will scrutinize the final draft before signing off for publication, what we have is effectively "the scientists’ cut."

Past impacts reports from the IPCC were based around attempts to produce detailed forecasts of local climate in future decades and somewhat mechanistic assessments of what this would mean for society. But the new report is much more wary, especially of putting numbers on likely changes. Many previously firm-sounding forecasts have disappeared since the last major IPCC climate-impacts report in 2007, such as spreading droughts and crop losses in Africa and more violent hurricanes in the Atlantic.

The reason for avoiding precise forecasts is twofold. First, overly precise predictions got the authors of the 2007 report into trouble. The most famous faux pas was the claim that Himalayan glaciers would be gone by 2035, when 2350 is a more likely date. But there were other unsubstantiated forecasts, such as that “projected reductions in [crop] yield in some countries [in Africa] could be as much as 50% by 2020” — a misinterpretation of a paper, which was not peer-reviewed, that looked at rain-fed agriculture in just three North African countries.

The hundreds of authors of the draft report have been silent for some time, following IPCC rules by refusing to discuss their draft with journalists. But their chairman, Chris Field of the Carnegie Institution in Stanford, California, told me soon after taking on the job in 2009 that he recognized serious mistakes had been made last time and that he was “committed to sufficient checking and cross-checking to ensure a truly error-free product next time.”

Another reason for the more measured tone is that hopes that better science and greater computer power would allow more precise forecasts than seven years ago have often proved wrong. For parts of the world, model forecasts of regional climate change are diverging rather than converging. The more we know, it seems, the less we know for sure.

Caution is the watchword. Take the treatment of Africa. Last time, the chapter on that continent began with a declaration that up to a quarter of a billion Africans “are projected to be exposed to increased water stress due to climate change.” This time, the leaked draft states simply that while “a reduction in precipitation is likely over North Africa ... projected rainfall change over sub-Saharan Africa is uncertain.”

 
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