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Poor forecasting undermines climate debate
01 May 2008
From New Scientist Print Edition.
Fred Pearce

Enlarge Figure 1

Enlarge Climate of doubt

"Politicians seem to think that the science is a done deal," says Tim Palmer. "I don't want to undermine the IPCC, but the forecasts, especially for regional climate change, are immensely uncertain."

Palmer is a leading climate modeller at the European Centre for Medium-Range Weather Forecasts in Reading, UK, and he does not doubt that the Intergovernmental Panel on Climate Change (IPCC) has done a good job alerting the world to the problem of global climate change. But he and his fellow climate scientists are acutely aware that the IPCC's predictions of how the global change will affect local climates are little more than guesswork. They fear that if the IPCC's predictions turn out to be wrong, it will provoke a crisis in confidence that undermines the whole climate change debate.

On top of this, some climate scientists believe that even the IPCC's global forecasts leave much to be desired. In particular, they say that because the IPCC cannot take the most recent research into account, its predictions are too conservative.

Next week, climate modellers from around the world will meet in Reading at the World Modelling Summit for Climate Prediction, held under the auspices of the UN, to try to improve our forecasting abilities. Its declared aim is to "prepare a blueprint to launch a revolution in climate prediction", including measures that will allow us to predict how the climate will be affected locally as well as globally.

The organisers say that this will require the computing power brought to bear on the problem to be increased "by a factor of 1000". One option likely to be discussed is the creation of a global climate modelling centre - a climatological equivalent of international collaborations like the CERN particle physics centre in Europe.

Meanwhile representatives of the world's nations were meeting in Bangkok, Thailand, last month, to begin detailed work on a treaty to replace the Kyoto protocol in 2013. They will be basing their discussions on the best predictions available from the IPCC, which means that by the time this son-of-Kyoto is in force, the science on which it is based will be eight years old.

European governments are pressing for an agreement that would keep atmospheric concentration of carbon dioxide below 450 parts per million. This compares with pre-industrial levels of 280 ppm and current levels of 380 ppm. This, they argue, will prevent warming by more than 2 °C, and so avoid "dangerous" climate change.

Yet many climate scientists wince at this. First, because the European governments like to claim that the IPCC backs these targets, when in fact the IPCC goes out of its way to say that setting targets is a job for politicians. And second, because nobody knows either whether 450 ppm will hold warming below 2 °C, or whether this amount of warming will turn out to be safe. "It's horrifying when you see things boiled down to simple terms like a 2 °C warming. That will mean hugely different things for different places," Palmer says.

One reason the IPCC's official reports are slow to bridge this gap is the panel's policy of only considering published peer-reviewed research that is available when its review process gets under way. This means the current report, published last year, takes no account of research published after early 2005.

An increasingly scary debate about the state of the Greenland ice sheet is almost entirely absent in the 2007 report, for instance (see "What if the ice goes?"). Other recent research suggests that warming may be accelerating beyond IPCC predictions: first, because higher temperatures are releasing greenhouse gases from forests, soils and permafrost; and second, because the ocean's ability to absorb CO2 seems to have declined in the past decade.

Equally worrying is the fact that climatologists are losing confidence in the ability of existing models to work out what global warming will do to atmospheric circulation - and hence to local weather patterns like rainfall. The most recent IPCC report made a number of regional predictions. It felt able to do so because it was generally assumed that if most models agreed on future climate in, say, the Amazon rainforest or western Europe, then they were probably right.

Palmer disputes this. In a paper in the April edition of the Bulletin of the American Meteorological Society he warns that models often share the same biases and blind spots about features of the climate system that are critical for regional forecasts. They cannot reproduce El Niños in the Pacific Ocean, for instance. Nor can they simulate the weather systems that bring drought to the Sahel region of Africa, or the Atlantic storm tracks and blocking high-pressure zones that determine whether western Europe is wet or dry.

Last year, a panel on climate modelling that was preparing the ground for next week's summit concluded that current models "have serious limitations" and that their uncertainties "compromise the goal of providing society with reliable predictions of regional climate change". The panel, chaired by Jagadish Shukla of George Mason University in Claverton, Maryland, dismissed many current regional predictions as "laughable".

But whatever the uncertainties at the local level, the big picture remains clear. Our planet is straying into unknown climatic territory, with consequences that we probably have to accept are almost impossible to predict.

One of these unknowns was highlighted last month in the preprint of a paper James Hansen of the NASA Goddard Institute for Space Studies has submitted to the journal Science (www.arxiv.org/abs/0804.1126). Looking back 50 million years, to a time when falling CO2 levels in the atmosphere reached 425 ppm - a level we are likely to reach within two decades - he says that was the moment Antarctica got its ice cap. This suggests that the planet may have a tipping point at around that level, give or take 75 ppm, and that by going above it we could render Antarctica ice-free once again. That would raise sea levels by around 60 metres.

Hansen concludes that far from aiming to limit rising CO2 concentrations to a ceiling of 450 ppm, as currently suggested, the world should set a long-term target of getting back down to 350 ppm. A few decades with CO2 above that figure might not matter, but "it would be foolish to allow CO2 to stay in the danger zone for centuries," he says. "If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects."

These developments in climate research raise fears that the IPCC will be left stranded, too distant from the cutting edge of research to be of much use in guiding action over climate change. Some researchers now argue that it should produce more regular, up-to-date reviews of climate science, but at a meeting in Budapest in early April the panel decided to stick with its current policy. Its next full assessment is due in 2013 or 2014. Who knows where the world will be by then?

From issue 2654 of New Scientist magazine, 01 May 2008, page 8-9

What if the ice goes? For hundreds of millions of people living near the world's coastlines, a rise in sea level is the biggest danger posed by climate change. Last year's IPCC report puts the likely rise in the coming century at between 18 and 59 centimetres, and most countries are basing their decisions on climate policy on that assumption. But it could be much more. Even as the IPCC report was being finalised, new research showed sea levels were rising 50 per cent faster than the report assumed. And new modelling presented last month at the conference of the European Geosciences Union in Vienna, Austria, by Svetlana Jevrejeva of the Proudman Oceanographic Laboratory in Liverpool, UK, predicted the rise in the 21st century would be three times the IPCC prediction, at up to 1.5 metres. The new element here is the discovery that as the atmosphere warms, the ice sheets on Greenland and West Antarctica may not melt gradually as had been predicted. Instead, they may break up rapidly as meltwater penetrates the ice. Researchers at the Woods Hole Oceanographic Institution in Massachusetts reported last month that they had watched a lake in Greenland 3 kilometres across empty down a kilometre-deep crack in the ice within 90 minutes - a discharge that, they said, "exceeded the flow of Niagara Falls". The possibility that such events could cause the break-up of the Greenland ice sheet is an increasingly controversial topic among glaciologists. Not all believe even large numbers of "Greenland Niagaras" would destabilise the entire sheet, but the possibility remains that it could disappear, which would raise sea levels by 6 metres. Meanwhile the old certainties of gradual melting over thousands of years remain in the IPCC report. It is doubtful whether they still hold water.

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