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The United Nations’ forecast of how quickly global sea levels will rise this century is vital in determining how much money might be needed to combat the phenomenon. But predictions by researchers vary wildly, and the attempt to find consensus has become fractious.

It is a number which will ultimately establish how billions in taxpayer money will be spent — and it is one which is the subject of heated debate, both among politicians and scientists.

When the next report from the United Nations Intergovernmental Panel on Climate Change (IPCC) is issued in two years, it will include a forecast for how high the world’s oceans might rise by 2100. With 146 million people in the world currently living less than one meter above sea level, the forecast will be vital in determining how much money governments must spend on measures to protect people from the rising waters and to resettle those in the most acute danger.

Eighteen scientists from 10 countries are involved in the task, and their first step is to determine which of the myriad studies relating to climate change’s effect on ocean levels to consider. In the end, they are to establish a possible range, with the maximum being the most decisive — and most contested — number. Even more challenging, the estimates currently differ by almost five meters (16.5 feet).

The last IPCC report, which was issued in 2007, forecast an ocean level rise of up to 59 centimeters by the end of the century. Now, the UN experts must once again sift through hundreds of reports, and the haggling over their findings is not unlike the bargaining for the best price at the bazaar. On the one hand, researchers have published forecasts that are far higher than the result reported in the last IPCC report. On the other, sea level measurements have yet to prove any meaningful rise though there is agreement that they are, on global average, rising.

Outdoing Each Other’s Predictions

In recent days, the debate over the IPCC forecast has heated up. Some 4,000 experts gathered in Melbourne, Australia last week for a meeting of the International Union of Geodesy and Geophysics (IUGG). And it seems almost as though they are in competition to outdo one-another’s predictions.

NASA climate researcher James Hansen, for example, warns in a paper published this month that sea levels could rise by five meters in the next 90 years — nine times higher than the maximum cited in the last IPCC report. He insists that he has found indications that sea levels in the future could rise by as much as five centimeters per year.

Hansen, say some climatologists, is risking his reputation with such an extreme forecast. Three years ago, researchers found that a rise of over two meters per century is impossible because so much ice simply can’t melt in such a short time. Furthermore, current measurements show a rise of just three millimeters per year.

An additional recent study, written by Jim Houston from the Engineer Research Center in Vicksburg, Mississippi and Bob Dean from the University of Florida in Gainsville for the Journal of Coastal Research, argues that sea levels have risen steadily for the last 100 years — and that there has been no acceleration at all in recent years.

A reply was not long in coming. In the current issue of the Journal of Coastal Research, Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research argues that Houston and Dean only included sea level calculations beginning in the 1930s. He says that if one chooses a year from the previous century, an accelerated rise can be seen.

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