Sydney Morning Herald. July 27, 2009
The UN's climate panel has been backed over a key question as to how far global warming will drive up sea levels this century.
The UN experts are right that the oceans are unlikely to rise by an order of metres by 2100, as some scientists have feared, the study published on Sunday says.
But, its authors caution, low-lying countries and delta areas could still face potentially catastrophic flooding if the upper range of the new estimate proves right.
In a landmark report in 2007, the UN's Intergovernmental Panel on Climate Change (IPCC) predicted oceans would rise by 18-59 centimetres by 2100.
The increase would depend on warming, estimated at between 1.1 and 6.4 degrees Celsius this century, which in turn depends on how much man-made greenhouse gas is poured into the atmosphere.
It based the calculation on thermal expansion of the seas - when a liquid is warmed, it grows in volume.
Harder to calculate, the IPCC admitted, was how far meltwater from glaciers and icesheets on land would boost sea levels.
It ventured a provisional calculation, suggesting contributions from those sources could push the upper limit to 76 centimetres.
The new paper, led by Mark Siddall of Britain's University of Bristol, used data from fossilised coral and from ice-core measurements to reconstruct sea-level fluctuations over the past 22,000 years, from the height of the last Ice Age to the balmy era of today.
This century, they calculate, the seas will rise by between seven and 82 centimetres, all sources included, on the basis of a 1.1-6.4 degree warming - an estimated increase that is in the same ballpark as the IPCC's.
The study appeared in the journal Nature Geoscience on Sunday.
"Given that the two approaches are entirely independent of each other, this result strengthens the confidence with which one may interpret the IPCC results," said Siddall.
But, he said, no one should be fooled into thinking the flooding threat was over.
"The fact that this number is smaller than other numbers does not mean that this is not potentially a massive and very important sea level rise," Siddall said.
"Fifty centimetres of rise would be very, very dangerous for Bangladesh, it would be very dangerous for all low-lying areas. And not only that, the 50 centimetres is the global mean. Locally, it could be as high as a metre, perhaps even higher, because water is pushed into different places by the effect of gravity."
He added: "Extreme flood effects will definitely become more frequent. If you rise by 50 centimetres, floods that once happened every 100 years then become once a decade."
Siddall also pointed out that sea levels would inevitably rise even higher after the 21st century because of inertial effect.
It takes decades for atmospheric warming to translate into a warming of the seas because of the vast volume of the ocean, he said.
Thus the 22nd century and beyond will feel the impacts of the warming of the 21st century.
The IPCC's estimates on sea levels have been repeatedly challenged since the Fourth Assessment Report was published in 2007.
Several studies have suggested that runoff from the Greenland and Antarctic icesheets - which hold the world's biggest stores of freshwater - will be much higher than the panel suspected.
One paper, published in April by Paul Blanchon, a geoscientist at Mexico's National University, said that, in the distant past, the seas suddenly rose by 3 metres within a very short time.
There was "a distinct possibility" that a step change of this kind could happen within the next 100 years, said Blanchon.