Sea level rises could far exceed IPCC estimates
Could our coastlines disappear underwater much sooner than we think? The controversial view that sea levels could rise at a rate of more than 1 metre per century has found support from a new study of a long-melted ice sheet.
In reconstructing the events at the end of the last ice age, Anders Carlson of the University of Wisconsin-Madison and colleagues found that the Laurentide ice sheet, which covered most of North America between 95,000 and 7000 years ago, rapidly disintegrated.
The researchers began by studying beryllium isotopes in rocks to determine how the outer edges of the two final chunks of the Laurentide ice sheet retreated. They found that the ice retreated rapidly between 9000 and 8500 years ago, stabilised, and then made its final rapid retreat between 7600 and 6800 years ago.
The team calculated the volume of water that would have been released in each of these melting stages, and the rate at which it must have raised sea levels. They concluded that levels would have climbed 1.3 metres per century in the earlier period, and 0.7 metres per century in the final melt.
Carlson then used a sophisticated computer model — one that is used to forecast future climate change — to check the results. The model predicted an average sea level rise of 1.3 metres per century.
"The forces that led to the demise of the Laurentide ice sheet in a very rapid way are comparable to the forces the same computer models predict we will experience this century if we do not rapidly curb greenhouse gas emissions," says Gavin Schmidt of the NASA Goddard Institute for Space Studies, who collaborated with Carlson on the study.
For Mark Siddall of the University of Bristol, UK, and Michael Kaplan of the Lamont Doherty Earth Observatory in New York, however, there remain many differences between what happened nearly 10,000 years ago and the climate change Earth is currently experiencing.
"To what extent this dynamic response of the Laurentide ice sheet to past temperature change can be considered analogous to present and future reduction of the Greenland ice sheet remains unresolved," they say in an associated commentary. "But their work suggests that future reductions of the Greenland ice sheet on the order of one metre per century are not out of the question."
If Carlson's estimates are correct, they show that 2007 predictions made by the Intergovernmental Panel on Climate Change — a sea-level rise of between 18 centimetres and 59 cm by 2100 — are very conservative, as the IPCC acknowledged at the time.
Millions at risk
Sea-level rises of at least a metre per century were also predicted by the head of the NASA Goddard Institute for Space Studies, James Hansen. Hansen believes that our climate will soon hit tipping points — points of no return — beyond which the ice sheets will rapidly disintegrate.
Carlson says his team's findings are proof that large ice sheets can disintegrate very rapidly. What's more, he says the forces that caused the Laurentide ice sheet to disintegrate are equivalent to the ones that threaten the Greenland ice sheet today.
Some say the water contained within the Antarctic and Greenland ice sheets will not contribute to rises in sea levels for many centuries. Together, the ice sheets hold enough water to raise global levels by 70 metres. Carlson's study considers the Greenland ice sheet only.
If the 1-metre-per-century predictions are right, then homes of at least 145 million people, most of them in Asia, could be beneath the waves by 2108.
In the course of preparing the 2006 Stern Report, economists at the Tyndall Centre in the UK multiplied the at-risk population by their respective economic value, in terms of 1995 GDP per capita. The crude estimate put a gross monetary value of $944 billion to the risk.
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