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14,000 years ago, sea level rose approximately 20 meters in 400 years, scientist says amid rapid ice-sheet disintegration fears

Posted by gmarkets on 3 September, 2007

There were glaciologists who anticipated long response times, because their ice sheet models had been designed to match past climate changes, wrote James Hansen in New Scientist (28/7/2007, p.33). Unprecedented rapid ice-sheet disintegration possible: However, work by my group shows that the typical 6000-year timescale for ice sheet disintegration in the past reflects the gradual changes in Earth’s orbit that drove climate changes at the time, rather than any inherent limit for how long it takes ice sheets to disintegrate, Hansen wrote. Indeed, the palaeoclimate record contains numerous examples of ice sheets yielding sea level rises of several metres per century when forcings were smaller than that of the business-as-usual scenario.

One metre sea rise in 20 years? For example, about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years. There was growing evidence that the global warming already under way could bring a comparably rapid rise in sea level.

Less heat radiation escapes Earth: The process begins with human-made greenhouse gases, which cause the atmosphere to be more opaque to infrared radiation, thus decreasing radiation of heat to space. As a result, the Earth was gaining more heat than it was losing: currently 0.5 to 1 watts per square metre. This planetary energy imbalance was sufficient to melt ice corresponding to 1 metre of sea level rise per decade, if the extra energy were used entirely for that purpose – and the energy imbalance could double if emissions keep growing.

Extra energy to cause albedo flip? So where is the extra energy going? A small part of it is warming the atmosphere and thus contributing to one key feedback on the ice sheets: the “albedo flip” that occurs when snow and ice begin to melt.

Reference: James Hansen heads NASA’s Goddard Institute for Space Studies in New York. A physicist and astronomer by training, he began his career studying the clouds on Venus. Since the late 1970’s he has been studying and modelling the human impact on Earth’s climate, and has published more than 100 papers. This article is based on a paper in the open-access journal Environmental Research Letters (DOI: 10.1088/1748-9326/2/2/024002)

New Scientist, 28/7/2007, p. 33


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