Melting ice sheet may accelerate rise in sea level
By the end of the century sea levels may be rising three times as
fast as they are at present, as a result of rapid melting of the
Greenland ice sheet.
The vast Greenland ice sheet could begin to melt more rapidly than
expected towards the end of the century, accelerating the rise in sea
levels as a result of global warming, scientists warned yesterday.
Water running off the ice sheet could triple the current rate of sea
level rise to around 9mm a year, leading to a global rise of almost 1
metre per century, the researchers found.
Sea levels are already on the rise as a result of increasing
temperatures, because the oceans expand as they warm up, but until now
scientists have had a poor understanding of how quickly ice sheets such
as those in Greenland and Antarctica will begin to disappear.
There are signs that the Greenland ice sheet, which covers 1.7
million square kilometres of land, has already begun to melt faster than
expected. The reason is thought to be surface water on the ice sheet
trickling down through fissures to the underlying bedrock, making the
ice sheet less stable, and the loss of buttressing ice shelves along the
coastline.
Climate scientists are uncertain how susceptible ice sheets are to
global warming, largely because they have never witnessed one disappear,
so researchers led by Anders Carlson at the University of
Wisconsin-Madison decided to look back to the end of the last ice age
for clues.
Around 20,000 years ago, when the last ice age was at its peak, a
giant mass of frozen water called the Laurentide ice sheet covered much
of what is now North America. The ice sheet, which was three miles thick
in some places, had almost completely melted 6,500 years ago as the
world warmed as part of its natural cycle.
At the time, surface air temperatures were similar to those that
climate scientists predict for 2100.
The researchers used evidence in the geological record and computer
simulations to reconstruct the demise of the Laurentide ice sheet, which
was the last ice sheet to completely disappear in the northern
hemisphere.
They dated boulders and fossilised organisms left on fresh ground as
the ice sheet retreated, and found that it went through two periods of
rapid melting. Computer simulations revealed that around 9,000 years
ago, water melting off the ice sheet caused sea levels to rise by about
7 metres at a rate of around 1.3cm a year. The second stage of rapid
melting began 7,500 years ago, when sea levels rose by 5 metres at a
rate of around 0.7cm a year.
The reconstruction suggests that the Greenland ice sheet may melt in
a similar fashion. "We have never seen an ice sheet retreat
significantly or even disappear before, yet this may happen for the
Greenland ice sheet in the coming centuries to millenia," said Carlson,
whose study appears in the journal Nature Geoscience.
"We're not talking about something catastrophic, but we could see a
much bigger response in terms of sea level from the Greenland ice sheet
over the next 100 years than what is currently predicted," Carlson
added.
The most recent report from the UN's Intergovernmental Panel on
Climate Change predicts that sea levels will have risen by around 10cm
at most by 2100, but according to Carlson's analysis, rapid melting of
the Greenland ice sheet could cause much greater rises.
"For planning purposes, we should see the IPCC projections as
conservative," he said. "We think this is a very low estimate of what
the Greenland ice sheet will contribute to sea level."
In an accompanying article, Mark Sidall at Bristol University
describes how a 1 metre rise in sea level would submerge an estimated
2.2m square kilometres of land, largely in Asia, and displace around 145
million people at a global cost of $944 billion.
He points out, though, that while the Laurentide ice sheet completely
vanished at the end of the last ice age, the Greenland ice sheet
remains, suggesting it is more resistant to warming. "To what extent
this dynamic response of the Laurentide ice sheet to past temperature
change can be considered analagous to present and future reduction of
the Greenland ice sheet remains unresolved," he writes. "But [the
researchers'] work suggests that future reductions of the Greenland ice
sheet on the order of 1 metre per century are not out of the question."
- Guardian,UK
|