Present Greenland ice sheet (Illustrations courtesy Bette Otto-Bliesner, NCAR.)

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Research suggests many coastal cities
will be flooded by the end of the century

25 March 2006: London, Tokyo, Mumbai and New York, together with thousands of other communities around the world, could be flooded by the end of this century according to research published in March 2006. Two studies suggest that global warming has a much more dramatic effect on sea levels than previously thought. Ice sheets across both the Arctic and Antarctic could melt more quickly than expected this century, with Arctic summers by 2100 being as warm as they were nearly 130,000 years ago, when sea levels eventually rose up to six meters higher than today.

Bette Otto-Bliesner from the National Center for Atmospheric Research (NCAR) and Jonathan Overpeck (University of Arozona) based their findings on data from ancient coral reefs, ice cores, and other natural climate records, as well as output from the NCAR-based Community Climate System Model (CCSM), a powerful tool for simulating past, present, and future climates. "Although the focus of our work is polar, the implications are global," says Ms Otto-Bliesner. "These ice sheets have melted before and sea levels rose. The warmth needed isn't that much above present conditions."

The two studies show that greenhouse gas increases over the next century could warm the Arctic by three to five degrees Celsius during the summer. This is roughly as warm as it was 130,000 years ago, between the most recent ice age and the previous one. The warm Arctic summers during the last interglacial period were caused by changes in Earth's tilt and orbit.

Bette Otto-Bliesner said that the research modelling used by the research team suggested that during the interglacial period, meltwater from Greenland and other Arctic sources raised sea level by as much as 3.5 meters. However, coral records indicate that the sea level actually rose four to six metres or more. Jonathan Overpeck concluded that Antarctic melting must have produced the remainder of the sea-level rise.

These studies are the first to link Arctic and Antarctic melting in the last interglacial period. Marine diatoms and beryllium isotopes found beneath the West Antarctic Ice Sheet indicate that parts of the ice disappeared at some point over the last several hundred thousand years.

Mr Overpeck theorizes that the rise in sea levels produced by Arctic warming and melting could have helped destabilize ice shelves at the edge of the Antarctic ice sheet and led to their collapse. “If such a process occurred today, it would be accelerated by global-scale greenhouse-induced warming year round,” Mr Overpeck added. “In the Arctic, melting would likely be hastened by pollution that darkens snow and enables it to absorb more sunlight.”

In the last few years sea level has begun rising more rapidly, now at a rate of about an inch per decade. Recent studies have also found accelerated rates of glacial retreat along the margins of both the Greenland and West Antarctic ice sheets.

A rise in sea level as suggested by the research would result in major changes to the world map. Large parts of countries like Bangladesh or the Netherlands would have to be abandoned. Most coastal cities would face severe flooding and many Pacific Island communities would completely disappear.

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