Contents

Idea

One among the expected side effects of global warming is a global increase in sea level, or sea level rise. Sea levels are expected to increase for a number of reasons, including melting ice and the fact that warmer water occupies a greater volume. This is expected to affect low-lying land areas along coasts, including river deltas and barrier islands.

Estimates

According to the NRC climate stabilization targets report, global sea level has risen by about 0.2 meters since 1870. The sea level rise by 2100 is expected to be at least 0.6 meters due to thermal expansion and loss of ice from glaciers and small ice caps. This could be enough to permanently displace as many as 3 million people — and raise the risk of floods for many millions more.

Ice loss is also occurring in parts of Greenland and Antarctica. If all the ice in Greenland were to melt, it would cause an additional sea level rise of 7.2 meters, and if all the ice in the West Antarctic Ice Sheet were to melt, it would cause a further rise of 4.8 meters (see below). However, the amount of sea level rise in the next century remain uncertain, because the rate of melting of these bodies of ice is hard to predict.

Greenland

If the entire $2.85 \cdot 10^6$ km3 of ice in Greenland were to melt, it would lead to a global sea level rise of 7.2 m. This would inundate most of the world’s coastal cities and remove several small island countries from the face of the Earth, since island nations such as Tuvalu and Maldives have a maximum altitude below or just above this level:

Antarctica

It is estimated that the volume of the Antarctic ice sheet is about 25.4 million km3, and the West Antarctic Ice Sheet (or WAIS) contains just under 10% of this, or 2.2 million km3:

• Matthew B. Lythe and David G. Vaughan, BEDMAP: A new ice thickness and subglacial topographic model of Antarctica, Journal of Geophysical Research 106 (June 2001), 11335–11352.

Large parts of the West Antarctic Ice Sheet (or WAIS) sit on a bed which is below sea level and slopes downward inland. This slope, and the low isostatic head, mean that the ice sheet is theoretically unstable: a small retreat could in theory destabilize the entire WAIS leading to rapid disintegration. However, current computer models do not include the physics necessary to simulate this process, and observations do not provide guidance, so predictions as to its rate of retreat remain uncertain.

In January 2006, in a UK government-commissioned report, the head of the British Antarctic Survey, Chris Rapley, warned that this huge west Antarctic ice sheet may be starting to disintegrate. Rapley said a previous Intergovernmental Panel on Climate Change (IPCC) report that played down the worries of the ice sheet’s stability should be revised. “The last IPCC report characterized Antarctica as a slumbering giant in terms of climate change,” he wrote. “I would say it is now an awakened giant. There is real concern.”

(Note that the IPCC report did not use the words “slumbering giant”.)

Rapley said, “Parts of the Antarctic ice sheet that rest on bedrock below sea level have begun to discharge ice fast enough to make a significant contribution to sea level rise. Understanding the reason for this change is urgent in order to be able to predict how much ice may ultimately be discharged and over what timescale. Current computer models do not include the effect of liquid water on ice sheet sliding and flow, and so provide only conservative estimates of future behaviour.”

It has been argued that a collapse of the WAIS could raise global sea levels by approximately 3.3 meters:

• J. L. Bamber, R.E.M. Riva, B.L.A. Vermeersen and A.M. LeBroq, Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet, Science324 (2009), 901.

However, these authors claim there would be important regional variations, with the maximum increase concentrated along the Pacific and Atlantic seaboard of the United States, where the value is about 25% greater than the global mean, even for the case of a partial collapse.

If the entire West Antarctic Ice Sheet were to melt, this would contribute 4.8 m to global sea level:

• J. L. Bamber, R.E.M. Riva, B.L.A. Vermeersen and A.M. LeBroq,, Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet (supporting online material), Science 324 (2009), 901.

Questions

How much sea level rise should we expect from Greenland and the West Antarctic Ice Sheet within the next, say, 10 years?

What are the best adaptive measures? Floating cities?

References

category: climate