Robert W. Corell
Twenty thousand years ago, during the Last Glacial Maximum when the global average surface temperature was roughly 4—5°C (about 7—9°F) colder than at present, sea level was about 130 m (nearly 400 ft) below its current level due to the glacial build-up. This ice, which was piled 2 km (over a mile) deep over much of northern North America and Europe, melted away over 6000-10,000 years, so roughly at a rate of 1-2 m (about 3-6 ft) per century.
During the Eemian, which was the last interglacial and was centered about 127,000 years ago, sea level, as estimated from the elevations of beaches on a number of tropical islands, was about 4-6 m (13-20 ft) higher than at present. Reconstructions of the global average surface temperature suggest that it was perhaps 1-2°C (2-4°F) warmer than at present (perhaps a bit more in high northern latitudes) during this interglacial period, which was of quite short duration, and estimates indicate that the pace of sea level rise leading to that point was also near 1 m (3 ft) per century, and perhaps even a bit faster. Based on ice core evidence, it appears that Greenland was covered with only half as large an ice sheet as at present, suggesting that a significant fraction of the West Antarctic Ice Sheet must also have melted.
The last time that global average temperature was roughly 5°C (9°F) higher than at present was roughly 20 million years ago. At that time, there was virtually no ice stored on land, and sea level must therefore have been nearly 70 m (roughly 225 ft) above its present level (over 7 m from Greenland, 61 m from the West Antarctic and the rest from mountain glaciers). While some of this change was likely related to the continents being in different locations, that the IPCC projections of warming through the 21st century range from about 1-6°C suggests that society faces the risk of rises in sea level that will be much greater than the 0.15-0.2 m experienced during the 20th century. Indeed, over the last decade, the first global satellite estimate has sea level currently rising at a rate of about 0.3 m per century.
So, what about the future? In IPCC's Third Assessment Report, it was projected that sea level would rise between 0.09 and 0.88 m from 1990 to 2100. The most recent IPCC assessment, published in early 2007, based on improved estimates of the effects of warming on thermal expansion of ocean waters and melting of mountain glaciers, and the effects of altered balances in the build-up and decay of ice and snow, narrowed the range to 0.18-0.59 m, depending upon emission scenario and climate sensitivity. While it is important to note that the minimum estimate doubled, little reassurance should be taken from the upper estimate being lower because the IPCC, in each of these estimates, notes that it has left out some important terms. What IPCC has done in generating its estimate is to focus only on the terms that it believes can be effectively estimated, and it has left out the terms that climate models cannot yet represent well. As it turns out, these are just the terms that could cause rapid loss of mass from the ice sheets by (a) meltwater carving tunnels in the ice sheet and changing its density, and (b) meltwater lubricating the base of the glaciers and allowing faster rates of flow. IPCC suggests that these terms could add to the pace of sea level rise, but there is uncertainty about how much this might be, and so the terms were left out of the numerical estimates.
The three papers in Part 2 address a number of aspects of this issue, including evaluating the potential that the pace of change could speed up. In Chapter 4, Dr Claire Parkinson focuses on the increasing pace of reduction in the amount of sea ice, which, although this will not directly affect sea level, could dramatically affect the climate that would, in turn, affect the conditions experienced by mountain glaciers, ice caps and ice sheets, the melting of which would affect sea level. In Chapter 5, Dr Eric Rignot provides the latest information on the state of the Greenland Ice Sheet, indicating that it is losing mass at an increasing rate. In Chapter 6, Dr Robert Bindschadler reports on the state of the Antarctic Ice Sheet, also indicating that it seems poised for change.
While the newest IPCC estimate suggests that we likely have some time to deal with the climate change issue before significant sea rise occurs, the deteriorating state of the Greenland and West Antarctic ice sheets, the increasing pace of warming in high latitudes due to the retreat of sea ice, and the rapid rates of sea level rise evident in the geological record make it quite plausible that human-induced climate change could now be committing the world to much more rapid and greater sea level rise than had been considered likely less than a decade ago.
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