Could a Little Warming Raise Oceans by 20 Feet?

Based on the Earth’s history, it’s possible.

THOSE OF you who followed the news last week may have seen disturbing headlines warning that we may be headed for ocean levels a full 20 feet (6 meters) higher than they are at present — this article from CBS News was typical. The odd thing about this round of press is that we already more or less knew this; there was no actual news.

The articles were prompted instead by a scientific review of what we know about the relationship between rising temperatures and rising oceans that has occurred in the past. Rather than performing a new analysis, the review simply looked at existing studies in the literature and tried to pull them all together into a single, coherent picture. And that picture suggests that ocean levels have been much higher following only a slight rise in temperature.

The problem is that all of the periods that might be informative about the present warming occurred at least tens of thousands of years ago. We can’t directly measure things like temperature, ocean levels, or ice sheet volumes. Instead, we have to infer these things from proxies: measurements we can make from samples preserved from this time.

For example, heavier isotopes of oxygen tend to end up in ice more readily, so changes in their presence can help provide an indication of how much of the world’s water is locked up in ice sheets. And gases trapped in ice cores provide some hint of the climate and a direct measure of the greenhouse gases in the atmosphere. You can also get a relatively direct measure of past ocean levels from the location of corals or, in some cases, the presence of past shorelines. From all of these, it’s possible to build a picture of the climate, the ocean levels, and how much water was locked up in the ice sheets. But the error bars on all of this are rather large.

Still, it’s rather important we try to find out. Melting lots of ice takes a long time, and we’re still not sure how much additional melting we’ve already committed to. An understanding of this relationship in the past can help us understand both that and where we’re likely to be headed. As the authors put it, “Given the recent increases in greenhouse gases and global mean temperature, the present ice sheets are out of equilibrium with the climate, raising important questions regarding their potential future contribution to sea-level rise.”

The authors looked at three different warm periods. One was the Mid-Pliocene warm period, which occurred three million years ago, when atmospheric CO₂ levels were similar to their present values. Most climate records suggest temperatures were a few degrees warmer than present (between 1 and 4 degrees Celsius), but some Arctic records suggest elevated temperatures of up to 8 degrees Celsius, which could have severe consequences for the Greenland ice sheet. Coastal records suggest sea levels rose by more than 15 meters, while models of the ice sheets in Greenland and Antarctica suggest those contributed 13m to this rise.

The second period considered was an unusually long interglacial period about 400,000 years ago. Greenhouse gases reached levels similar to those found in preindustrial times, while temperature estimates suggest a climate that was 1 to 2 degrees Celsius warmer than present. But pollen suggests that southern Greenland was covered in a spruce forest, and sea level estimates suggest a rise of at least 6 meters — the source of the 20 foot rise that made the headlines.

Another interglacial, coming about 120,000 years ago, saw global temperatures only slightly warmer than present, but the Arctic and Antarctic appeared to be unusually warm (in the area of 5 degrees Celsius). Sea levels again reached at least six meters above present. And some records suggest fluctuations of a meter or so occurred once the high point was reached.

So, most of these estimates suggest we’re in for a lot higher sea level given that we’re committed to emitting enough carbon to easily surpass the temperatures involved in the past warm periods. But it’s not entirely that simple. Those warm periods were caused by changes in the pattern of sunlight that brings more to the Northern Hemisphere — potentially explaining the high Arctic temperatures and the severe impacts on Greenland. (That doesn’t, of course, explain the evidence of high Antarctic temperatures.) And, as noted above, the error bars are still quite large.

At the same time, the uncertainty shouldn’t be viewed as cause for comfort. Most of the 6 meter figures that seem so dramatic come from the low end of the uncertainty ranges.

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Source: Ars Technica