How much future sea level rise? More evidence from models and ice sheet observations.

Also in this week’s Science are two articles that further strengthen the case that ice sheets are quite sensitive to warming climate. A paper by Göran Ekström et al. shows that the increased speed of Greenland glaciers occurs in distinct lurches (observed as micro “ice-quakes”) that are strongly seasonal, with the greatest number occuring in late summer. This provides evidence that meltwater plays an important role in the acceleration of Greenland’s glaciers. Essentially, the idea is that surface melting that occurs in the summer can make its way quickly down to the glacier bed, lubricating the bed and allowing the glaciers to slide more rapidly. The “ice quakes” occur because the rough bedrock surface causes the glaciers to stick; they only accelerate when enough hydraulic pressure has built up to help float the glacier over the bumps. This is strong evidence that climate, not merely “internal ice sheet dynamics”, has contributed to the recent increases in Greenland’s glaciers. Indeed, a doubling of the rate of quakes has occurred over the past five years, just as the aerial extent of surface melting has increased.

Finally, in a very nice bit of work Velicogna and Wahr use data from the “Gravity Recovery and Climate Experiment” (GRACE) satellites to show that the Antarctic ice sheet has been losing mass at a rate of 150 +/- 80 km3 each year since 2002. That’s equivalent to about 0.4 mm of sea level rise each year. This is about twice other recent estimates, while IPCC 2001 actually gives negative 0.1 mm/yr. What is especially nice about Velicogna and Wahr’s study is that by using gravity measurements they have measured mass changes directly, avoiding the problem of virtually all previous measurements of ice sheet mass change, which usually measure either input (snowfall) or loss (calving, melting, or thinning of the ice), but not both at once.

What does all this news mean in practice? Reading the editorials in Science, and quotations from various researchers in newspaper articles, one might be under the impression that we should now expect “catastrophic sea-level rise” (as Science’s Richard Kerr writes). Of course, what is catastrophic to the eye of a geologist may be an event taking thousands of years. In the Otto-Bliesner et al. simulations, it takes 2000-3000 years for Greenland to melt back to its LIG minimum size. And while we don’t advocate sticking with the typical politician’s time frame of 4 or 5 years, the new results do not require us to revise projections of sea level rise over the next century or so. This is because even with Arctic temperature continuing to rise rapidly, there will still be significant delay before the process of ice sheet melting and thinning is complete. There is uncertainty in this delay time, but this is already taken into account in IPCC uncertainty estimates. It is also important to remember that the data showing accelerating mass loss in Antarctica and rapid glacier flow in Greenland only reflect a very few years of measurements — the GRACE satellite has only been in operation since 2002, so it provides only a snapshot of Antarctic mass changes. We don’t really know whether these observations reflect the long term trend.

On the other hand, none of the new evidence points in the direction of smaller rates of sea level rise in the future, and probably nudge us closer to the upper end of the IPCC predictions. Those who have already been ignoring or naysaying those predictions now have even less of a leg to stand on. Coastal managers, real estate developers, and insurance companies, at the least, would be wise to continue to take such predictions seriously.** As Don Kennedy and Brooks Hanson write in the lead Editorial, “accelerated glacial melting and larger changes in sea level should be looked at as probable events, not as hypothetical possibilities.”

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*Note that we don’t actually have good constrains on the rate of sea level rise from the penultimate glacial period (~140,000 years ago) to the last interglacial (LIG, ~125,000 years ago). However, we have very good data on the more recent glacial-to-interglacial transition, between about 14,000 and 7,000 years ago. During that time, sea levels rose at an average rate of about 11 mm/year, and at rates much higher than that for short intervals.

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