Water, water everywhere...

Two new studies on sea level rise appearing this week deserve our attention, one in Nature Geoscience, the other in Science. Both conclude the IPCC's estimates of no more than 59 cm of rising waters by 2100 should be tossed out the window. We've been hearing this since early 2007, when the latest IPCC reports were released ;;;; specifically, complaints that the cut-off point for research that could be considered was early 2006, and so much has come out since then that we really shouldn't be paying too much attention to IPCC numbers. But most of those comments were based on observations that hadn't yet been synthesized, analyzed and peer-reviewed. Now they have. And it's not good news.

First, the Nature Geoscience study, "Rapid early Holocene deglaciation of the Laurentide ice sheet" by Anders E. Carlson of the University of Wisconsin, along with colleagues from NASA, the Woods Hole Oceanographic Institution, California Institute of Technology, University of British Columbia, Vancouver and University of New Hampshire. Their study of what might happen to Greenland's ice sheet (GIS) is based on what happened to the Laurentide ice sheet, which covered most of Canada 10,000 years ago. It concludes that "current projections of GIS melt rates for the coming century may be only minimum estimates even without considering positive feedbacks from ice-sheet dynamics."

The Laurentide sheet was much bigger, and the authors acknowledge the difficulties in comparing it with Greenland today. Still, their calculations show that by the end of this century, if we don't stop pushing up the planet's temperature, melting Greenlandic ice could result in an increase in sea level of about 1.3 meters. "We conclude that we could be grossly underestimating how much the Greenland ice sheet could melt by the end of this century," Carlson told Nature's Amanda Leigh Haag.

And that would be very bad, flooding something like a quarter of Bangladesh, for example.

The Science paper, "Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise" by W. T. Pfeffer at the University of Colorado and colleagues at the University of Montana and the University of California San Diego, takes a bigger-picture approach. Their study comes with even more caveats, trying to predict the consequences of all sources of melting ice, including Greenland, Antarctic, alpine glaciers and good old-fashioned thermal expansion of already-liquid water. As a result they come up with a wide range of possibilities, but even their low-end estimate of what we could see by 2100 is frightening: "we suggest that an improved estimate of the range of SLR to 2100 including increased ice dynamics lies between 0.8 and 2.0 m."

Greenland, according to Pfeffer's group, Greenland contributes only between 16 and 54 cm to the total. Antarctic meltwater between 15 and 62 cm, glaciers 4-47 cm and thermal expansion another 30 cm.

So there's some disagreement between the two, but both suggest that we're looking at serious problems in the not-too-distant future. The good news, at least, according to Pfeffer et al, is that "increases in excess of 2 meters are physically untenable" this century. There are those who will disagree with such absolutist language. But that's not the point. Just one meter by 2100 would be an "unmitigated catastrophe for the planet" as Joe Romm describes it. Even 80 cm would make life difficult for most coastal communities.

There's plenty of work left for glaciologists, of course. In the requisite plea for more research funding that is inevitably tagged onto every paper, Pfeffer et al write:

We emphasize that assumptions made to arrive here contain substantial uncertainties, and many other scenarios and combinations of contributions could be considered.... Hence, these values give a context and starting point for refinements.... Without better knowledge of the number, size, and catchment areas of marine-based outlet glaciers in the GIC category, improvements on the estimates made here will be very difficult.

The only thing I would add is, sea level isn't the only negative effect from global warming. Even if the ice caps managed to remain in the solid phase, we'd still have acidification of the oceans and consequent loss of critical elements of the marine ecosystems, including corals; biodiversity decline; drought-related crop failure and associated problems feeding people; and more destructive hurricanes. For starters.

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Anders E. Carlson, Allegra N. LeGrande, Delia W. Oppo, Rosemarie E. Came, Gavin A. Schmidt, Faron S. Anslow, Joseph M. Licciardi, Elizabeth A. Obbink (2008). Rapid early Holocene deglaciation of the Laurentide ice sheet Nature Geoscience, 1 (9), 620-624 DOI: 10.1038/ngeo285

W. T. Pfeffer, J. T. Harper, S. O'Neel (2008). Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise Science, 321 (5894), 1340-1343