In a paper by Jim Hansen that we discussed last week, he called for a consensus statement to be issued on global warming, West Antarctica and sea level rise, from relevant scientific experts. A group of scientists have beat him to the punch issuing a consensus statement last week:

Polar ice experts from Europe and the United States, meeting to pursue greater scientific consensus over the fate of the world’s largest fresh water reservoir, the West Antarctic Ice Sheet, conclude their three-day meeting at The University of Texas at Austin’s Jackson School of Geosciences with the following statement:

Read the whole thing, but the take home point is that there remain substantial uncertainties, as indicated in the following parts of the consensus statement:

*Satellite observations show that both the grounded ice sheet and the floating ice shelves of the Amundsen Sea Embayment have thinned over the last decades.

*Ongoing thinning in the grounded ice sheet is already contributing to sea-level rise.

*The thinning of the ice has occurred because melting beneath the ice shelves has increased, reducing the friction holding back the grounded ice sheet and causing faster flow.

*Oceanic changes have caused the increased ice-shelf melting. The observed average warming of the global ocean has not yet notably affected the waters reaching the base of the ice shelves. However, recent changes in winds around Antarctica caused by human influence and/or natural variability may be changing ocean currents, moving warmer waters under the ice shelves.

*Our understanding of ice-sheet flow suggests the possibility that too much melting beneath ice shelves will lead to “runaway” thinning of the grounded ice sheet. Current understanding is too limited to know whether, when, or how rapidly this might happen, but discussions at the meeting included the possibility of several feet of sea-level rise over a few centuries from changes in this region.

What is the policy significance of this statement?

1. Decisions about climate change will have to be made is the face of considerable uncertainties as those related to sea level rise will not be reduced anytime soon.

2. There is the “possibility of several feet of sea-level rise over a few centuries from changes in this region.” This contrasts strongly with Jim Hansen’s assertion that

“Spatial and temporal fluctuations are normal, short-term
expectations for Greenland glaciers are different from long-term expectations for West Antarctica. Integration via the gravity satellite measurements puts individual glacier fluctuations in proper
perspective. The broader picture gives strong indication that ice sheets will, and are already beginning to, respond in a nonlinear fashion to global warming.There is enough information now, in my opinion, to make it a near certainty that IPCC BAU climate forcing scenarios would lead to disastrous multi-meter sea level rise on the century time scale” (PDF).

Who will eventually be proven correct? I have no idea. Nor do I think that it matters at all from the perspective of policy (compare Naomi Oreskes’ views on this subject here) . The significance of this difference in views has more to do with issue of scientific advice than sea level rise itself — if scientists create an expectation that our decision making should be guided by consensus views of relevant experts, then they should take care when abandoning that approach to scientific advice when less politically convenient.

3. Bottom line: Sea level rose about 20 cm over the past century. The IPCC expects that it may rise from less than that amount to about 60 cm over the next century. RealClimate says they see another 40 cm possible in the IPCC report. And there is a long thin probability tail of higher amounts with low probability and high consequences.

So, What are the Next Steps?

**Mitigation actions can have only a small effect on sea level rise in the short term (e.g., from no impact decades hences to small impacts a century out) and thus respond primarily to the longer term threats of sea level rise more than a century in the future.

**The most (and only) effective responses to sea level rise over the century timescale will be adaptive. So once again we see that mitigation and adaptation respond to fundamentally different aspects of the climate issue. Mitigation and adaptation should occur in parallel.

**Mitigation and adaptation policies to in response to sea level rise have been looked at (notably in the work of Richard Tol, e.g., here), but far more needs to be done (e.g., as argued here in PDF and here in PDF). Sea level rise has, like so many aspects of the climate issue, fallen into the trap of the “prediction game” at the expense of research on the possible consequences of alternative policies in response to that which we already know. The sea level rise issue risks becoming simply a “poster child” for mitigation rather than an issue deserving more attention itself.

So my recommendation is: Rather than arguing over which predictions of the climate future (or which scientists) are correct, we know enough about the general trend in sea level rise and the possibility of a large increase to begin thinking and talking in far more depth about a wide range of possible policy responses, especially adaptation. Such discussions have to go beyond the simplistic use of future sea level rise as a argument to mitigate. Under any scenario of mitigation the adaptation challenge remains essentially the same, and similarly under any adaptation efforts the mitigation challenge does not change. Absolutely no science on the prediction of future sea level rise will alter this basic reality.