Sunday, February 08, 2009

Levelling the science of sea-level rise

One of my biggest pet peeves with the climate change communication world is widespread use of sea-level rise ‘maps’. There are countless maps and animations out there. Think the simulated flooding of New York in An Inconvenient Truth or, for a local example, the Sierra Club’s post-Greenland map of Vancouver in which my home becomes coveted waterfront property. Anyone with a digital elevation dataset and some GIS skills can draw a map of what land will “disappear” if the sea level rises by 6 m, or 6 km for that matter.

Leave aside for now the uncertainty about future rates of sea-level rise, the usual beef with graphic representations of sea level rise. Spend a few weeks in a coral atoll and you’ll know the real problem with these simplistic graphics. The sea is not actually level. And there’s no scientific reason to think that the rise will be.
A part of this issue is tackled in a terrific new paper by Mitrovica et al. in last week’s Science.

The paper estimates the regional variation in sea level rise that would occur from the melting of the West Antarctic Ice Sheet (WAIS). Unlike the brute force mapping exercises, Mitrovica et al. consider the actual physics of the ice melt, including gravitational attraction of the ice sheet, migration of shorelines and the effect of all that ice on the Earth’s rotations. For example:

The rapid melting of ice sheets and glaciers leads to a sea-level change that departs dramatically from the assumption of a uniform redistribution of meltwater (4). An ice sheet exerts a gravitational attraction on the nearby ocean and thus draws water toward it. If the ice sheet melts, this attraction will be reduced, and water will migrate away from the ice sheet. The net effect, despite the increase in the total volume of the oceans after a melting event, is that sea level will actually fall within ~2000 km of the collapsing ice sheet and progressively increase as one moves further from this region. Each ice reservoir will produce a distinct geometry, or fingerprint, of sea-level change... Although the physics of fingerprinting has been embraced in studies of past sea-level change, it has been largely ignored in discussions of future projections.

The conclusion which drew media attention (the Globe and Mail) and a bold-ed and italicized post from Joe Romm is that sea-level rise from melting of WAIS will be higher along will the coastlines of North America, including cities like Washington, DC, New York, and Vancouver.

The media coverage is not wrong, but misses the point. The paper demonstrates how melting of an ice sheet leads to uneven sea level rise, using WAIS as an example. In reality, if all of WAIS were to melt, so presumably would some or all of the Greenland Ice Sheet, the East Antarctica Ice Sheet and the mountain glaciers. The melting of all that other ice would also influence the geography of sea level rise. The take home message of the paper is not we need to start sandbagging along English Bay beaches here in Vancouver. The message is that the scientific community – and I assume by extension the environmental community – needs to remember that the oceans’ rise will not be level. The concluding sentences of the paper:

Any robust assessment of the sea-level hazard associated with the loss of major ice reservoirs must, of course, account for other potential sources of meltwater, namely Greenland, the East Antarctic, and mountain glaciers. Nevertheless, future projections should avoid simple, eustatic estimates and be based on a suitably complete sea-level theory.

Amen to that.

[You can give thanks for this post to my father, who notified me of the Mitrovica et al. press coverage, and who many of you know through the macro-economic advice he periodically delivers in the Toronto Star, The Globe and Mail and on the CBC. Forget my deranged comments on economics – he is the one they really should be calling to fix the federal budget.]


Anonymous said...

I made the same point in the comments at Climate Progress.

That said, the Mitrovica et al results might not be that far from reality (or at least closer than a "flat" calculation) if it's the case that the WAIS could be lost without major contributions from the EAIS or GIS (recognizing that this would be a very temporary situation with respect to the latter). That sounds plausible, but I suppose MIS-11 could be a partial argument against (unless maybe the pattern and timing of melt would be different enough now, e.g. because of warm current undermining of the WAIS).

(Just so we're clear, my expertise on this is zilch.)

Anonymous said...

Are there any land/sea maps out there that show what land is exposed, by lower sea levels?

Simon Donner said...

Anonymous - most climate or anthropology textbooks will have a figure of land exposed during the last glacial maximum (21 kya)