How high and how fast?

How high and fast will sea levels rise? An important piece (PDF) by Stefan Rahmstorf in Science concludes:

A rise of over 1 m by 2100 for strong warming scenarios cannot be ruled out, because all that such a rise would require is that the linear relation of the rate of sea-level rise and temperature, which was found to be valid in the 20th century, remains valid in the 21st century.

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These scenarios, which are really nothing more than business-as-usual emissions plus amplifying carbon-cycle feedbacks, would give us sea level rising at 6 inches a decade in 2100. In such an inundated world, “adaptation” is almost meaningless term.

And this isn’t even the worst case scenario, because ice sheet dynamics are clearly non-linear, as NASA’s James Hansen makes clear in a new article (PDF).

But didn’t the UN’s Intergovernmental Panel on Climate Change just lower their projections for sea level rise in the recently-released Fourth Assessment Report (AR4)? No. That mistaken view was the result of a lot of misreporting by the media. Rahmstorf clears up the confusion in a lengthy post on Realclimate, which concludes:

The main conclusion of this analysis is that sea level uncertainty is not smaller now than it was at the time of the TAR [Third Assessment Report], and that quoting the 18-59 cm range of sea level rise, as many media articles have done, is not telling the full story. 59 cm is unfortunately not the “worst case”. It does not include the full ice sheet uncertainty, which could add 20 cm or even more. It does not cover the full “likely” temperature range given in the AR4 (up to 6.4 degrees C) — correcting for that could again roughly add 20 cm. It does not account for the fact that past sea level rise is underestimated by the models for reasons that are unclear. Considering these issues, a sea level rise exceeding one metre can in my view by no means ruled out.

The subject is a complicated one, and the IPCC has not done a good job of clearing up the confusion. But everyone needs to become knowledgeable on this potentially devastating climate impact.