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Sea Level Changes of the Past: Portents of the Future?
PALSEA (the PALeo SEA level working group: Abe-Ouchi, A., Andersen, M., Antonioli, F., Bamber, J., Bard, E., Clark, J., Clark, P., Deschamps, P., Dutton, A., Elliot, M., Gallup, C., Gomez, N., Gregory, J., Huybers, P., Kawamura, K., Kelly, M., Lambeck, K., Lowell, T., Mitrovica, J., Otto-Bleisner, B., Richards, D., Siddall, M., Stanford, J., Stirling, C., Stocker, T., Thomas, A., Thompson, W., Torbjorn, T., Vazquez Riveiros, N., Waelbroeck, C., Yokoyama, Y. and Yu, S.) 2009. The sea-level conundrum: case studies from palaeo-archives. Journal of Quaternary Science 25: 19-25.

Climate alarmists such as Al Gore and James Hansen have long talked about a warming-induced global sea level rise over the 21st century being measured in meters. But just how likely is an increase of that magnitude?

What was done
In an effort to place some reasonable potential limits (both high and low) on possible future sea-level rise, members of the PALeo SEA Level Working Group (PALSEA) recently looked to the past for some answers.

What was learned
Turning to the IPCC -- the most recent report of which predicts a global warming of somewhere between 1.1 and 6.3C for the 21st century -- the PALSEA group writes that "the last time that a global warming of comparable magnitude occurred was during the termination of the last glacial period," which consisted of "a series of short, sharp steps on millennial to centennial timescales." Hence, they looked at what is known about sea level change during the Bolling-Allerod and post-Younger Dryas/early Holocene periods, because they say "the magnitude and rate of warming during these periods are most closely analogous to the magnitude and rate of anthropogenic warming [that is predicted to occur] over the coming centuries;" and this comparison ruled out any type of exponentially increasing sea level response, pointing more towards an asymptotic response, where the sea level rise is high initially but gradually levels off.

For even greater realism, the PALSEA team next turned to warm periods of the Holocene, since the earth is now at a much higher "starting" temperature than it was during the termination of the last great ice age. Considering what is known about eustatic sea level between 9 and 8.5 ka BP and between 7.6 and 6.8 ka BP (increases of 1.3 and 0.7 m per century, respectively), they state that a "rapid demise of ice sheets in a climate similar to today is certainly a possibility," but that "an improved understanding of ice sheet dynamics is required before one can conclude that the Greenland or West Antarctic ice sheets will behave in a similar fashion in the future."

Turning finally to previous interglacials, the 32-member research group notes that some studies have placed peak sea levels during the last interglacial period somewhere in the range of 3-6 m above modern sea level about 126 ka BP, but only "several thousand years after proxy records of temperature reached interglacial levels."

What it means
The PALSEA scientists say that "using palaeo-data and direct observations, it is possible to put loose limits on just how rapidly we might expect sea-level rise to occur over the next century" if the worst-case warming scenario of the IPCC were actually to occur, placing the projected rise somewhere between the lower limit of 20th-century sea-level rise (0.12 m per century) and the sea-level rise at the conclusion of the termination of the last glacial period (1 m per century). Interestingly, this range significantly exceeds (at the high end) that reported in the IPCC's Fourth Assessment Report (-0.01 to 0.17 m over the current century); but it is still a far cry from the multiple "meters" suggested by the world's two most outspoken climate alarmists.

Reviewed 14 April 2010