Tuesday, December 18, 2007

Marine Isotope Stage 5e

High rates of sea-level rise during the last interglacial period : Abstract : Nature Geoscience
Article : Nature Geoscience
Interview : Scitizen.com

Nature Geoscience has published an online Letter by E.J. Rohling, K. Grant, Ch. Hemleben, M. Siddall, B.A.A. Hoogakker, M. Bolshaw & M. Kucera. Their Letter concerns Marine Isotope Stage 5e, roughly from 124 thru 119 kiloyears ago, during the Eemian or last interglacial period.

In the near future humanity’s unburied carbon emissions are expected to raise global mean temperatures a minimum of 2 Celsius degrees. MIS-5e is the most recent period when our planet was already interglacial in climate, warmed comparably, and underwent changes in sea level.

Calibrating coral data with stable oxygen isotope measurements of central Red Sea sediment cores for “tight stratigraphic control” of the relative ages of core features, Rohling et al. were able to chart the sea rise rates and levels that occurred through MIS-5e. Their average rise rate finding was 1.6 meters per century, occasionally spiking to at least 2.5m per century. Detailed stratigraphic descriptions of coastal/reef architecture enabled Rohling et al. to postulate a +5m sea-level highstand 123 kyr ago, a +9m highstand 121.5 kyr ago, a lesser highstand 119.5 kyr ago, and sea-level drops in between them.

Rohling et al. equate this 1.6m rise rate with melting one Greenland-sized ice sheet every four centuries = just about double the highest rate predicted for the coming century by the recent Fourth Assessment Report of the Intergovernmental Panel on Climate Change. If the melts and highstands of our near future and those of MIS-5e are similar enough, these Rohling et al. findings will have joined a growing body of new indications that polar melt and sea rise may be speedier than the IPCC AR4 predicted.

So how similar will MIS-5e be to our future? Rohling et al. have presented the first detailed information on sea rise rates due to fluctuations within an interglacial. Previous work had focused instead on the sea rise or fall rates due to the transitions between glacial and interglacial periods; such transitional rates can top 5m per century. But the cause of MIS-5e warmth was orbital forcing by insolation, whereas the warming predicted for present and future comes from rapid atmospheric increase of carbon dioxide and other heat-holding gases. This disparity in cause, as pointed out by Rohling et al., could yield some divergent results: “MIS-5e ice-volume responses may have differed in detail from future responses.”

The MIS-5e average sea rise rate of 1.6 meters per century, if duplicated in the real near future, would seriously damage coastal cities and ecosystems. Yet, because today’s Earth has more ice to melt and faster warming to melt it than the Eemian Earth had, it may be the slowest average we can plausibly expect for the next hundred years and many more centuries to come. I think we have to treat that 1.6m figure as our lower bracket when we try to choose a rise rate for the Meltwater fictional geochronology.

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