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Sea level rise: not so fast

In the latest salvo of the scientific debate over future sea level rise, a new report counters claims that rapidly swelling seas will soak estimates published by the UN climate planel in 2007.

A major “it’s worse than we thought” story out of March’s Copenhagen Climate Congress, for example, was that sea level could climb more than a metre by 2100 – seemingly far worse than the rise of up to 59 centimetres indicated in the 2007 report from the Intergivernmental Panel on Climate Change (IPCC). This was in fact something of a straw-man comparison, since the IPCC total explicitly excluded the impacts of accelerated glacier melt, and the new studies were attempting to add these impacts in.

But the latest study suggests that even considering glacier effects, the 2100 rise is likely to be well under a metre. A trio of researchers – Mark Siddall of Columbia University in New York, Thomas Stocker of the University of Bern (current co-chair of IPCC Working Group I) and Peter Clark of Oregon State University – used a new method that looks to the past to inform this future projection.

They built a simple model that simulates sea level changes over the last 22,000 years, as reconstructed from fossil corals. The model accurately reflects observed changes in sea level from that date right through the twentieth century, and includes the contribution from glaciers. When they turned to the twenty-first century, their model showed a 7- to 82-centimetre rise by 2100, based on a range of IPCC emissions scenarios expected to produce temperature rises of 1.1-6.4 degrees centigrade. The study was published this week in Nature Geoscience; there’s a nice news report on it over at AFP.

The group points out that the IPCC tentatively said that accelerating glacier melt could bring the total rise as high as 76 centimetres. They write that their 7- to 82-centimetre range “is slightly larger than the estimates from the IPCC models of 18-76 cm, but is sufficiently similar to increase confidence in the projection.”

Though their projections of sea level rise are on the low end, the extra ocean they project is nothing to sneeze at, as Siddall tells AFP:

“Fifty centimetres (20 inches) of rise would be very, very dangerous for Bangladesh, it would be very dangerous for all low-lying areas. And not only that, the 50 centimetres (20 inches) is the global mean. Locally, it could be as high as a metre (3.25 feet), perhaps even higher, because water is pushed into different places by the effect of gravity.”

The model also shows that the long-term effects of twentieth-century warming will continue to push up seas for many centuries beyond 2100.

The research goes into the bubbling stew of recent sea level rise projections. The IPCC met earlier this month to plan their next assessment report, due in 2014, and according to chair Rajendra Pachauri this is one of the foremost questions that assessment will try to sort out.

Anna Barnett


  1. Report this comment

    sidd said:

    The timescale governing sea level rise response to temperature forcing is fitted as 3KYr. The ANDRILL data indicate WAIS collapse in the order of 1KYr. Can we reconcile these estimates? Although admittedly the subject paper does not cover the ANDRILL timescales.

    Is it possible that either there is something like the Weertman instability absent in the last 22KY BP? Or along the lines of the Hansen argument, Siddall et al. have modelled the timescale of the external forcing and not the internal timescale of icesheet collapse, especially given forcings as large as we see and project today ?

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    Supriyono said:

    I have observed that the cooling of the soil surface begun in the early of 80s and may be long before it and maximum cooling observed in the late of 1997, where between 1982-1997 the soil surface drop into lowest level, 20 degrees, and the impacts not so simple, the hydrologycal cycle changed, most of the rain drop not in the usual places anymore but in the lower altitudes so the land slided & the water floods back to the sea, sea level rise & warmer reachs the polar area

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