Misinterpreting a retraction of rising sea level predictions

A new skeptic argument has emerged that upon close inspection, is a polar opposite to the scientific reality. This week, scientists who published a 2009 paper on sea level rise retracted their prediction due to errors in their methodology. This has led some to claim sea levels are no longer predicted to rise. This interpretation was helped no doubt by the unfortunate Guardian headline "Climate scientists withdraw journal claims of rising sea levels". However, when you read the article and peruse the peer-reviewed science on future sea level, you learn that the opposite is the case.

The IPCC 4th Assessment Report predicted sea level will rise between 18 to 59 cm by the year 2100. Many consider this a conservative estimate as observed sea level rise is tracking at the top range of IPCC estimates (Rahmstorf 2007, Allison 2009). However, a study led by Mark Siddall examined how sea levels have changed over the past 22,000 years in response to temperature change (Siddall 2009). This enabled them to predict how sea level would respond to future warming, estimating sea level rise between 7 to 82 cm by the year 2100. Siddall's paper concluded that this increased confidence in the IPCC projections.

However, a later study using similar methods to Siddall 2009 came to dramatically different results, estimating sea level rise of 75 to 190 cm by 2100 (Vermeer & Rahmstorf 2009). Why the discrepancy? Judging by the acknowledgement in Siddall's retraction, one speculates that Vermeer and Rahmstorf discovered flaws in Siddall's methodology and notified the authors. Siddall saw that the errors undermined their results and retracted their paper. So we have two papers using similar methods - one predicting low sea level rise, the other predicting high sea level rise. The low sea level rise is found to be in error. While some are spinning this result to imply no sea level rise, in actuality it increases our confidence in high sea level rise.

Vermeer's results are confirmed by another study that approach the sea level question from a different angle, examining the dynamics of calving glaciers (Pfeffer 2008). The conclusion was a predicted sea level rise of 80 cm to 2 metres by 2100. Further evidence of the ice sheets' high sensitivity to warmer temperature comes from paleoclimate studies of the last interglacial period 125,000 years ago. At that time, global temperatures were around 2 degrees warmer than now. This is the amount of warming expected for some of the lower emission scenarios. At that time of the last interglacial, sea levels were at least 6 metres higher than present levels. So while we expect sea levels to rise up to 2 metres by 2100, they will continue to rise afterwards to at least 6 metres.

Future sea level rise will be one of the most serious impacts of global warming on humanity, with much of the world's population clustered around coastlines. That my daughter will see sea level rise of 1 to 2 metres in her lifetime is for me, an unhappy prospect. This scientific reality is a stark contrast to the 'Now You Can Forget About Those Rising Seas' attitude. Despite the serious picture painted by the peer-reviewed science, these kinds of misinterpretations turn the climate debate in an almost farcical direction. One could blame the Guardian for a carelessly worded headline. More blame should be apportioned to those who pontificate from their soapboxes without bothering to acquaint themselves with the science. That skeptics allow themselves to be tossed and turned by media headlines is the very antithesis of genuine skepticism.

Posted by John Cook on Thursday, 25 February, 2010

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