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Misinterpreting a retraction of rising sea level predictions

Posted on 25 February 2010 by John Cook

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.

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Comments 101 to 107 out of 107:

  1. Oops. kwinters (67) wrote: "Though my analysis here is much too simplistic, it certainly helps convince me that Siddall's results appear quite reasonable. " I meant to say (Vermeer & Rahmstorf 2009) appears quite reasonable. I think Siddall was too low in his retracted paper (which is why he retracted it).
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  2. kwinters79, with your equations, the projected 2100 sea level rise for these three periods are Period1 => 73 cm Period2 => 84 cm Period3 => 148 cm The equations give your values by the year 2106 (with minor rounding errors) relative to the 6710.8 mm tide gauge level, which is indeed the average for the years between 1856-1876. Not 21 years though, only 20, for the year 1661 is missing (Civil war? The New York Draft Riots occurred in 1863. Some gadget from tide gauge needed for war? Personnel drafted?). However, we get a better view of the quality of data, if we calculated projected 2106 sea level rise based on some more subperiods. I also supply a second column, sea level rise relative to the 1990-2008 average. It is 37.4 cm higher than your reference level, but it has already happened in New York and the city is still alive. 1900-1920: +15722 mm +15348 mm 1900-1930: -1711 mm -2085 mm 1900-1940: +2566 mm +2192 mm 1900-1950: +2313 mm +1939 mm 1900-1960: +1792 mm +1418 mm 1900-1970: +1204 mm +831 mm 1900-1980: +817 mm +443 mm 1900-1990: +557 mm +184 mm 1900-2000: +696 mm +322 mm 1900-2008: +675 mm +302 mm 1910-2008: +596 mm +223 mm 1920-2008: +534 mm +160 mm 1930-2008: +511 mm +138 mm 1940-2008: +754 mm +380 mm 1950-2008: +879 mm +505 mm 1960-2008: +1155 mm +782 mm 1970-2008: +1588 mm +1214 mm 1980-2008: -184 mm -557 mm 1990-2008: -2862 mm -3236 mm Had we faith in this method, the 1980-2008 trend would project an almost two feet sea level drop by the year 2106 and more than 3 m (10 feet) if the judgment is bases on the last two decades. Quite scary, ships stranding at low tide, New York harbor gets unusable. On the other hand, if we only saw data for 1900-1920, would fancy a 15 m rise by 2106. Manhattan submerged. Whenever trends are so sensitive to endpoint selection it does not make much sense to use them. To illustrate it, I give 2106 sea level projections with a finer resolution, relative to present (1990-2008 average). 1970-2008: +1214 mm 1972-2008: +1142 mm 1974-2008: +388 mm 1976-2008: -136 mm 1978-2008: +397 mm 1980-2008: -557 mm Depending on endpoint selection, sea level would either rise by four feet or drop by two. If we consider the 1927-1991 period, for which tide gauge data seem to be most reliable, we get a 64.7 cm drop by 2106 relative to present. It is reasonable to use as much data as one has got. Based on the period 1856-2008, projected sea level rise is 38.2 cm (relative to present). It is basically a linear rate of 2.3 mm/year (acceleration is negligible, 6.7 micron/yr^2). It is not much, considering the difference between low and high tides at the Battery can easily exceed 6 feet. We can see "recent rate of sea level rise acceleration" is meaningless if it is based on a single site. However, for the US alone one can readily find 2523 more gauges. The PSMSL has 1166 gauges, 75 US sites. Looks like if one is not interested in the deep past, at least ten times more gauges could be used (e.g. for the last two or three decades). It is just a data collection problem, which should be easy and cheap in the Internet age.
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  3. Berényi Péter at 23:45 PM on 28 February 2010 That was a nice illustration, thanks. The big unknown here and one that has proven tough to model is of course what's going to happen to major ice sheets in coming years. Prognostications of sea level rise based on what's happened so far are missing this important component. Progress is being made in modeling ice sheet behavior but from my reading so far it's a tricky task because each ice sheet is arranged differently as to details. We'll see! Some of us, anyway...
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  4. Martin Vermeer (who along with Stefan Rahmstorf uncovered the errors which led to the retraction of the Siddall paper) gives a correct explanation of the problem with the paper. Apparently my explanation isn't wrong but "a separate and interesting issue but not significant" (compared to the BIG erro)r.
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  5. Dr. Nils-Axel Mörner has studied sea level and its effects on coastal areas for some 35 years. Recently retired as director of the Paleogeophysics and Geodynamics Department at Stockholm University, Mörner is past president (1999-2003) of the INQUA Commission on Sea Level Changes and Coastal Evolution, and leader of the Maldives Sea Level Project. The uncompromising verdict of Dr Mörner is that all this talk about the sea rising is nothing but a colossal scare story - Sea Levels Are Not Rising! Read more at
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  6. climatejournal at 15:12 PM on 1 March, 2010 Depending on which of his publications you read, Dr. Mörner concludes either that sea level is rising or is not, though of late he seems to have settled on no rise at all. This is at variance with -all- instrumental records, and meanwhile Dr. Mörner does not explain how he reaches these his own disparate conclusions. Let me be careful to point out that I'm not engaging in ad hominem attack but rather am trying to establish context when I refer to Dr. Mörner's publications on dowsing to suggest that he is not necessarily the most reliable source for the final word on scientific matters, though he does in fact have an extensive publication record on less controversial topics. Finally, I find it ironic that Dr. Mörner's title for his essay which is popular in some circles, "The Greatest Lie Ever Told", would be unacceptable in the sort of polite company John Cook has assembled here.
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  7. RealClimate has a new post about predictions of sea level rise, which refers to the retraction of the Siddell paper. Stefan is trying to contrast the bizarre and distorted way that the media has been hyping various trivial and/or misrepresented claims of errors in the IPCC AR4, but has completely ignored cases where the AR4 is "erring" on the side of underestimating the severity of impacts from AGW. As Stefan points out, there are internal pressures within IPCC to be "conservative" and to risk understating rather than overstating most claims. This is something that very few outsiders seem to understand.
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