Stefan Rahmstorf and the consensus of experts on sea level -vs- reality, reality wins

From Stefan Rahmstorf and the Potsdam Institute for Climate Impact Research (PIK)

Expert assessment: Sea-level rise could exceed 1 meter in this century

In contrast, for a scenario with strong emissions reductions, experts expect a sea-level rise of 40-60 centimeters by 2100 and 60-100 centimeters by 2300. The survey was conducted by a team of scientists from the USA and Germany.

“While the results for the scenario with climate mitigation suggest a good chance of limiting future sea-level rise to one meter, the high emissions scenario would threaten the survival of some coastal cities and low-lying islands,” says Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research. “From a risk management perspective, projections of future sea-level rise are of major importance for coastal planning, and for weighing options of different levels of ambition in reducing greenhouse-gas emissions.”

Projecting sea-level rise, however, comes with large uncertainties, since the physical processes causing the rise are complex. They include the expansion of ocean water as it warms, the melting of mountain glaciers and ice caps and of the two large ice sheets in Greenland and Antarctica, and the pumping of ground water for irrigation purposes. Different modeling approaches yield widely differing answers. The recently published IPCC report had to revise its projections upwards by about 60 percent compared to the previous report published in 2007, and other assessments of sea-level rise compiled by groups of scientists resulted in even higher projections. The observed sea-level rise as measured by satellites over the past two decades has exceeded earlier expectations.

Largest elicitation on sea-level rise ever: 90 key experts from 18 countries

“It this therefore useful to know what the larger community of sea-level experts thinks, and we make this transparent to the public,” says lead author Benjamin Horton from the Institute of Marine and Coastal Sciences at Rutgers University in New Jersey. “We report the largest elicitation on future sea-level rise conducted from ninety objectively selected experts from 18 countries.” The experts were identified from peer-reviewed literature published since 2007 using the publication database ‘Web of Science’ of Thomson Reuters, an online scientific indexing service, to make sure they are all active researchers in this area. 90 international experts, all of whom published at least six peer-reviewed papers on the topic of sea-level during the past 5 years, provided their probabilistic assessment.

The survey finds most experts expecting a higher rise than the latest IPCC projections of 28-98 centimeters by the year 2100. Two thirds (65%) of the respondents gave a higher value than the IPCC for the upper end of this range, confirming that IPCC reports tend to be conservative in their assessment.

The experts were also asked for a “high-end” estimate below which they expect sea-level to stay with 95 percent certainty until the year 2100. This high-end value is relevant for coastal planning. For unmitigated emissions, half of the experts (51%) gave 1.5 meters or more and a quarter (27%) 2 meters or more. The high-end value in the year 2300 was given as 4.0 meters or higher by the majority of experts (58%).

While we tend to look at projections with a focus on the relatively short period until 2100, sea-level rise will obviously not stop at that date. “Overall, the results for 2300 by the expert survey as well as the IPCC illustrate the risk that temperature increases from unmitigated emissions could commit coastal populations to a long-term, multi-meter sea-level rise,” says Rahmstorf. “They do, however, illustrate also the potential for escaping such large sea-level rise through substantial reductions of emissions.”

Article: B. P. Horton, S. Rahmstorf, S. E. Engelhart, A.C.Kemp: Expert assessment of sea-level rise by AD 2100 and AD 2300. Quaternary Science Reviews (2013). [doi: 10.1016/j.quascirev.2013.11.002]

Link to the article when it goes online: http://dx.doi.org/10.1016/j.quascirev.2013.11.002

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The basic premise of Stefan Rahmstorf’s claim is that sea level rise will accelerate before the end of the century. So far there has been no evidence of acceleration, it appears entirely linear no matter whether we look at tide gauges or satellite measurements.

The image below (From Holgate 2007 On the decadal rates of sea level change during the twentieth century in GRL) shows Holgate’s reconstruction of the sea level rise rate for the 20th century from the highest quality tide gauge data.

As you can see, the sea level rise rate widely varied during the 20th century.  It reached about 4 mm/year around 1911, and again in the 1930s, 1950s and around 1980.   It was much lower in the 1920s, 1940s, 1960s and mid-1980s.

Holgate concludes:

Based on a selection of nine long, high quality tide gauge records, the mean rate of sea level rise over the period 1904–2003wasfoundtobe1.74±0.16mm/yr after correction for GIA using the ICE-4G model [Peltier, 2001] and for inverse barometer effects using HadSLP2 [Allan and Ansell, 2006]. The mean rate of rise was greater in the first half of this period than the latter half, though the difference in rates was not found to be significant. The useof a reduced number of high quality sea level records was found to be as suitable in this type of analysis as using a larger number of regionally averaged gauges.

For satellite measurements there also doesn’t seem to be any acceleration.

German veteran meteorologist Klaus-Eckart Puls has done an analysis of sea level rise. Contrary to many claims, we see that sea level rise has decelerated markedly since 2003.

– See more at: http://notrickszone.com/2012/12/06/meteorologist-klaus-eckart-puls-sea-level-rise-has-slowed-34-over-the-last-decade/#sthash.h1npSYgJ.dpuf

So, neither tide gauges nor satellite measurements suggest acceleration is occurring. Even if we use the worst case value, 3.2 mm/year cited by CU in a linear calculation…

Related articles

• IPCC’s New Estimates for Increased Sea-Level Rise (yaleclimatemediaforum.org)
• Scientists Find That Sea Level Rise Is Much Slower Than Expected…No Human Fingerprint (notrickszone.com)
• IPCC sea level exaggeration (wattsupwiththat.com)