Sea-level rise: What the experts expect

In the long run, sea-level rise will be one of the most serious consequences of global warming. But how fast will sea levels rise? Model simulations are still associated with considerable uncertainty – too complex and varied are the processes that contribute to the increase. A just-published survey of 90 sea-level experts from 18 countries now reveals what amount of sea-level rise the wider expert community expects. With successful, strong mitigation measures, the experts expect a likely rise of 40-60 cm in this century and 60-100 cm by the year 2300. With unmitigated warming, however, the likely range is 70-120 cm by 2100 and two to three meters by the year 2300.

IMG_4484Complex problems often cannot simply be answered with computer models. Experts form their views on a topic from the totality of their expertise – which includes knowledge of observational findings and model results, as well as their understanding of the methodological strengths and weaknesses of the various studies. Such expertise results from years of study of a topic, through ones own research, through following the scientific literature and through the ongoing critical discussion process with colleagues at conferences.

For many topics it would be interesting for the public to know what the expert community thinks. If I had a dangerous disease, I would give a lot to learn what the best specialists from around the world think about it. Mostly, however, this expertise is not transparent to outsiders. The media only offer a rather selective window into experts’ minds.

More transparency can be achieved through systematic surveys of experts. The International Council of Scientific Academies (InterAcademy Council, IAC) in its review of IPCC procedures recommended in 2010: “Where practical, formal expert elicitation procedures should be used to obtain subjective probabilities for key results”. We took this advice and last November conducted a broad expert survey on future sea-level rise, in the context of a research project funded by NOAA. Lead author is Ben Horton (Rutgers University), the further authors are Simon Engelhart (University of Rhode Island) and Andrew Kemp (Tufts University).

The credibility of such surveys stands and falls with the selection of experts (see Gavin’s article A new survey of scientists). It is important to identify relevant experts using objective criteria. For us, formal criteria such as professorships were not relevant; our objective was to reach active sea-level researchers. To this end we used the scientific publication database Web of Science of Thomson Reuters and let it generate a list of the 500 researchers who had published the most papers for the search term “sea level” in the last five years in the peer-reviewed literature. It was found that at least 6 publications were required for a scientist to make it onto this list. For 360 of those experts we were able to find email addresses. We asked those for their estimates of the sea-level rise from 2000 to 2100 and 2300, both the “likely” rise (17th to 83rd percentile) and the range of the 5th to the 95th percentile (the 95th percentile is the increase which with 95 % probability will not be exceeded, according to the expert). 90 experts from 18 countries provided their responses.

Sea-level: a bit of context

For context, the following figure from the current IPCC report summarizes the sea-level evolution:

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