Sea-level rise: What the experts expect

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:

IPCC_AR5_13.27

Figure 1: Sea level rise according to the IPCC report of 2013. Shown is the past history of sea level since the year 1700 from proxy data (sediments, purple) and multiple records from tide gauge measurements. Light blue are the satellite data (from 1993). The two future scenarios mentioned in the text (RCP8.5 and RCP2.6) are shown in red and blue, with their “likely” uncertainty range according to the IPCC (meaning a 66 % probability to remain inside this range). Source: IPCC AR5 Fig. 13.27.

A more detailed discussion of the IPCC sea level numbers can be found here. The red and blue future scenarios correspond (to good approximation) to the two climate scenarios on which we surveyed the experts: blue a scenario with effective climate mitigation, red a scenario with a further unabated growth of emissions into the 22nd Century.

The survey results

The following graph shows what the surveyed experts expect for these two scenarios up to the year 2100:

Horton_SLR_Survey

Figure 2: Sea level rise over the period 2000-2100 for two warming scenarios (red RCP8.5, blue RCP3). The ranges show the average numbers given across all the experts. The inner (darker) range shows the 17 to 83 percentile values, the outer range the 5 to 95th percentiles. For comparison we see the NOAA projections of December 2012 (dashed lines) and the new IPCC projections (bars on the right). Since this graph shows the increase from the year 2000, about 25 cm should be added for a direct numerical comparison with the previous graph.

The experts gave a median rise of 40-60 cm for the blue climate scenario and 70-120 cm for the red scenario. Most of the experts thus expect a higher rise than the IPCC – about two-thirds (65%) give the upper limit for the red ‘likely’ range a higher value than the IPCC, even though the IPCC has increased its projections by ~60% since its last report of 2007. In expert circles the IPCC reports are widely considered to be conservative; this is empirical confirmation.

The following table shows all the median values:

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