Quaternary Science Reviews 84 (2014) 1e6

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Expert assessment of sea-level rise by AD 2100 and AD 2300
Benjamin P. Horton a, b, *, Stefan Rahmstorf c, Simon E. Engelhart d, Andrew C. Kemp e
a

Institute of Marine and Coastal Sciences, School of Environmental and Biological Sciences, Rutgers University, New Brunswick, NJ 08901, USA
Division of Earth Sciences and Earth Observatory of Singapore, Nanyang Technological University, 639798, Singapore
c
Potsdam Institute for Climate Impact Research, Telegrafenberg A62, 14473 Potsdam, Germany
d
Department of Geosciences, University of Rhode Island, Kingston, RI 02881, USA
e
Department of Earth and Ocean Sciences, Tufts University, Medford, MA 02155, USA
b

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 21 October 2013
Accepted 1 November 2013
Available online

Large uncertainty surrounds projections of global sea-level rise, resulting from uncertainty about future
warming and an incomplete understanding of the complex processes and feedback mechanisms that
cause sea level to rise. Consequently, existing models produce widely differing predictions of sea-level
rise even for the same temperature scenario. Here we present results of a broad survey of 90 experts
who were amongst the most active scientific publishers on the topic of sea level in recent years. They
provided a probabilistic assessment of sea-level rise by AD 2100 and AD 2300 under two contrasting
temperature scenarios. For the low scenario, which limits warming to <2   C above pre-industrial temperature and has slowly falling temperature after AD 2050, the median ‘likely’ range provided by the
experts is 0.4e0.6 m by AD 2100 and 0.6e1.0 m by AD 2300, suggesting a good chance to limit future sealevel rise to <1.0 m if climate mitigation measures are successfully implemented. In contrast, for the high
warming scenario (4.5   C by AD 2100 and 8   C in AD 2300) the median likely ranges are 0.7e1.2 m by AD
2100 and 2.0e3.0 m by AD 2300, calling into question the future survival of some coastal cities and lowlying island nations.
Ó 2013 Elsevier Ltd. All rights reserved.

Keywords:
Expert elicitation
Survey
IPCC

1. Introduction
Beginning in the late 19th or early 20th century, the rate of
global sea-level rise increased sharply above the relatively stable
background rates of the previous w2000 years (e.g. Church and
White, 2006; Engelhart et al., 2009; Church and White, 2011;
Gehrels and Woodworth, 2012; IPCC, 2013). This onset of modern
sea-level rise coincided with increasing global temperature (e.g.
Kemp et al., 2011). While there is widespread agreement that the
rate of sea-level rise will continue to increase during the 21st
century, great uncertainty surrounds its future magnitude. The
Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report (AR5) projected global sea-level rise to AD 2100 forced
by different emission scenarios (IPCC, 2013). Projected sea-level
rise under each scenario is the sum of individual contributions
from steric changes and melting of glaciers and ice caps, the
Greenland Ice Sheet, the Antarctic Ice Sheet, and land water

* Corresponding author. Institute of Marine and Coastal Sciences, School of
Environmental and Biological Sciences, Rutgers University, 71 Dudley Road, New
Brunswick, NJ 08901-8525, USA. Tel.: þ1 848 932 3430.
E-mail address: bphorton@marine.rutgers.edu (B.P. Horton).
0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.quascirev.2013.11.002

storage. These projections are derived from process-based models
and assessment of glacier and ice sheet contributions. The IPCC AR5
projected a ‘likely’ (i.e. 66% likelihood range) global-average sealevel rise of 28e61 cm for a scenario of drastic emissions reductions
(RCP 2.6) and 52e98 cm in case of unmitigated growth of emissions
(RCP 8.5) by AD 2100 (relative to AD 1986e2005). This marks a
substantial upward revision (w60%) compared to the IPCC 4th
assessment report published in 2007. The process-based models
used in the 4th report substantially underestimated the observed
past sea-level rise (Rahmstorf et al., 2007, 2012a).
Process-based predictions of sea-level rise are limited by uncertainties surrounding the response of the Greenland and West
Antarctic ice sheets (Pfeffer et al., 2008; Rignot et al., 2011;
Pritchard et al., 2012), steric changes (Domingues et al., 2008;
Marcelja, 2010), contributions from mountain glaciers (Raper and
Braithwaite, 2009), as well as from groundwater pumping for irrigation purposes and storage of water in reservoirs (Konikow, 2011;
Pokhrel et al., 2012; Wada et al., 2012). In large part because of the
limitations of physical process-based models, IPCC AR5 does not
offer “very likely” (90% likelihood range) sea-level projections, but
concluded that “there is currently insufficient evidence to evaluate the
probability of specific levels above the assessed likely range” (Summary for Policy Makers, p. 18).

 B.P. Horton et al. / Quaternary Science Reviews 84 (2014) 1e6

Semi-empirical models linking global temperature and sea level
provide a complementary approach for estimating future sea-level
rise (e.g. Rahmstorf, 2007; Grinsted et al., 2009; Vermeer and
Rahmstorf, 2009). Semi-empirical models are calibrated with data
from the past to constrain how sea level responded to changing
temperatures. Projections are made by driving the model with a
scenario of future warming. They are robust to the choice of input
data and statistical technique (Rahmstorf et al., 2012b) and successfully predicted the out-of-sample 20th century sea-level rise
when calibrated with data up to AD 1900 (Bittermann et al., 2013).
However, it is unknown if the historic relationship between sealevel rise and temperature will continue to hold in the future
(Rahmstorf, 2010). Semi-empirical models predict a larger sea-level
rise than the IPCC AR5 by AD 2100 under the same scenario of
future temperature rise.
The wide range of sea-level projections from process-based and
semi-empirical models is reflected in recent assessments of sealevel rise. By AD 2100 the 2009 Antarctic Science Report anticipated up to 1.4 m (Turner et al., 2009), the 2011 Arctic Monitoring
and Assessment Program 0.9e1.6 m (AMAP, 2012), the 2012 U.S.
National Research Council report 0.5e1.4 m (NRC, 2012), and the
2012 World Bank Climate Report 0.27e1.23 m (World Bank, 2012),
although there are some differences in the underlying scenario
assumptions and exact time intervals used. Most recently, sea-level
scenarios prepared by the National Oceanic and Atmospheric
Administration (NOAA) for the U.S. National Climate Assessment
projected a mid-range of 0.5e1.2 m, with plausible lower and upper
limits of 0.2 m and 2.0 m (Parris et al., 2012). Assessments like the
ones cited may be affected by cultural and institutional processes
(Oppenheimer et al., 2007; O’Reilly et al., 2012; Brysse et al., 2013).
As our modeling capacity is immature and different modeling
approaches yield conflicting results (an issue known as structural
model uncertainty; O’Reilly et al., 2011), expert elicitation is a useful
approach to gauge the range of views held in the scientific community (Cooke, 1991; National Research Council, 1994; Arkes et al.,
1997; USEPA, 2011). Expert elicitations yield no new scientific results, but they make the views of scientists transparent to a wider
public, which is important in situations where policy decisions (e.g.
coastal planning and hazard mitigation) must be taken based on
the limited (and sometimes conflicting) scientific information
available (Dewispelare et al., 1995). Such elicitations are also a
valuable tool for quantifying uncertainty (Arkes et al., 1997; USEPA,
2011). The Inter Academy Council (2010), in its review for the
United Nations on the process and procedures of the IPCC, therefore
recommended that “Where practical, formal expert elicitation procedures should be used to obtain subjective probabilities for key results” (p. 41). Whereas the IPCC did not include an expert elicitation
on sea-level rise in AR5, we present one here.
Expert elicitations can be divided into “deep” and “broad” types
(National Research Council, 1994). A “deep” elicitation compiles the
views of a small number of experts in considerable detail (e.g.
Morgan and Keith, 1995; Cooke and Goossens, 2004; Zickfeld et al.,
2007; Bamber and Aspinall, 2013). In contrast, a “broad” elicitation
asks a large number of experts a small number of questions, aiming
for wide participation by minimizing the required time investment
for participation (e.g. Keeney and von Winterfeldt, 1991; Hoffmann
et al., 2006; Wardekker et al., 2010). Broad elicitations are appropriate for interdisciplinary problems that involve large uncertainties (Hoffmann et al., 2006) like sea-level prediction. Such an
elicitation asks carefully phrased questions that prompt a subjective (Bayesian) probability assessment from the respondent, since
statements about uncertain issues cannot by definition be made
with certainty (USEPA, 2011; Knol et al., 2010). Therefore, responses
reflect the degree of uncertainty perceived by the experts (Clemen
and Winkler, 1999). Here, we report the results from an

anonymous, broad elicitation to determine the professional judgments from members of the scientific community about global sealevel rise for the periods AD 2000e2100 and AD 2000e2300.
2. Materials and methods
A key element of an expert elicitation is the choice of experts
(e.g. USEPA, 2011; Knol et al., 2010). We objectively selected sealevel experts identified from the peer-reviewed literature using
the scientific publication database Web of Science of Thompson
Reuters. We searched (on the 19th September 2012) for all papers
in peer-reviewed journals on the index term “sea level” published
since AD 2007 to identify the 500 scientists who (co-)authored the
greatest number of these papers. Thus, we obtained a sample of 500
experts that arguably includes the most active scientific publishers
on the subject of sea level and all of whom had published at least
six peer-reviewed on “sea level” since AD 2007. We found e-mail
addresses for 360 of these experts and accordingly sent out invitations to participate in the survey on 16th November 2012, with
a unique identifier to ensure anonymity and avoid duplicate responses. Of those invited, 36% (131) participated, which is typical
for this type of internet-based survey (e.g. Wardekker et al., 2010).
The main reason given for declining to participate was a (perceived)
lack of expertise in predicting future sea-level rise. We could not
analyze 41 responses from participants because they either left all
boxes blank or filled with a question mark, or the responses were
logically inconsistent (e.g. gave a higher probability for exceeding a
1.0 m sea-level rise than a 0.8 m rise). Not all survey respondents
completed every percentile box.
The ninety international sea-level experts provided their probabilistic assessment of global sea-level rise, given two temperature
scenarios derived from the upper and lower extremes of the
Representative Concentration Pathways (RCPs) scenarios
(Meinshausen et al., 2011; Fig. 1). This conditional approach separates uncertainty about future temperature from that about sealevel rise (for the exact phrasing of the questions see
Supplementary Note S1). In the RCP 3-PD greenhouse gas scenario
there is warming of w1   C from AD 2000 to AD 2060 followed by a

Warming relative to pre-industrial (oC)

2

8

Global mean
surface temperature

7
6

RCP8.5

5
4
3

RCP3-PD

2
1
0

1900

2000

2100

2200

2300

Year (AD)
Fig. 1. Scenarios of global temperature changes up to AD 2300. The two scenarios are
reproduced from Meinshausen et al. (2011) and were presented to survey participants
as basis for their assessment of future sea-level rise. These temperature projections
correspond to the lower (RCP3-PD; blue) and upper (RCP8.5; red) greenhouse gas
scenarios included in the Representative Concentration Pathways (RCP) and their
extension to AD 2300 (Moss et al., 2010; Meinshausen et al., 2011). The thin green line
shows observed global temperature.

 B.P. Horton et al. / Quaternary Science Reviews 84 (2014) 1e6

slow cooling up to AD 2300, with high probability the global
temperature stays below the international policy limit of 2   C above
the pre-industrial level throughout. In the RCP 8.5 emission scenario there is warming of 4.5   C by AD 2100 and a further 3.5   C
during the years AD 2100 to AD 2300.

Table 1
The median ‘likely’ ranges and ‘very likely’ ranges of sea-level rise for AD 2100 and
AD 2300 from the expert elicitation.
Year

Scenario

Likely range (m)
(17the83rd
percentiles)

Very likely range (m)
(5the95th percentiles)

AD 2100

Lower
Upper
Lower
Upper

0.4e0.6
0.7e1.2
0.6e1.0
2.0e3.0

0.25e0.7
0.5e1.5
0.5e1.2
1.3e4.0

3. Results and discussion
AD 2300

The experts provided two probability ranges for each case. By
definition, sea-level rise is assessed to have a 66% probability to be
within the inner range (17th to 83rd percentiles), which thus corresponds to the IPCC’s definition of a ‘likely’ range. With an
assessed probability of 90%, sea-level rise will be within the outer
range (5th to 95th percentiles), corresponding to the IPCC’s definition of a ‘very likely’ range. Fig. 2 presents box plots of the survey
results, and the median ‘likely’ ranges and ‘very likely’ ranges across
all experts are provided in Table 1. The mean ranges are moderately
higher (typically by w10%) than the median ranges because the
distribution is slightly skewed towards higher values. The AD 2100
ranges given by the experts are higher than those projected by the
IPCC AR5. IPCC AR5 predicted a ‘likely range’ of 0.28e0.61 m under
RCP 2.6 (numbers for RCP 3.0 are not provided by IPCC, but would
be similar) and 0.52e0.98 m under RCP 8.5 by AD 2100. The IPCC
AR5 did not provide ‘very likely’ predictions.
We found no significant correlation between the sea-level
ranges given by the experts and their self-reported Hirsch (H)

3

(RCP 3)
(RCP 8.5)
(RCP 3)
(RCP 8.5)

Index (Hirsch, 2005) (a common ranking measure of the scientific
publication record; Fig. 3). This is an important test for broad expert
surveys, because it can reveal differences among the views of experts with different degrees of expertise, experience, or scientific
standing. That we found no correlation is perhaps expected, given
that we only invited experts with a strong publication record on sea
level. We also looked for a possible effect of the country or region
where the experts are based (Fig. 3). The respondents, working in
18 countries, were equally split between North America and Europe
(46% each), with only 8% from the rest of the world (Brazil, China,
Japan, and Australia). Given the small number of experts from
outside North America and Europe, only a comparison between
these two regions was justified. North American experts returned
slightly greater sea-level rise estimates than their European colleagues in all instances, but the difference was not statistically
significant.

Fig. 2. Box plots of survey results from all experts who provided at least partial responses to questions. The number of respondents for each of the four questions is shown in the top
left corner; it is lower than the total of 90 participants since not all answered each question. Participants were asked to estimate likely (17the83rd percentiles) and very likely (5the
95th percentiles) sea-level rise under two temperature scenarios and at two time points (AD 2100 and AD 2300), resulting in four sets of responses. Shaded boxes represent the
range between the first and third quantiles of responses. Dashed horizontal line within the box is the median response. Whiskers (solid lines) represent two standard deviations of
the responses. Filled circles show individual responses that are beyond two standard deviations of the median.

 4

B.P. Horton et al. / Quaternary Science Reviews 84 (2014) 1e6

We compared sea-level scenarios obtained from this expert
elicitation with the recent NOAA scenarios (Parris et al., 2012) and
those of the IPCC AR5 (Fig. 4). To add an exemplary time evolution we fitted a quadratic time dependence as was used in the
NOAA scenarios. Our survey results are consistent with these
scenarios; there is no systematic high or low bias of one relative
to the other. NOAA’s intermediate-low and intermediate-high
scenarios (0.5 m and 1.2 m) are near the mean values of the
expert survey for the low and high temperature scenarios. The
highest and lowest predictions from NOAA (0.2 m and 2.0 m) are
outside of the mean ‘very likely’ ranges provided by our experts,
but within the full range of predictions returned by our experts
(Figs. 2 and 4).
The results of our survey are broadly consistent with those of a
recent deep elicitation focused on ice sheets (Bamber and Aspinall,
2013) that projected sea-level rise of 0.33e1.32 m by AD 2100 under
an intermediate emissions scenario (RCP 4.5). However, our experts
estimated a much greater sea-level rise than a deep elicitation
published in 1996 (Titus and Narayanan, 1996), which found only a
1% chance of exceeding 1.0 m of sea-level rise by AD 2100 with
temperatures increases of up to 4.7   C, which exceed the warming
scenarios used here. This difference likely reflects an evolution of
expert opinion over time in light of scientific advances, including
new observational data for sea level and ice sheet mass loss, the
further evolution of process-based models and development of
semi-empirical models. A recent study analyzing the ranges of
projected sea-level rise in the scientific literature from AD 1989 to

Anticipated Global Sea-Level Rise (m)

Fig. 3. Survey results from experts who provided at least partial responses to questions stratified by their self-reported Hirsch (H) Index and location (North America; NA or Europe;
EU). Dashed lines represent the mean range of values respondents provided for the 5the95th percentiles (very likely sea-level rise). Solid lines represent the mean range of values
provided for the 17the83rd percentiles (likely sea-level rise). The number of respondents is presented for each case.

mean of all respondents
very likely
likely
NOAA Assessment (Parris et al., 2012)
Highest
Intermediate - high
Intermediate - low
Lowest

Year (AD)
Fig. 4. Scenarios of future sea-level rise generated from survey results for two contrasting temperature scenarios (RCP3-PD; blue and RCP8; red). Shading represents
mean likely and very likely ranges. The evolution of sea-level rise from AD 2000 to the
respondent estimates for AD 2100 was described by a quadratic time dependence. The
current NOAA sea-level scenarios (Parris et al., 2012), published after our survey was
conducted, are shown for comparison as dashed lines. Likely projections from IPCC
AR5 for AD 2100 (relative to AD 2000) are shown for RCP 2.6 and 8.5 as vertical bars on
the right.

 B.P. Horton et al. / Quaternary Science Reviews 84 (2014) 1e6

AD 2009 found that projections published in the scientific literature
have increased substantially over that period (Rick et al., 2011).
4. Conclusions
Our survey reflects the substantial uncertainty that remains in
predicting the magnitude of future sea-level rise. Most experts
estimate a larger sea-level rise by AD 2100 than the IPCC AR5
projects. The body of expert opinion is more in line with the recent
NOAA sea-level scenarios. We expect on average about 0.5 m of sealevel rise for the low and 1.0 m for the upper temperature scenario.
Thirteen experts estimated a 17% probability of exceeding 2.0 m of
sea-level rise by AD 2100 under the upper temperature scenario.
For the rise from AD 2000 to AD 2300, the median ‘likely’ range
is 0.6e1.0 m for the low temperature scenario. This is significantly
more optimistic than the semi-empirical model result of Schaeffer
et al. (2012) that estimated 2.0 m of global rise for the same temperature scenario. The IPCC AR5 model spread for low emission
scenarios (includes RCP 2.6) is 0.41e0.85 m by AD 2300. Our survey
results and the IPCC AR5 assessment thus suggest that stringent
mitigation measures could deliver a good chance to keep sea-level
rise below the 1 m guardrail proposed by the German Advisory
Council on Global Change (WBGU, 2006). For the high-temperature
scenario, the median “likely” range given by the experts is 2.0e
3.0 m, with some experts estimating a rise of up to 14.0 m (Fig. 2).
The IPCC AR5 model spread for the high emission scenario (includes RCP 8.5) is 0.92 me3.59 m by AD 2300. Overall the results for
AD 2300 illustrate the risk that temperature increases from unmitigated emissions could commit coastal populations to a longterm, multi-meter sea-level rise that would have catastrophic impacts on many coastal cities and low-lying lands. However, they
also illustrate the potential for escaping such large sea-level rise
through successful reduction of emissions.
Acknowledgments
This research was funded by NOAA Grant NA11OAR4310101 and
NSF Award 1052848. We greatly appreciate the scientific community for completing the survey. This paper is a contribution to IGCP
project 588 and PALSEA2.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.quascirev.2013.11.002.
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