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. 2013 Aug 20;110(34):13745-50.
doi: 10.1073/pnas.1219414110. Epub 2013 Jul 15.

The multimillennial sea-level commitment of global warming

Anders Levermann  1 , Peter U Clark, Ben Marzeion, Glenn A Milne, David Pollard, Valentina Radic, Alexander Robinson
Affiliations

The multimillennial sea-level commitment of global warming

Anders Levermann et al. Proc Natl Acad Sci U S A. .

Abstract

Global mean sea level has been steadily rising over the last century, is projected to increase by the end of this century, and will continue to rise beyond the year 2100 unless the current global mean temperature trend is reversed. Inertia in the climate and global carbon system, however, causes the global mean temperature to decline slowly even after greenhouse gas emissions have ceased, raising the question of how much sea-level commitment is expected for different levels of global mean temperature increase above preindustrial levels. Although sea-level rise over the last century has been dominated by ocean warming and loss of glaciers, the sensitivity suggested from records of past sea levels indicates important contributions should also be expected from the Greenland and Antarctic Ice Sheets. Uncertainties in the paleo-reconstructions, however, necessitate additional strategies to better constrain the sea-level commitment. Here we combine paleo-evidence with simulations from physical models to estimate the future sea-level commitment on a multimillennial time scale and compute associated regional sea-level patterns. Oceanic thermal expansion and the Antarctic Ice Sheet contribute quasi-linearly, with 0.4 m °C(-1) and 1.2 m °C(-1) of warming, respectively. The saturation of the contribution from glaciers is overcompensated by the nonlinear response of the Greenland Ice Sheet. As a consequence we are committed to a sea-level rise of approximately 2.3 m °C(-1) within the next 2,000 y. Considering the lifetime of anthropogenic greenhouse gases, this imposes the need for fundamental adaptation strategies on multicentennial time scales.

Keywords: climate change; climate impacts; sea-level change.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Sea-level commitment per degree of warming as obtained from physical model simulations of (A) ocean warming, (B) mountain glaciers and ice caps, and (C) the Greenland and (D) the Antarctic Ice Sheets. (E) The corresponding total sea-level commitment, which is consistent with paleo-estimates from past warm periods (PI, preindustrial, Plio, mid-Pliocene; see text for discussion). Temperatures are relative to preindustrial. Dashed lines and large dots provide linear approximations: (A) sea-level rise for a spatially homogeneous increase in ocean temperature; (A, D, E) constant slopes of 0.42, 1.2, and 1.8 and 2.3 m/°C. Shading as well as boxes represent the uncertainty range as discussed in the text. (A–C) Thin lines provide the individual simulation results from different models (A and B) or different parameter combinations (C). The small black dots in D represent 1,000-y averages of the 5-million-year simulation of Antarctica following ref. .
Fig. 2.
Fig. 2.
Sea-level commitment per degree of warming as in Fig. 1 except for sea-level contributions after 2,000 y. Results were obtained from physical model simulations of (A) ocean warming, (B) mountain glaciers and ice caps, and (C) the Greenland and (D) the Antarctic Ice Sheets. (E) The corresponding total sea-level commitment. Temperatures are relative to preindustrial. Dashed lines and large black dots provide linear approximations: (A) sea-level rise for a homogeneous increase in temperature; (D and E) constant slopes of 1.2 and 2.3 m/°C. Shading represents the uncertainty range as discussed in the text. The large green dot in C corresponds to the sea-level contribution of Greenland (following ref. 3) after 1,000 y of integration with 560 ppm atmospheric CO2 concentration and a standard polar amplification of 1.5, which translates Greenland temperatures to the global mean temperature.
Fig. 3.
Fig. 3.
Regional patterns of sea-level change computed using an isostatic surface loading model (see text for details) for scenarios of 1, 2, 3, and 4 °C of warming. These results are based on the assumption that ice thinning was uniform over the ice sheets and progressed linearly for 2,000 y. The contribution of glaciers and ice caps and ocean thermal expansion to the spatial pattern is not included; however, their global mean sea-level contribution is included (Table 1). These results are based on a spherically symmetric Earth model with 1D, depth-dependent viscosity structure. This structure comprises an elastic outer layer of thickness 96 km (to simulate the lithosphere) and two sublithosphere layers with viscosity values 5 × 1020 Pas (96 km to 660 km depth) and 1022 Pas (660 km to 2,900 km). These parameters are compatible with those inferred in a number of studies (e.g. refs and 73). The white contour in each frame denotes the global mean sea-level value (Table 1).
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