The multimillennial sea-level commitment of global warming

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 sealevel 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.climate change | climate impacts | sea-level change S ea-level projections show a robust, albeit highly uncertain, increase by the end of this century (1, 2), and there is strong evidence that sea level will continue to rise beyond the year 2100 unless the current global mean temperature trend is reversed (3-6). At the same time, inertia in the climate and global carbon system causes the global mean temperature to decline slowly even after greenhouse gas emissions have ceased (6), raising the question of how much sea-level rise we are committed to on a multimillennial time scale for different levels of global mean temperature increase. During the 20th century, sea level rose by approximately 0.2 m (7, 8), and it is estimated to rise by significantly less than 2 m by 2100, even for the strongest scenarios considered (9). At the same time, past climate records suggest a sea-level sensitivity of as much as several meters per degree of warming during previous intervals of Earth history when global temperatures were similar to or warmer than present (10, 11). Although sea-level rise over the last century has been dominated by ocean warming and loss of glaciers (7), the sensitivity suggested from records of past sea level indicates important contributions from the Greenland and Antarctic Ice Sheets. Because of the uncertainties in the paleo-reconstructions, however, additional strategies are req...

Future sea level rise constrained by observations and long-term commitment

“…Two different models (20,50) have been used to estimate the glacier equilibrium sea level sensitivity globally (5). Forced by atmospheric data from 4 and 15 different climate models, respectively, they provide 19 different sensitivity curves for six levels of global warming, as shown in Fig.…”

“…The SMB change from Antarctica is therefore estimated via dS ais,smb dt ðtÞ = −ð1 − κÞ · γ · ΔT [5] where ΔT denotes the global mean temperature change, γ is the snowfall sensitivity, and κ is the fraction lost due to the increase in dynamic discharge. We use a constant κ of 0.25 within this study.…”

“…A relatively constant commitment of α ais,dyn 1.2 m sea level per degree of warming is deduced from correlating global mean temperature with ice volume (figure 1D in ref. 5), S eq,ais,dyn = α ais,dyn · ΔT [6] We account for the uncertainty that originates from forcing data, ice physics, and memory of the ice sheet by sampling α from the interval [1.0-1.5] m per degree Celsius, which reflects the first standard deviation of the model simulations on which the relation is based. For all contributions, we apply Monte Carlo sampling to project future sea level rise including observational and climate system uncertainties.…”

“…The long-term multicentennial to millennial sensitivity of the main individual sea level contributors to global temperature changes can be constrained by paleoclimatic data and is more easily computed with currently available process-based large-scale models than are decadal to centennial variations (5,11). In addition, there is an increasing number of observations available for the historical individual contributions to sea level rise, which capture the early response to global temperature changes.…”

Similitude of ice dynamics against scaling of geometry and physical parameters

“…The scaling behavior of ice sheets, which here is analyzed in a conceptual way, might be of use to better understand the large-scale evolution of the polar ice sheets. Of particular interest is the scaling of the ice-sheet response time (Levermann et al, 2013 against the background of Antarctic instabilities (Weertman, 1974;Schoof, 2007b;Rignot et al, 2014;Fogwill et al, 2014;Mengel and Levermann, 2014). The timescales of possible rapid ice discharge due to instability in the past (Pollard and DeConto, 2009;Pollard et al, 2015) and future (Favier et al, 2014;Joughin et al, 2014;Feldmann and Levermann, 2015b) are highly uncertain.…”