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The Paris Climate Agreement and future sea-level rise from Antarctica

Abstract

The Paris Agreement aims to limit global mean warming in the twenty-first century to less than 2 degrees Celsius above preindustrial levels, and to promote further efforts to limit warming to 1.5 degrees Celsius1. The amount of greenhouse gas emissions in coming decades will be consequential for global mean sea level (GMSL) on century and longer timescales through a combination of ocean thermal expansion and loss of land ice2. The Antarctic Ice Sheet (AIS) is Earth’s largest land ice reservoir (equivalent to 57.9 metres of GMSL)3, and its ice loss is accelerating4. Extensive regions of the AIS are grounded below sea level and susceptible to dynamical instabilities5,6,7,8 that are capable of producing very rapid retreat8. Yet the potential for the implementation of the Paris Agreement temperature targets to slow or stop the onset of these instabilities has not been directly tested with physics-based models. Here we use an observationally calibrated ice sheet–shelf model to show that with global warming limited to 2 degrees Celsius or less, Antarctic ice loss will continue at a pace similar to today’s throughout the twenty-first century. However, scenarios more consistent with current policies (allowing 3 degrees Celsius of warming) give an abrupt jump in the pace of Antarctic ice loss after around 2060, contributing about 0.5 centimetres GMSL rise per year by 2100—an order of magnitude faster than today4. More fossil-fuel-intensive scenarios9 result in even greater acceleration. Ice-sheet retreat initiated by the thinning and loss of buttressing ice shelves continues for centuries, regardless of bedrock and sea-level feedback mechanisms10,11,12 or geoengineered carbon dioxide reduction. These results demonstrate the possibility that rapid and unstoppable sea-level rise from Antarctica will be triggered if Paris Agreement targets are exceeded.

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Fig. 1: Antarctic contribution to future GMSL rise.
Fig. 2: Ice-sheet evolution following the +3 °C global warming emissions trajectory.
Fig. 3: AIS thresholds and commitments to GMSL rise with delayed mitigation.

Data availability

Model-generated data associated with this work are available with this paper. Three-dimensional ice-sheet model output associated with Fig. 2 and Extended Data Figs. 3, 5 are available at the ScholarWorks@UMASS Amherst repository (https://doi.org/10.7275/j005-r778). Climate model forcing used in our main ensembles and meltwater-feedback simulations (Fig. 1) are reported in refs. 46,80Source data are provided with this paper.

Code availability

The modified ice-sheet model codes based on ref. 51 are available from the corresponding author. CESM1.2.2 GCM87 is available from NCAR (https://www.cesm.ucar.edu/models/cesm1.2/) and the RCM is reported in ref. 79. The Earth–sea level model is described in refs. 12,49.

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Acknowledgements

We thank T. Naish for guidance on Pliocene sea-level targets. This research was supported by the NSF under awards 1664013, 2035080, 1443347 and 1559040, and by a grant to the NASA Sea Level Change Team 80NSSC17K0698.

Author information

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Authors

Contributions

R.M.D. and D.P. conceived the model experiments and developed the main model codes with conceptual input from R.B.A.; R.M.D. and D.P. wrote the manuscript with input from R.B.A., I.V., E.G., N.G., and S.S.; I.V. provided GRACE mass change estimates; E.G. contributed to climate forcing scenarios; S.S. and A.C. provided CESM1.2.2 climatologies; N.G. collaborated on coupled ice–Earth simulations; A.D. provided palaeo sea-level target ranges; D.L. compiled CMIP5 and CMIP6 GCM results; and D.M.G., E.L.A. and R.E.K. developed the statistical model described in Supplementary Information.

Corresponding author

Correspondence to Robert M. DeConto.

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The authors declare no competing interests.

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Peer review information Nature thanks Jacqueline Austermann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Ensemble observational targets.

196 simulations (grey lines), each using a unique combination of hydrofracturing and ice-cliff calving parameters (Extended Data Table 1) compared with observations (blue dashed boxes). Solid blue lines show simulations without hydrofracturing and ice-cliff calving. Red lines show simulations with maximum parameter values in our main ensemble. Additional simulations (black lines) allow ice-cliff calving rates of up to 26 km yr−1, twice the maximum value used in our main ensembles. The vertical heights of the blue boxes represent the likely range of observations. Changes in ice mass above floatation are shown in equivalent GMSL. a, Simulated annual contributions to GMSL in the RCP8.5 ensemble compared with the 1992–2017 IMBIE4 observational average (0.15–0.46 mm yr−1; dashed blue box). b, LIG ensemble simulations from 130 to 125 kyr ago. The height of the dashed blue box shows the LIG target range (3.1–6.1 m), the width represents ~1,000-yr age uncertainty34. c, The same LIG simulations as in b, showing the rate of GMSL change contributed by Antarctica, smoothed over a 25-yr window. The peak in the early LIG is mainly caused by marine-based ice loss in West Antarctica. d, The same as b, except for warmer mid-Pliocene conditions. Maximum ice loss is compared with observational estimates of 11–21 m (refs. 35,36; blue dashed lines). Note the saturation of the simulated GMSL values near the top of the LIG and Pliocene ensemble range, and the failure of the model to produce realistic LIG or Pliocene sea levels without hydrofracturing and ice-cliff calving enabled (blue lines). Source data

Extended Data Fig. 2 RCP8.5 ensembles calibrated with alternative GRACE estimates.

a, b, The fan charts show the time-evolving uncertainty and range around the median ensemble value (black line) in 10% increments. RCP8.5 ice-sheet model ensembles calibrated with GRACE estimates of annual mass change averaged from 2002–2017 using alternative GIA corrections (Methods). Use of GIA corrections produces estimates of mass loss between 2002 and 2017 of 0.2–0.54 mm yr−1 (a) and 0.39–0.53 mm yr−1 (b). The more restrictive and higher range of GRACE estimates in b skews the distribution and shifts the ensemble median values of GMSL upwards from 27 cm to 30 cm in 2100 and from 4.44 m to 4.94 m in 2200.

Extended Data Fig. 3 Last Interglacial and Pliocene ice-sheet simulations.

ae, Ice-sheet simulations with the updated model physics used in our future ensembles and driven with the same LIG and Pliocene climate forcing used in ref. 8. Simulations without hydrofracturing and ice-cliff calving (a, b, d) correspond to blue lines in Extended Data Fig. 1. Simulations using maximum hydrofracturing and ice-cliff calving parameters (c, e) correspond to red lines in Extended Data Fig. 1. a, Modern (1950) ice-sheet simulation. b, c, LIG simulations run from 130 to 125 kyr ago are shown at 125 kyr ago. Values at the top of each panel are the maximum GMSL contribution between 129 and 128 kyr ago. Values in parentheses are the GMSL contribution at 125 kyr ago. d, e, Warm Pliocene simulations. Values shown are the maximum GMSL achieved during the simulations. Smaller values in parentheses show GMSL contributions after 5,000 model years (Extended Data Fig. 2d). Ice mass gain after peak retreat is caused by post-retreat bedrock rebound and enhanced precipitation in the warm Pliocene atmosphere.

Extended Data Fig. 4 RCP8.5 ensembles calibrated with modern and palaeo observations.

The fan charts show the time-evolving uncertainty and range around the median ensemble value (black line) in 10% increments. Mean and median ensemble values are shown at 2100. a, Raw ensemble with a range of plausible model parameters based on glaciological observations (Extended Data Table 1). b, The ensemble trimmed with IMBIE4 (1992–2017) estimates of ice mass change. c, The ensemble trimmed with IMBIE rates of ice mass change plus LIG sea-level constraints between 129 and 128 kyr ago34. d, The same as c, except with the addition of maximum mid-Pliocene sea-level constraints35,36 (Extended Data Fig. 1). Future ensembles in the main text (Fig. 1, Table 1) use the combined IMBIE + LIG + Pliocene history matching constraints as shown in d. Source data

Extended Data Fig. 5 Future retreat of Thwaites Glacier (TG) and Pine Island Glacier (PIG) with +3 °C global warming.

The Amundsen Sea sector of the ice sheet in a nested, high-resolution (1 km) simulation using average calibrated values of hydrofracturing and ice-cliff calving parameters (CALVLIQ = 107 m−1 yr2; VCLIF = 7.7 km yr−1), consistent with those used in CESM1.2.2-forced simulations (Fig. 1h) and CDR simulations (Fig. 3, Table 1). ac, The ice sheet in 2050. df, The ice sheet in 2100. a, d, Ice-sheet geometry and annually averaged ice-cliff calving rates at thick, weakly buttressed grounding lines. The solid line in all panels is the grounding line and the dashed line is its initial position. Note that simulated ice-cliff calving rates are generally much slower than the maximum allowable value of 7.7 km yr−1. Ice shelves downstream of calving ice cliffs are the equivalent of weak mélange, incapable of stopping calving64. b, e, Ice surface speed showing streaming and fast flow just upstream of calving ice cliffs where driving stresses are greatest. c, f, Change in ice thickness relative to the initial state. g, GMSL contributions within the nested domain at model spatial resolutions spanning 1–10 km.

Extended Data Fig. 6 Antarctic contribution to sea level under standard RCP forcing.

ac, The fan charts show the time-evolving uncertainty and range around the median ensemble value (thick black line) in 10% increments. The RCP ensembles use the same IMBIE, LIG and Pliocene observational constraints applied to the simulations in Fig. 1. GMSL contributions in simulations without hydrofracturing or ice-cliff calving (excluded from the validated ensembles) are shown for East Antarctica (thin blue line), West Antarctica (thin red line) and the total Antarctic contribution (thin black line). a, RCP2.6; b, RCP4.5; and c, RCP8.5. Source data

Extended Data Fig. 7 Long-term magnitudes and rates of GMSL rise contributed by Antarctica.

a, Ensemble median (50th percentile) projections of GMSL rise contributed by Antarctica with emissions forcing consistent with the +1.5 °C and +2.0 °C Paris Agreement ambitions, versus a +3.0 °C scenario closer to current NDCs. b, Median (50th percentile) rates of GMSL rise in the same emissions scenarios as in a, illustrating a sharp jump in ice loss in the warmer +3.0 °C scenario after 2060 (also see Fig. 1), and reduced net ice loss before 2060 (black line) caused by increased snowfall. c, Ensemble median (50th percentile) projections of GMSL rise contributed by Antarctica with emissions forcing consistent with standard RCP scenarios, highlighting the potential for extreme GMSL rise under high (RCP8.5) emissions. d, Ensemble median (50th percentile) rates of GMSL rise in the same RCP scenarios as shown in c. Note the much larger vertical-axis scales in c and d relative to a and b.

Extended Data Fig. 8 Coupled ice–Earth–sea level model simulations.

ac, Simulations without hydrofracturing and ice-cliff calving processes. df, Simulations with hydrofracturing and ice-cliff calving enabled (Methods). GMSL contributions are from the WAIS only. Various Earth viscosity profiles (coloured lines) are compared with the ice-sheet model’s standard ELRA formulation (black line). The most extreme viscosity profile (blue line) assumes a thin lithosphere and very weak underlying mantle, like that observed in the Amundsen sea10, but extended continent-wide. a, RCP2.6 without hydrofracturing or ice-cliff calving. b, RCP2.6 with hydrofracturing and ice-cliff calving. c, RCP4.5 without hydrofracturing or ice-cliff calving. d, RCP4.5 with hydrofracturing and ice-cliff calving. e, RCP8.5 without hydrofracturing or ice-cliff calving. f, RCP8.5 with hydrofracturing and ice-cliff calving.

Extended Data Table 1 Model ensemble parameter values
Extended Data Table 2 Antarctic sea-level contributions with alternative maximum ice-cliff calving rates

Supplementary information

Supplementary Information

This file contains Supplementary Methods, Supplementary Notes, Supplementary Figures 1–6, and Supplementary Tables 1–2. The Supplementary Information shows 1) uncertainty in future Antarctic climate forcing, 2) an alternative ice shelf hydrofracturing scheme, an improved formulation of buttressing at grounding lines, and 4) statistical emulation of our physical model ensembles.

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DeConto, R.M., Pollard, D., Alley, R.B. et al. The Paris Climate Agreement and future sea-level rise from Antarctica. Nature 593, 83–89 (2021). https://doi.org/10.1038/s41586-021-03427-0

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