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Antarctic ice dynamics amplified by Northern Hemisphere sea-level forcing

Nature volume 587, pages 600–604 (2020)Cite this article

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Abstract

Sea-level rise due to ice loss in the Northern Hemisphere in response to insolation and greenhouse gas forcing is thought to have caused grounding-line retreat of marine-based sectors of the Antarctic Ice Sheet (AIS)1,2,3. Such interhemispheric sea-level forcing may explain the synchronous evolution of global ice sheets over ice-age cycles. Recent studies that indicate that the AIS experienced substantial millennial-scale variability during and after the last deglaciation4,5,6,7 (roughly 20,000 to 9,000 years ago) provide further evidence of this sea-level forcing. However, global sea-level change as a result of mass loss from ice sheets is strongly nonuniform, owing to gravitational, deformational and Earth rotational effects8, suggesting that the response of AIS grounding lines to Northern Hemisphere sea-level forcing is more complicated than previously modelled1,2,6. Here, using an ice-sheet model coupled to a global sea-level model, we show that AIS dynamics are amplified by Northern Hemisphere sea-level forcing. As a result of this interhemispheric interaction, a large or rapid Northern Hemisphere sea-level forcing enhances grounding-line advance and associated mass gain of the AIS during glaciation, and grounding-line retreat and mass loss during deglaciation. Relative to models without these interactions, the inclusion of Northern Hemisphere sea-level forcing in our model increases the volume of the AIS during the Last Glacial Maximum (about 26,000 to 20,000 years ago), triggers an earlier retreat of the grounding line and leads to millennial-scale variability throughout the last deglaciation. These findings are consistent with geologic reconstructions of the extent of the AIS during the Last Glacial Maximum and subsequent ice-sheet retreat, and with relative sea-level change in Antarctica3,4,5,6,7,9,10.

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Fig. 1: Contributions to deglacial sea-level changes in Antarctica.
Fig. 2: Timing of Northern Hemisphere sea-level forcing and its influence on Antarctic ice volume changes.
Fig. 3: Enhanced Antarctic ice loss during MWP-1A and the early Holocene.
Fig. 4: Agreement of predicted sea-level and ice-cover changes with geological records in the Ross Sea sector.

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Data availability

The datasets generated for this publication are available in the PANGAEA database (https://doi.org/10.1594/PANGAEA.919498) and as source data for Extended Data Fig. 9. The modelling results are available in the OSF database (https://osf.io/g5ur2/?view_only=8acbf1e38c184d9c8f09811c8bbef036). Source data are provided with this paper.

Code availability

The coupled ice-sheet–sea-level model used is reported in refs. 26,28; the PSU 3D ice-sheet model is reported in ref. 37. Ice-sheet and sea-level models are available on request from the authors of the references listed.

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Acknowledgements

N.G. and H.K.H. were supported by the Natural Sciences and Engineering Research Council (NSERC), the Canada Research Chair’s programme and the Canadian Foundation for Innovation, M.E.W. by the Deutsche Forschungsgemeinschaft (DFG; grant numbers We2039/8-1 and We 2039/17-1), and J.X.M. by NASA grant NNX17AE17G and Harvard University. We thank G. Tseng for assistance with exploratory research that informed this study, and D. Pollard for insight on and use of the PSU ice-sheet model.

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada

    Natalya Gomez & Holly K. Han

  2. Department of Geochemistry and Petrology, Institute for Geosciences, University of Bonn, Bonn, Germany

    Michael E. Weber

  3. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA

    Peter U. Clark

  4. School of Geography and Environmental Sciences, University of Ulster, Coleraine, UK

    Peter U. Clark

  5. Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA

    Jerry X. Mitrovica

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Contributions

N.G. contributed the numerical modelling and analysis; H.K.H. prepared model input; M.E.W. contributed iceberg-rafted debris records and, together with P.U.C. and J.X.M., other published data and related discussion. All authors contributed to developing the idea and to writing and refining the manuscript.

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Correspondence to Natalya Gomez.

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The authors declare no competing interests.

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Peer review information Nature thanks Frank Pattyn 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 Sensitivity of results to ice and Earth model parameters.

a, Changes in Antarctic ice volume predicted in simulations with evolving (solid lines) and fixed (dashed lines) Northern Hemisphere ice mass, and adopting the LVZ Earth model (Methods). Blue lines are identical to those in Fig. 2b, corresponding to a basal sliding coefficient of b = 10−5 m yr−1 Pa−2; red lines correspond to a basal sliding coefficient of b = 10−6 m yr−1 Pa−2. The black dotted line shows changes in Northern Hemisphere ice volume (right axis; in metres of global-mean sea level (gmsl) equivalent) prescribed in the ICE5G27 ice history. b, As in a, but adopting the HV Earth model (Methods). Blue (a) and red (b) vertical bands represent the timing of MWP and AID events, as in Fig. 2a, c, respectively.

Extended Data Fig. 2 Evolution of Antarctic ice cover with and without Northern Hemisphere sea-level forcing.

a, b, Thickness of grounded ice (in metres) and extent of ice shelves, at 30 ka, 20 ka, 10 ka and the present day, predicted from simulations that include variations in the Northern Hemisphere ice sheets represented by the ICE5G27 ice history (a) and from simulations in which ice cover in the Northern Hemisphere remains fixed (b). Black lines show the grounding lines. c, The difference in grounded ice thickness between simulations in a and b, representing the effect of sea-level changes associated with Northern Hemisphere ice sheets on the evolution of the AIS. Green and black lines represent the positions of the grounding lines with (a) and without (b) the Northern Hemisphere sea-level forcing included.

Extended Data Fig. 3 Influence of Northern Hemisphere sea-level forcing on Antarctic ice cover during the deglaciation.

The colour scale indicates the difference in the thickness (in metres) of grounded ice, at the indicated times, between simulations that include variations in the Northern Hemisphere ice sheets from ICE5G27 ice history and in which ice cover in the Northern Hemisphere remains fixed throughout the simulation. Differences are displayed as in Fig. 3c, but every 1 kyr for the past 19 kyr. Green and black lines represent the positions of the grounding lines with and without the Northern Hemisphere sea-level forcing included, respectively.

Extended Data Fig. 4 Antarctic ice-volume changes in the Ross Sea and Weddell Sea sectors.

a, Changes in ice volume in the Weddell Sea sector predicted in simulations with fixed (red) and evolving (black) Northern Hemisphere ice from the ICE5G27 ice history. b, As in a, but for the Ross Sea sector. c, Blue lines outline the areas included in the calculations in a and b; colour scale indicates the change in ice thickness (in metres) from 20 ka to the present day in the simulations that include Northern Hemisphere ice-cover changes from ICE5G27.

Extended Data Fig. 5 Influence of Northern Hemisphere sea-level forcing on the rate of Antarctic ice loss.

a–e, Rate of change of Antarctic ice volume, including grounded and floating ice, calculated with a 100-year running mean, predicted from simulations including (black) and excluding (red) Northern Hemisphere ice-cover changes, using the ice histories indicated in the legend (Methods). The mean and standard deviation of these five panels are shown in Fig. 3a.

Extended Data Fig. 6 Patterns of sea-level change for Antarctic ice loss during MWP-1A and the early Holocene.

a, b, Predicted sea-level change, normalized by the global-mean sea-level-equivalent associated with Antarctic ice loss during MWP-1A (a) and the early Holocene (including MWP-1B; b). Calculations are associated with simulations that include Northern Hemisphere ice cover changes given by ICE5G27. The patterns of sea-level change and the global mean sea-level equivalent used in the normalization are calculated over the time windows indicated by the green vertical bands in Fig. 2b. Green and magenta asterisks indicate the locations of the far-field relative sea-level records in Tahiti and Barbados.

Extended Data Fig. 7 Sensitivity of the Weddell Sea sector to geographic variability in sea-level forcing.

a, Same as Fig. 3b, but zoomed in on the Weddell Sea region, where geographically variable sea-level changes associated with Northern Hemisphere ice loss are largest (Fig. 1c). The colour scale shows the change in ice thickness predicted from a simulation adopting the ICE5G27 ice history in the Northern Hemisphere, which includes geographically variable sea-level changes associated with gravitational, deformational and Earth rotational effects activated by ice-cover changes globally, during MWP-1A (14.5–13.5 ka). Grey and black lines indicate the grounding-line position at the start and end of the time interval, respectively. b, The difference between a and the same calculation but adopting the simulation with globally uniform sea-level change from the Northern Hemisphere. The black line is as in a; the blue line indicates the grounding-line position at the end of the time interval for the uniform sea-level simulation. c, Antarctic ice-volume variations from simulations with geographically variable (black) and uniform (red) sea-level changes associated with Northern Hemisphere ice loss over the MWP-1A interval. d–f, As in a–c, but for the early Holocene interval (11.5–9 ka). In this case, d is the same as Fig. 3d, but zoomed in on the Weddell Sea region. The uniform sea-level change is calculated relative to modern topography and scaled such that the total contribution to global sea-level change from the Northern Hemisphere over the last deglaciation (since 21 ka) is 95.5 m, in agreement with ref. 27.

Extended Data Fig. 8 Predicted Antarctic ice-volume changes and global-mean sea-level contributions.

a, Changes in AIS volume predicted in a simulation with Northern Hemisphere ice cover fixed at the 40 ka configuration within ICE5G27 (solid red line) and in simulations with evolving Northern Hemisphere ice adopting the ICE5G27 (solid black line), ICE6GC31 (dashed black line) and ANU30 (cyan line) ice histories, as well as two composite ice histories in which ice cover over North America and Greenland in ICE5G has been replaced by regional GLAC1D29 models (blue lines). The dashed red line represents a simulation in which the Northern Hemisphere ice sheets are fixed at the modern configuration rather than at the 40 ka configuration throughout the simulation. In this case, marine-based sectors of the AIS start on even shallower bedrock, and hence the predicted ice-sheet growth is larger at the LGM, while the ice loss during the deglaciation occurs later and is of even smaller magnitude than in the original simulation. Note that this is not a realistic starting configuration. b, As in a, but expressed as a global-mean sea-level-equivalent (GMSLE) relative to the modern state. This is calculated by taking the ice above floatation thickness in Antarctica relative to the palaeo bedrock topography at each time step in the model, and dividing by the area of the modern ocean. Note that a and b are not directly proportional because as the bedrock topography in Antarctica evolves the volume of ice above floatation in marine sectors also changes. Blue (a) and red (b) vertical bands represent the timing of MWP and AID events, as in Fig. 2a, c, respectively.

Extended Data Fig. 9 Age model comparison and uncertainty for IBRD flux record from Iceberg Alley.

a, Age difference between the AICC 201255,56 and EDML154 age models. b, Age uncertainty in the AICC 2012 age model. c, IBRD flux time series adopting the AICC 2012 (black line, as in Figs. 2c, 4b) and EDML1/EDC3 (blue dotted line) age scales. The IBRD stack is composed of records from sites MD07-3133 and MD07-3134. It is presented here for 20–0 ka and was combined with previous data for 27–7 ka4 and 8–0 ka24. Vertical brown bars indicate AID events 1–74 on the AICC 2012 age scale. Blue vertical bars indicate MWP-1A21 and MWP-1B22. Horizontal black error bars show propagated uncertainties for the upper and lower bounds of each AID event for errors in tie-point correlation to EDML4 and uncertainties of the AICC 2012 age model.

Source data.

Extended Data Fig. 10 Comparison of predicted and observed ice-thickness changes in the Weddell Sea region.

a, b, Predicted (lines) and observed (error bars) ice thickness (in metres) above the modern thickness at sites 11–13 (a) and 14, 15 (b) from ref. 35. Predictions are from simulations in which Northern Hemisphere ice cover is evolving according to ICE5G27 (black lines) or is fixed (blue lines). Error bars show cosmogenic exposure age data with 2σ uncertainty from ref. 35. c, Map of predicted ice thickness at 12 ka, in the simulation with ICE5G27. The locations of the relevant sites in the Weddell Sea and Ross Sea (see Extended Data Fig. 11) regions are indicated. See Methods for further discussion of these results.

Extended Data Fig. 11 Comparison of predicted and observed ice-thickness changes in the Ross Sea region.

a, Predicted (lines) and observed (2σ error bars) ice thickness (in metres) above the modern thickness at Scott Coast site S (black) and sites 1 (red) and 3–5 (shades of blue) from ref. 35. The locations of the sites are indicated in b–e. Predictions are from simulations in which Northern Hemisphere ice cover is evolving according to ICE5G27 (solid lines) or is fixed (dashed lines). Observations are cosmogenic exposure age data from ref. 35. Red vertical bands represent the timing of AID events 1 and 2, as in Fig. 2c. b, Map of predicted ice thickness 12 ka in the Ross Sea, in the simulation with evolving Northern Hemisphere ice. c–e, The difference in ice thickness between 12 ka (b) and 11 ka (c), 10 ka (d) and 9 ka (e). See Methods for further discussion of these results.

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Gomez, N., Weber, M.E., Clark, P.U. et al. Antarctic ice dynamics amplified by Northern Hemisphere sea-level forcing. Nature 587, 600–604 (2020). https://doi.org/10.1038/s41586-020-2916-2

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