We can also expect sea-level rise to continue after the end of 21st century due to the long response times of the ice sheets and ocean. In fact, the next ~80 years is a very short time window when considering the long timescales of processes operating in the climate system and the total sea-level rise we might eventually experience.

The paleo perspective

Much of our understanding of potential long-term sea-level changes comes from the paleo record. Evidence from past warm periods in Earth's history (i.e. when global temperatures were almost as warm or warmer than now) shows that sea level was substantially higher than at present. During the last interglacial 125,000 years ago, for example, global temperatures were close to those we see today, but global sea level peaked somewhere between 6 and 9 m higher (Kopp et al. 2009). Other interglacial periods over the past 3 million years similarly indicate that global sea levels were higher during periods when the Earth was only a few degrees warmer. Studying these past warm periods can, therefore, help us better understand what might happen in the future. But it is important to bear in mind that the nature or forcing of past climate changes was different to the global warming occurring now – so these past warm periods do not represent a perfect analogue for future changes on our planet.

TEMPERATURE: Peak global mean temperature, atmospheric CO2, maximum global mean sea level, and source(s) of meltwater. Light blue shading indicates uncertainty of global mean sea level maximum. Red pie charts over Greenland and Antarctica denote fraction (not location) of ice retreat. Taken from Dutton et al. (2015).

What were the contributing sources of these higher sea levels in the past? As the potential contribution from mountain glaciers and thermal expansion is limited (to around 1 m or so) it implies that the Greenland and Antarctic ice sheets were somewhat smaller than at present. In fact, as shown in the diagram above, it could be that ice on Greenland all but disappeared during some of these past warm periods. Identifying exactly which of the ice sheets contributed how much is a tricky question to answer and the focus of ongoing research.

Our sea level commitment

When discussing long-term sea-level changes scientists often refer to our sea level commitment. It is the long-term (typically over a several thousand-year timeframe) sea-level rise we can expect from a constant climate forcing. This gives us an idea of the total sea-level rise we will eventually experience once the ice sheets and oceans start to come into equilibrium with the climate. As discussed above, the potential multi-meter contribution from the ice sheets is of most concern. The Greenland and Antarctic ice sheets contain enough ice to raise global sea levels by around 7 m and 58 m, respectively (Vaughan et al., 2013).

Research on projections of long-term sea-level changes suggests that, depending on our future greenhouse gas emissions, we are committed to a global sea-level rise of between ~25 and ~50 m over the next 10,000 years (Clark et al., 2016). The results of this study (see figure below) show that much of this global sea-level rise could be realized within the next few thousand years, with rates of rise reaching 2 to 4 m/century. Thus, as for what happens after 2100, it is clear we are looking at a large rise in global sea level as the ice sheets respond to sustained warming. Greenhouse gas emissions made today and over the remainder of the 21st century will play a crucial role in determining the size and speed of this rise, as well as our eventual sea level commitment.

PAST AND FUTURE GLOBAL SEA-LEVEL CHANGES: Paleo records of global sea-level rise over the past 20,000 years are from Lambeck et al. (2014). Future projections for the next 10,000 years from Clark et al. (2016) are for a warming of ~2°C (green) and ~7.5°C (red) relative to the preindustrial. Observed global temperatures are currently ~1°C above preindustrial levels.

References

P.U. Clark, J.D. Shakun, S.A. Marcott, A.C. Mix, M. Eby, S. Kulp, A. Levermann, G.A. Milne, P.L. Pfister, B.D. Santer, D.P. Schrag, S. Solomon, T.F. Stocker, B.H. Strauss, A.J. Weaver, R. Winkelmann, D. Archer, E. Bard, A. Goldner, K. Lambeck, R.T. Pierrehumbert, G. Plattner (2016). Consequences of twenty-first-century policy for multi-millennial climate and sea-level change, Nature Climate Change, vol. 6, pp. 360-369, 2016. doi: 10.1038/nclimate2923.

A. Dutton, A. E. Carlson, A. J. Long, G. A. Milne, P. U. Clark, R. DeConto, B. P. Horton, S. Rahmstorf, M. E. Raymo. (2015). Sea-level rise due to polar ice-sheet mass loss during past warm periods. Science 349, 6244.

R. E. Kopp, F. J. Simons, J. X. Mitrovica, A. C. Maloof, M. Oppenheimer. (2009) Probabilistic assessment of sea level during the last interglacial stage. Nature 462, 863.

Lambeck, K., Rouby, H., Purcell, A., Sun, Y., Sambridge, M. (2014). Sea level and global ice volumes from the Last Glacial Maximum to the Holocene. Proc. Natl Acad. Sci. USA 111, 15296–15303.

D. G. Vaughan, J. C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, T. Murray, F. Paul, J. Ren, E. Rignot, O. Solomina, K. Steffen, T. Zhang. (2013). Observations: Cryosphere. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.