Sea level rise

Introduction | Holocene sea level rise | Current observations | Predicting the future | References | Comments |


Global sea levels are currently rising at an average rate of 1.8 mm per year since 1961, and 3.1 mm per year since 1993. The main contributions for this rise are from melting glaciers and ice caps and thermal expansion of the ocean. In line with this, the extent of polar snow and ice cover has been receding[1]. One of the main targets of glaciologists and glacial geologists is to better understand rates of sea level rise, to allow better predictions of future change[2]. The IPCC future estimations of sea level rise do not take into account dynamic changes in glaciers (such as the impacts of ice shelf collapse or marine ice sheet instability). Understanding Holocene rates of sea level rise contextualises present rates of change. Finally, understanding the rate of sea level rise around Antarctica can be used to constrain past ice volumes.

Holocene sea level rise

When an ice mass grows on land, it depresses the crust, and raises the relative local sea level[3]. As the ice melts, the crust rebounds. This is called isostatic uplift. For example, Scotland is still rebounding after the last great ice age in Britain[4, 5]. Places like these, which were depressed during the last glaciation, are called near field sites. We can use near field sites to reconstruct the past volume of ice (because we know the viscosity of the crust and how much mass is needed to depress it be a certain amount).

Sea level variation during the last post-glacial period. Credit: Robert A. Rhode, Global Warming Art Project, Wikimedia Commons.

The story is more complicated, however, because when there is a lot of ice in the world (a high global ice volume, for example, during the Last Glacial Maximum, ~18,000 years ago[6]), global sea levels are lower. This is eustatic sea level change: the water is locked up in ice sheets, instead of in the oceans. Tectonically stable places far away from places with high ice volumes during the Last Glacial Maximum are called far field sites, because they had no isostatic depression during the last glaciation[7]. These places measure the global change in sea level over the last glacial cycle (sea levels were about 120 m lower during the last glaciation).

So, far field sites constrain global sea level changes, and near field sites constrain ice volumes. However, it is complex, and regional interactions between isostatic and eustatic sea level change gives us local rates of relative sea level change. Scientists can use raised beaches, dated with a variety of methods, to constrain local rates of relative sea level change[8, 9]. On islands, hollows can accumulate marine sediments and organisms. When these are uplifted above sea level, they accumulate lacustrine (fresh-water lake) organisms and sediments. Using radiocarbon dating and biostratigraphy, and taking into account global eustatic sea level rise, scientists can calculate when the region was uplifted, and by how much[10-12].

Current observations

Recent sea level rise. Credit: Bruce C. Douglas (1997). “Global Sea Rise: A Redetermination”. Surveys in Geophysics 18: 279–292. DOI:10.1023/A:1006544227856. Image from Global Warming art project. Wikimedia Commons

The IPCC currently estimates global sea level rise to be around 1.8 ± 0.5 mm per annum. The melting of mountain glaciers and ice caps accounts for quite a lot of this rise[13], and this may be because smaller glaciers, which also tend to be steeper, are more sensitive to climate warming[14]. Over the last 15 years, glaciers around the Antarctic Peninsula and in southern South America together have contributed 0.19 ± 0.045 mm per year to sea level rise[15].


Predicting the future

Future sea level projections to 2100 from the IPCC. Commonwealth Scientific and Industrial Research Organisation (CSIRO). Creative Commons Attribution

The IPCC predicts future sea level rise based on presents rates of melting and predictions of future carbon emissions and warming. However, there are large uncertainties (visible on the graph), because the dynamic interaction of ice sheets to climate change needs to be better understood[16]. Predictions to 2100 range from 20 cm to 2 m[16]. The best estimate is 0.6 m, mostly from thermal expansion of the oceans and glacier melt. Accelerated ice velocities, marine ice sheet instabilities and ice shelf collapse all form part of the large uncertainties in estimating future global sea level rise.

A collapse of the West Antarctic Ice Sheet would raise sea levels by about 3.3 m[17]. Although it is unlikely, if the entire Antarctic Ice Sheet melted, it would raise sea levels by about 60 m [2]. You can read more about Antarctica’s contribution to global sea level rise in this blog post.

Impact of sea level rise

Explore the impact of sea level rise in the USA to 2100 using this cool interactive feature from Climate Central.

Further reading

Further reading: This nice, open-access paper by Van den Broeke et al., 2011.

See the Climate Institute for more information.

You can make sea levels rise here, and see if your house gets flooded!

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1.            IPCC core writing team, 2007. Climate change 2007: synthesis report. IPCC, 52 pp.

2.            Alley, R.B., Clark, P.U., Huybrechts, P., and Joughin, I., 2005. Ice-sheet and sea-level changes. Science, 2005. 310(5747): p. 456-460.

3.            Shennan, I., Peltier, W.R., Drummond, R., and Horton, B., 2002. Global to local scale parameters determining relative sea-level changes and the post-glacial isostatic adjustment of Great Britain. Quaternary Science Reviews, 2002. 21(1-3): p. 397-408.

4.            Shennan, I., Bradley, S., Milne, G., Brooks, A., Bassett, S., and Hamilton, S., 2006. Relative sea-level changes, glacial isostatic modelling and ice sheet reconstructions from the British Isles since the Last Glacial Maximum. Journal of Quaternary Science, 2006. 21: p. 585-599.

5.            Shennan, I., Hamilton, S., Hillier, C., and Woodroffe, S., 2005. A 16 000-year record of near-field relative sea-level changes, northwest Scotland, United Kingdom. Quaternary International, 2005. 133-134: p. 95-106.

6.            Clark, P.U., Dyke, A.S., Shakun, J.D., Carlson, A.E., Clark, J., Wohlfarth, B., Mitrovica, J.X., Hostetler, S.W., and McCabe, A.M., 2009. The Last Glacial Maximum. Science, 2009. 325(5941): p. 710-714.

7.            Peltier, W.R. and Fairbanks, R.G., 2006. Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews, 2006. 25(23-24): p. 3322-3337.

8.            Fretwell, P.T., Hodgson, D.A., Watcham, E.P., Bentley, M.J., and Roberts, S.J., 2010. Holocene isostatic uplift of the South Shetland Islands, Antarctic Peninsula, modelled from raised beaches. Quaternary Science Reviews, 2010. 29(15-16): p. 1880-1893.

9.            Watcham, E.P., Bentley, M.J., Hodgson, D.A., Roberts, S.J., Fretwell, P.T., Lloyd, J.M., Larter, R.D., Whitehouse, P.L., Leng, M.J., Monien, P., and Moreton, S.G., 2011. A new Holocene relative sea level curve for the South Shetland Islands, Antarctica. Quaternary Science Reviews, 2011. 30(21-22): p. 3152-3170.

10.          Sterken, M., Roberts, S.J., Hodgson, D.A., Vyverman, W., Balbo, A.L., Sabbe, K., Moreton, S.G., and Verleyen, E., 2012. Holocene glacial and climate history of Prince Gustav Channel, northeastern Antarctic Peninsula. Quaternary Science Reviews, 2012. 31(0): p. 93-111.

11.          Roberts, S.J., Hodgson, D.A., Sterken, M., Whitehouse, P.L., Verleyen, E., Vyverman, W., Sabbe, K., Balbo, A., Bentley, M.J., and Moreton, S.G., 2011. Geological constraints on glacio-isostatic adjustment models of relative sea-level change during deglaciation of Prince Gustav Channel, Antarctic Peninsula. Quaternary Science Reviews, 2011. 30(25-26): 3603–3617.

12.          Roberts, S.J., Hodgson, D.A., Bentley, M.J., Sanderson, D.C.W., Milne, G., Smith, J.A., Verleyen, E., and Balbo, A., 2009. Holocene relative sea-level change and deglaciation on Alexander Island, Antarctic Peninsula, from elevated lake deltas. Geomorphology, 2009. 112(1–2): p. 122-134.

13.          Hock, R., de Woul, M., Radic, V., and Dyurgerov, M., 2009. Mountain glaciers and ice caps around Antarctica make a large sea-level rise contribution. Geophysical Research Letters, 2009. 36: p. L07501.

14.          Oerlemans, J. and Fortuin, J.P.F., 1992. Sensitivity of Glaciers and Small Ice Caps to Greenhouse Warming. Science, 1992. 258(5079): p. 115-117.

15.          Ivins, E.R., Watkins, M.M., Yuan, D.-N., Dietrich, R., Casassa, G., and Rülke, A., 2011. On-land ice loss and glacial isostatic adjustment at the Drake Passage: 2003-2009. J. Geophys. Res., 2011. 116(B2): p. B02403.

16.          Willis, J.K. and Church, J.A., 2012. Regional sea level projection. Science, 2012. 336: p. 550-551.

17.          Bamber, J.L., Riva, R.E.M., Vermeersen, B.L.A. and Le Brocq, A.M., 2009. Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet. Science, 324 (5929): 901-903.

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