An ocean of ups, downs, and much uncertainty: sea-level rise in the 21st century

One of the oft-cited results of global warming is sea-level rise. But how does it work? Human-induced climate change contributes to sea-level rise in two ways. Almost two-thirds of the predicted rise will be caused by thermal expansion. The ocean is warming, causing the water in it to expand. There’s only one way for this ‘larger’ water to go, and that’s up, leading to rising sea levels. The remaining third is due to the input of fresh water locked up in glaciers, ice caps, and the icesheets on land at the two poles: Greenland and Antarctica. (The melting sea ice in the Arctic is a serious problem for entirely different, but it will not contribute to sea level rise, just as an ice cube melting in a glass of water does not cause the glass to overflow).

These icesheets are the greatest sources of uncertainty, the reason why scientists always give a range when they talk about sea-level rise, and hedge their predictions with caveats. (Aside from the fact that it’s in their natures to do so ;-)). It is difficult to predict how they will behave as temperatures continue to increase. How fast will they melt? A recent study predicts that with a temperature rise of 4C, which we are almost guaranteed given the lack of a serious efforts to reduce greenhouse gas emissions, sea-level can be expected to rise between 0.5 and 2.0 meters.

Almost 600 million people live within 10 miles of the coasts worldwide. Many could see their homes submerged by century’s end, leading to massive migrations away from flooded areas. Areas that are not submerged will face increased salinization of aquifers and growing lands, greater threat from flood surges, and increased shoreline erosion.

Sea-level rise will not be even around the world, however. Although it seems odd – the water in a glass has a uniform surface, as do most lakes and ponds – the ocean doesn’t quite behave that way. Because of the enormous area it covers, the immense volume of water it holds, and all the bits and bumps in the way (you know, those continents and big islands, like Australia), the water doesn’t even out. Winds can sometimes blow in such a way to ‘pile up’ water against a shoreline, like during El Nino events and in upwelling zones.  The earth’s rotation also has an effect on the movement of the ocean and sea-levels around the world.

All these factors mean that the water will rise more in some areas, and may actually fall in others. (Just to make things more interesting, along the coast of some northern areas like Scandinavia, sea level is rising, but the land is rising, too! Since the end of the last ice age, when the massive weight of all that ice pushed the land down, these regions have been experiencing uplift).

Under any sea-level rise scenario, coastal wetlands worldwide are vulnerable.  A study released today predicts that under a slow rise scenario (the one put forth in the Fourth IPCC Assessment report in 2007), many wetlands along the US East Coast would completely disappear by the century’s end.  Under the rapid rise scenario, which is close to the one quoted above, most coastal wetlands worldwide will disappear. It’s a sobering thought.

Kirwan, M. L., G. R. Guntenspergen, A. D’Alpaos, J. T. Morris, S. M. Mudd, and S. Temmerman (2010), Limits on the adaptability of coastal marshes to rising sea level, Geophys. Res. Lett., 37, L23401, doi:10.1029/2010GL045489.

Nicholls, R. J., N. Marinova, J. A. Lowe, S. Brown, P. Vellinga, D. de Gusmão, J. Hinkel, and R. S. J. Tol (2010), Sea-level rise and its possible impacts given a ‘beyond 4°C world’ in the twenty-first century, Phil. Trans. R. Soc. A 2011 369, 161-181
doi: 10.1098/rsta.2010.0291


5 thoughts on “An ocean of ups, downs, and much uncertainty: sea-level rise in the 21st century

  1. Danielle,

    Why will both the slow rise and the fast rise scenarios result in a loss of significant (in the statistical sense) coastal wetlands?

    Why won’t such systems migrate landward as barrier beaches and barrier islands do on a shorter timeframe? I didn’t think there was anything special about the present sea level rise relative to the continential shelf….except for all the people and cities who reside there that is.


    1. Good question. I believe that they would, except there’s nowhere to go. The land close to the coast is so developed, especially on the East Coast, that migrating wetlands will have little room to spread inland.

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