B. Geerts and E. Linacre
During the twentieth century, the global-mean sea-level has risen, on average, by 10-20 cm (1). The three factors chiefly determining sea-level change are:
Warmer water assumes a larger volume, so sea surface temperatures (SSTs) affect sea levels. However it is not clear how the warming varies with depth in the ocean. The observed warming appears to be concentrated above the thermocline. Depending on the assumptions involved, the observed SST increase during the last century implies a sea-level rise of about 3-10 cm/century (2). The retreat of continental glaciers worldwide between 1900-61 yields an estimated sea level rise of 2-5 cm/century (2). No firm evidence exists that the volume of ice on Greenland and Antarctica is changing, but the melting of all this ice would raise sea levels by 7 m and 65 m, respectively. It is not clear how global warming would affect the polar icecaps: higher temperatures would increase snowfall near the poles, yet temperatures would more often be above 0°C over a larger region, enhancing melting. For instance, except along the coast, the surface air temperature over Antarctica is always well below freezing, but on the west side of Antarctica it has risen by 0.5 K per decade since the mid-1940’s (3).
There may be a slight decrease in accumulation of snow in Greenland, but a large increase of melting, as a result of global warming. Over Antarctica, there may be a significant increase of accumulation, and still virtually no melting. The Greenland discharge would raise global sea levels by 1.1 mm/a, whereas the Antarctic accumulation would lower levels by 0.9 mm/a (4). In other words, changes in the ice budget at opposite poles would more or less offset each other.
The upshot is a widely ranging set of estimates for sea-level rise of around 40 cm by the year 2100, largely due to a SST increase of about 2.5 K.
The amount of sea ice does not directly affect the sea level, not because sea ice generally is less a metre thick, but rather because it is floating. Indirectly, it does affect the sea level, through its effect on global temperatures. Its high albedo, compared with that of open water, leads to a positive temperature feedback as a result of solar heating. It inhibits the loss of heat from the ocean to the atmosphere, and therefore it enhances the ‘continentality’ of adjacent coastal areas. The formation of ice causes an increase of salinity in the adjacent water, since the ice formation excludes most of the salt. The increased salinity means a higher density, which fosters thermohaline subsidence of the water beneath the ice. Conversely, melting of the ice in summer leaves a layer of relatively low-density water on the sea surface.
The area of sea ice appears to be falling around the Arctic (1), but around Antarctica only in the Amundsen & Bellingshausen Sea (since 1973) (3). Gaps of open water within an area of sea ice are known as ‘polynyas’. There is substantial heat loss from polynyas to the atmosphere, and consequent subsidence of the cooled water.
(1) Houghton, J.T. et al., L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg & K. Maskell (eds) 1996. Climate Change 1995: the science of climate change. Contribution from the Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. (Cambridge Univ. Press) 572pp.
(2) Warwick, R.A. et al. 1996. Changes of sea level. In Houghton et al. 1996, 359 - 405.
(3) Church, J. 1997. Observations of DecCen variability in the southern hemisphere oceans. Publ. series 11 (ICPO, World Climate Res. Prog., World Meteor. Organ.), 40-52.
(4) Ohmura, A., M. Wild and L. Bengtsson 1996. A possible change in the mass balance of Greenland/Antarctic ice sheets in the coming century. J. Clim., 9, 2124-35.