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Consequences of climate change

Some of the likely consequences of climate change - shifts in temperature and rainfall that may create dustbowls and famine, and more frequent and violent and hurricanes – have been given considerable publicity. While these represent grave problems, it has been argued that it would benefit society more to carry on with economic business as usual, and simply adapt to the new climatic circumstances. We focus here on sea level change, the impact of which is likely to be on such a scale that adaptation cannot be presented as a preferred option.

Sea level change
Sea level has constantly fluctuated in the geological past: its highest recorded level was in the Cretaceous Period, some 80 million years ago, when CO2 levels were considerably higher than at present, and ice-caps were virtually absent from the earth. Then, sea level stood at least 200 metres higher than today, with most of the UK being submerged.

The sea level fluctuations of the Ice Age, as continental ice caps waxed and waned, are well known. Thus, 20 000 years ago, at the peak of the last glaciation, so much water had been extracted from the oceans that sea level stood some 120 metres below its present level, and Stone Age people walked across the floor of the North Sea.

The most recent analogue?
Less well known are the variable sea levels recorded in previous warm phases of the Ice Ages. For instance, in the most recent of these, some 125 000 years ago, sea level reached some 6 m higher than at present. Such a difference is geologically modest, and reflects relatively minor differences in the extent of melting of land ice. We emphasize that it occurred in a world where levels of greenhouse gases, unaffected by humans, were lower than at present.

So how much can sea level rise in a world where, say, the levels of CO2 are at twice pre-industrial levels and where global temperatures are between 2 and 5 degrees higher? We cannot predict this precisely, but sea level rises of a few to several tens of metres would not be geologically unusual.

There is general agreement on sea level during the previous (Eemian) and earlier interglacial phases being higher than at present by a few to several metres; at question is where the water came from (Greenland or Antarctica) and the precise history and speed of the sea level rise:
  • Cuffey, K.M. & Marshall, S.M. 2000. Substantial contribution to sea level rise during the last interglacial from the Greenland ice sheet. Nature, vol. 404, pp. 591-594.
  • Hearty, P.J. et al. 1999. A 20 m+ sea level middle Pleistocene highstand (Barmuda and the Bahamas) due to partial collapse of Antarctic ice. Geology, vol. 27, pp. 375-378.
  • Neumann, A.C. & Hearty, P.J. 1996. Rapid sea level changes at the close of the last interglacial (substage 5e) recorded in Bahamaian island geology. Geology, vol. 24, pp. 775-778.
  • Poore, R.Z. & Dowsett, H.J. 2001. Pleistocene reduction of polar ice-caps: Evidence from Carioco Basin marine sediments. Geology, vol. 29, pp. 71-74.