Paragraph 1. INCREASED TEMPERATURES

The average air temperature at the surface of the earth has risen this century, as has the temperature of ocean surface waters. Because water expands as it heats, a warmer ocean means higher sea levels. We cannot say definitely that the temperature rises are due to the greenhouse effect; the heating may be part of a “natural” variability over a long time-scale that we have not yet recognized I our short 100 years of recording. However, assuming the build up of greenhouse gases is responsible, and that the warming will continue. Scientists and inhabitants of low-lying coastal areas would like to know the extent of future sea level rises.

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Calculating this is not easy. Models used for the purpose have treated the oceans as passive, stationary and one-dimensional. Scientists have assumed that heat simply diffused into the sea from the atmosphere. Using basic physical laws, they then predict how much a known volume of water would expand for a given increase in temperature. But the oceans are not one-dimensional, and recent work by oceanographers, using a new model which takes into account a number of subtle facets of the sea-including vast and complex ocean currents-suggests that the rise in sea level may be less than some earlier estimates had predicted.

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An international forum on climate change, in 1986, produced figures for likely sea-level rises of 20 cm and 1.4 m, corresponding to atmospheric temperature increases of 1.5 and 4.5C respectively. Some scientists estimate that the ocean warming resulting from those temperature increases by the year 2050 would raise the sea level by between 10 cm and 40 cm. This model only takes into account the temperature effect on the oceans; it does not consider changes in sea level brought about by the melting of ice sheets and glaciers, and changes in groundwater storage. When we add on estimates of these, we arrive at figures for total sea-level rises of 15 cm and 70 cm respectively.

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It’s not easy trying to model accurately the enormous complexities of the ever-changing oceans, with their great volume, massive currents and sensitively to the influence of land masses and the atmosphere. For example, consider how heat enters the ocean. Does it just “diffuse” from the warmer air vertically into the water, and heat only the surface layer of the sea? (Warm water is less dense than cold, so it would not spread downwards). Conventional models of sea-level rise have considered that this the only method, but measurements have shown that the rate of heat transfer into the ocean by vertical diffusion is far lower in practice than the figures that many models have adopted.

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Much of the early work, for simplicity, ignored the fact that water in the oceans moves in three dimensions. By movement, of course, scientists don’t mean waves, which are too small individually to consider, but rather movement of vast volumes of water in huge currents. To understand the importance of this, we now need to consider another process- advection. Imagine smoke rising from a chimney. On a still day it will slowly spread out in all directions by means of diffusion. With a strong directional wind, however, it will all shift downwind, this process is advection-the transport of properties (notably heat and salinity in ocean) by the movement of bodies of air or water, rather than by conduction or diffusion.