In the past, sea level changes have varied widely on different time and
space scales. On larger time scales (ice ages), the global mean sea level
varied by more than a hundred meters, but after the latest deglaciation
event sea level changes have been considerably smaller.
According to most estimates based on tide-gauge measurements, the global mean sea level rise over the last hundred years was 10-20 cm. Melting of glaciers and small ice caps and thermal expansion of sea water are generally thought to be the main contributors to this rise, but there are still uncertainties about their relative importance.
Estimates of sea level rise due to thermal expansion during the period 1856-1991 are presented in this paper. The study differs from most previous studies in that the investigators used a zonally averaged ocean model, in which the ocean circulation is prescribed. The model consists of three ocean basins, representing the Atlantic, Pacific and Indian Oceans, which are connected by a circumpolar basin that represents the Southern Ocean.
The ocean surface forcing was based on observed sea surface temperatures instead of radiative forcing. Experiments were done with the three forcing types, which are of a global, a regional, and a more two-dimensional nature. In the model, only ocean temperatures were considered since the initial temperature field was assumed to be in thermal equilibrium. Because the ocean temperatures at the beginning of the selected period are unknown, the present-day temperatures were chosen as the initial temperature distribution.
Different parameterizations of the ocean heat mixing were considered in this study, namely diffusion by use of uniform diffusion coefficients, diffusion in which the vertical diffusivity was assumed to depend on the vertical stability of the water column, and isopycnal diffusion. Because all these parameterizations seem to be reasonable, differences in the model results are considered to reflect the existing uncertainties concerning ocean mixing. In this study, global mean sea level rise caused by thermal expansion over the past hundred years is estimated to be on the range 2.2-5.1 cm, a best estimate being 3.5 cm. These values are in close agreement with previous estimates obtained by means of simpler one-dimensional upwelling-diffusion models.
In spite of the slight differences in global sea level obtained by means of the presented model and the one-dimensional models, the authors argue that their model presents the advantage of a more realistic representation of the ocean thermohaline circulation. Furthermore, to estimate thermal expansion in the future, an atmospheric energy-balance model can be coupled easily to this ocean model, together with a sea ice model.
According to the authors, by means of such a climate model, one can examine the effect of several climate aspects on thermal expansion, which require some horizontal resolution such as the albedo-temperature feedback and possible changes in the sea ice coverage. However, a disadvantage of all upwelling-diffusion models compared with other models is the extremely simple representation of processes involved in the penetration and distribution of heat from the surface to the interior of the ocean. Since these simplifications make the upwelling-diffusion models cheap in computer time, they still are useful to perform a wide range of sensitivity experiments.
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