Abstract

Warming of the atmosphere as a result of an increased concentration of greenhouse gases is expected to lead to a significant rise is global sea level. We present estimates of the component of this sea level rise caused by thermal expansion of the ocean. These estimates are based on the idea that the upper layers of the main gyres of the ocean are ventilated by the subduction of water at higher latitudes and its subsequent equatorward and downward flow into the main thermocline along surfaces of constant “density”. In this mechanism, heat enters the ocean by an advection process rather than by vertical diffusion, as in previous estimates of the component of sea level rise that is caused by thermal expansion. After the heat initially enters the subtropical gyres by subduction, it is then redistributed to preserve gradients of the depth-integrated pressure field, by an adjustment involving low vertical-mode baroclinic waves. Estimates of historical sea level rise based on this simple ventilation scheme, when combined with estimates of nonpolar glacial melt, are about equal to the observed sea level rise. For a global mean 3.0°C (1.5°C, 4.5°C) temperature rise by 2050 (and with the spatial distribution predicted by three climate models), we estimate the component of sea level rise that is caused by thermal expansion to be about 0.2 to 0.3 m (0.1 m, 0.4 m) by 2050. Low-mode internal Rossby and Kelvin waves appear to be quite efficient at distributing the sea level rise evenly over the earth without major distortions to the thermocline. A delayed warming, as suggested by transient coupled ocean-atmosphere models, can be simulated by using a smaller temperature rise, say 1.5°C rather than 3.0°C, by 2050. Changes in sea level arising from variations in the wind field could be estimated, but so far our calculations are based on the assumption that the wind stress field does not change from its present value. We estimate the maximum rate of sea level rise caused by changes in deep water formation is 0.1 meter per century. Contributions from the cryosphere reported in the literature range from near zero to about 0.35 m. When added to the thermal expansion components, our total sea level rise scenario for 2050 for a temperature rise of 3.0°C (1.5°C to 4.5°C) is about 0.35 m (0.15 and 0.70 m).

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