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Sea-level change due to climate change

Final Report Summary - SEACHANGE (Sea-level change due to climate change)

The Seachange project aimed to improve scientific understanding of predicted changes in sea-level in coming decades due to changes in ocean temperature, salinity and circulation. These effects on sea-level are of comparable importance to the contributions from land ice (glaciers and ice-sheets). Warming of the ocean leads to expansion of sea water, and hence to global-mean sea-level rise, while changes in temperature, salinity and wind cause changes in regional sea-level, which may locally be substantially larger or smaller than the global-mean rise. Among the models used for climate projections for the 21st century assessed by the Intergovernmental Panel on Climate Change, there is a large spread in projections for both global and regional sea-level change. Understanding of the reasons for this spread is required in order to reduce it and hence make more reliable projections, which are necessary for assessment of impacts and actions for mitigation. We seek to gain improved understanding by comparison of results of different climate models and with observations.

In the Seachange project, we have studied global ocean temperature change and thermal expansion both in models and in observations. We have shown that global-mean sea-level rise due to thermal expansion in the 20th century may have been underestimated by models, because of not fully including the natural effect of occasional explosive volcanic eruptions. Correcting for this, and using recent estimates of changes in land ice during the twentieth century, we show how the observed global-mean sea-level rise can be accounted for. We expect that the geographical pattern of sea-level change in response to anthropogenic forcing will be detectable within the next decade. We argue that if society wishes to set targets for CO2 emissions in order to limit sea-level rise, it will be necessary to consider when the emissions occur, as well as their total, because emissions which occur in the nearer future lead to greater sea-level rise.

We show that a large part of the spread in projections of warming comes from the efficiency with which heat is removed from the surface into the deeper ocean, and its distribution within the deeper ocean. We find that the simulated physical processes of ocean heat uptake strongly depend on the formulation of the model. This points to a need for further work to refine and constrain the models. The Southern Ocean and the North Atlantic are regions where sea-level change is predicted to be particularly large, and where also the models have a large spread, implying uncertainty in projections. We find that a large part of this spread can be attributed to the projections of changes in surface fluxes from the atmosphere to the ocean. Sea-level change in the Southern Ocean is most strongly affected by changes in winds (surface momentum flux) and surface heating, while change in the North Atlantic is mainly affected by changes in surface heating, with additional effects due to the consequent weakening of the Atlantic meridional overturning circulation. Changes in surface water fluxes (precipitation, evaporation and river inflow) are relatively less influential on the pattern of sea-level change. The results of Seachange have stimulated the initiation of an international project to compare the results of climate models regarding the influences and physical processes determining ocean heat uptake and regional sea-level change.