Sea Level Rise Problems

Abstract

The greenhouse warming over the next 100 years has been calculated by means of the coupled atmosphere-ocean model of the Max-Planck-Institute of Meteorology Hamburg. The computations are based on the IPCC (Intergovernmental Panel on Climate Change) scenarios A (“Business as usual”) and D (“Accelerated policies”) and a “2×CO₂” experiment. As a reference state the “constant 1985 CO₂ concentration” has been chosen. The resulting global increase of the mean near bottom air temperature after 100 years is 2.6 K for scenario A, 0.6 K for D and 1.7 K for the “2×CO₂” scenario. The corresponding global sea level rise due to the thermal expansion of the ocean is 15 cm for scenario A, 5 cm for D and 16 cm for the “2×CO₂” experiment. These values seem to be small, but they have a strongly increasing tendency. The calculated changes of heat and salt fluxes at the sea surface have been used to drive an ocean general circulation model. Here a global mean of the temperature increase of 2.9°K within 50 years has been computed. However, the geographical distribution of the anomalies is very distinct: in tropical latitudes only small changes appear, where a maximum heating of more than 4.0°K occurs at the polar fronts. The largest sea level rise of more than 30 cm can thus be found in the latter regions. These global coupled atmosphere-ocean climate models typically have a horizontal resolution of about 500 km. Therefore they can only provide new climatologies, i.e. global atmospheric pressure and circulation patterns as well as large scale changes of the sea level, but not regional forecasts (e.g. for the North Sea with typical scales of 10 km). Thus, a “downscaling” is required. Appropriate methods are grid focusing and development of empirical transfer functions linking global atmospheric patterns and actual weather situations (such as local storms). Procedures are presented and examples given, demonstrating how the transient wind forcing in a regional sea can be derived from global climate models. Next, using a mesoscale resolving ocean model, possible extreme surges are predicted. The applications refer to the North Sea, but can be extended to other threatened areas of the world like Bangladesh.