Simulated changes in extreme events of the hydrological cycle under enhanced greenhouse conditions

In order to estimate a possible future change in the mean as well as in extremes associated with the hydrological cycle two simulations with the high resolution version of the ECHAM4 atmospheric general circulation model (T106, ca. 1.1 degrees) have been performed. These so-called time-slices cover the periods 1970-1999 and 2060-2089, respectively. The greenhouse gases have been prescribed according to observations until 1990 and according to the IPCC scenario IS92a thereafter. Sea surface temperatures and sea-ice distributions are taken from a corresponding coupled atmosphere-ocean model simulation with coarser horizontal resolution (T42, ca. 2.8 degrees).

The time-slice experiment reveals not only a general intensification of the mean hydrological cycle due to the enhanced concentrations of the greenhouse gases, but also an increase of the probability of heavy precipitation events as well as of the length of dry and wet spells in different parts of the globe. We have described the frequency and intensity of daily precipitation by fitting the time-series of daily precipitation by gamma distributions. By means of a model of the hydrological discharge, which has been applied to the respective data originating from the two time-slices, we have been able to estimate the effect of changes in precipitation and evaporation on the occurrence of floodings.