This study has not sought to examine the question of whether or not river flooding events have increased in recent years. The data are unavailable and the causes too complex to make a reasonable estimate of general trends in this area. Where data are available however we have reported on precipitation and streamflow trends.
The flooding phenomena examined here provide an insight into the potential impacts of future climate change on flooding rather than directly attributing individual events, however unusual, to the enhanced greenhouse effect. Trends in underlying parameters, such as streamflow in the USA, are consistent with GCM predictions but also lie within the bounds of natural variability.
The direct use of GCM output to drive hydrologic models is considered to be improper due to the coarse resolution (relative to river basin scale) of the spatial grids used by GCMs and the simplified GCM representations of sub-grid atmospheric processes. However, as the GCMs perform reasonably well in simulating the climate with respect to annual and seasonal averages over large regions, the simulations can be used to guide relevant changes that are applied to catchment scale hydrologic models.
The reliability of any estimation of the impact of climate change on surface hydrology is questionable because of the limitations of hydrological models. Values quoted here should, therefore, be considered as plausible changes in runoff based on the current state of science.
