Trends in river floods: why is there no clear signal in observations?
Svensson, C.; Hannaford, J.; Kundzewicz, A. W.; Marsh, T. J.. 2006 Trends in river floods: why is there no clear signal in observations? In: Frontiers in flood research, 8th Kovacs Colloquium, UNESCO, Paris, 30 June - 1 July 2006. IAHS, 1-18.
Floods are of great concern in many areas of the world, with the last decade seeing major fluvial flood events in, for example, Asia, Europe and North America. This has focused attention on whether or not these are a result of a changing climate. River flows calculated from outputs from global climate models often suggest that high river flows will increase in a warmer, future climate. However, the future projections are not necessarily in tune with the records collected so far—the observational evidence is more ambiguous. A recent study of trends in long time series of annual maximum river flows at 195 gauging stations worldwide suggests that the majority of these flow records (70%) do not exhibit any statistically significant trends. Trends in the remaining records are almost evenly split between having a positive and a negative direction. This paper discusses factors that influence the results of trend estimates of floods, and that contribute to the general lack of compelling observational evidence of any long-term increase in extreme river flows. Recent results of trend analysis of observed floods are outlined. Expected impacts of indirect anthropogenic climate change are discussed, and a summary is given of the direct impact of man’s influence on river flows in terms of catchment and river management. Different methodologies to detect trends are briefly outlined, and examples are given of how the choice of method can interact with climatological features to result in different estimates of trend. The examples illustrate the effects of using different types of flow indices and different periods of record. The effects on trend estimates of decadal-scale oscillations that have been shown to occur in many river flow records are discussed. Oscillations compound the problem of untangling trends from normal climatic variability as the cycles of the underlying climatic phenomena (e.g. the North Atlantic Oscillation) may also be predicted to change in a greenhouse gas-induced warmer climate. Initiatives to compile networks of pristine catchments with long river flow records are welcomed as a means of keeping scientific objectivity at the forefront of studies of change detection, an area of research riddled by uncertainty and speculation.