Sensitivity of modern and Holocene floods to climate change

Alluvial records of paleofloods show that natural floods resulting from exc essive rainfall, snowmelt, or from combined rainfall and snowmelt are highl y sensitive to even modest changes of climate equivalent or smaller than ch anges expected from potential future global warming in the 21st century. Th e high sensitivity results from effects of hemispheric or global-scale chan ges in circulation patterns of the ocean and atmosphere to influence the pa thway's and locations of air masses and storm tracks. Holocene paleoflood c hronologies from the Upper Mississippi Valley in the Midwest United States and from the Colorado River drainage of the Southwest United States show th at recurrence frequencies of large floods have been subject to abrupt chang es over time. These flood chronologies and hood chronologies observed for o ther middle-latitude regions suggest that recurrence frequencies of large h oods are increased when there is an increase in the number of waves and the ir amplitudes in the middle and upper tropospheric circum-polar westerly ci rculation. However, some middle-latitude regions on the western margins of continents experience increased frequencies of flooding during strong onsho re zonal westerly circulation. Flood chronologies from several regions sugg est that times of rapid climate change have a tendency to be associated wit h more frequent occurrences of large and extreme floods. The unusual high f requencies of large floods that have been observed in many regions since th e early 1950s are often attributed to land use change, but the rapid climat e forcing from the effects of increased atmospheric greenhouse gases may al so be a contributing factor. Paleoflood records provide information that is useful for better interpretation and calibration of modern short-term inst rumental records, and they provide unique event-scale information that is u seful fbr calibrating and testing geophysical models of past and anticipate d future climate conditions. (C) 1999 Elsevier Science Ltd. All rights rese rved.