Alluvial records of paleofloods show that natural floods resulting from excessive rainfall, snowmelt, or from combined rainfall and snowmelt are highly sensitive to even modest changes of climate equivalent or smaller than changes expected from potential future global warming in the 21st century. The high sensitivity results from effects of hemispheric or global-scale changes in circulation patterns of the ocean and atmosphere to influence the pathways and locations of air masses and storm tracks. Holocene paleoflood chronologies from the Upper Mississippi Valley in the Midwest United States and from the Colorado River drainage of the Southwest United States show that recurrence frequencies of large floods have been subject to abrupt changes over time. These flood chronologies and flood chronologies observed for other middle-latitude regions suggest that recurrence frequencies of large floods are increased when there is an increase in the number of waves and their amplitudes in the middle and upper tropospheric circum-polar westerly circulation. However, some middle-latitude regions on the western margins of continents experience increased frequencies of flooding during strong onshore zonal westerly circulation. Flood chronologies from several regions suggest that times of rapid climate change have a tendency to be associated with more frequent occurrences of large and extreme floods. The unusual high frequencies of large floods that have been observed in many regions since the early 1950s are often attributed to land use change, but the rapid climate forcing from the effects of increased atmospheric greenhouse gases may also be a contributing factor. Paleoflood records provide information that is useful for better interpretation and calibration of modern short-term instrumental records, and they provide unique event-scale information that is useful for calibrating and testing geophysical models of past and anticipated future climate conditions.

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