LARGE INCREASES IN FLOOD MAGNITUDE IN RESPONSE TO MODEST CHANGES IN CLIMATE

RECENT examinations of the possible hydrological response to global warming have emphasized changes in average conditions, rather than individual flooding events1-5. Historical accounts suggest, however, that such events may have had a considerable regional impact6-9 even in the face of any relatively modest climate change8. Here I present a 7,000-year geological record of overbank floods for upper Mississippi river tributaries in mid-continent North America, which provides concrete evidence for a high sensitivity of flood occurrence to changing climate. During a warmer, drier period between about 3,300 and 5,000 years ago, the largest, extremely rare floods were relatively small-the size of floods that now occur about once every fifty years. After approximately 3,300 years ago, when the climate became cooler and wetter, an abrupt shift in flood behaviour occurred, with frequent floods of a size that now recurs only once every 500 years or more. Still larger floods occurred between about AD 1250 and 1450, during the transition from the medieval warm interval to the cooler Little ice Age. All of these changes were apparently associated with changes in mean annual temperature of only about 1-2-degrees-C and changes in mean annual precipitation of less-than-or-equal-to 10-20%.