Letters to Nature

Nature 415, 514-517 (31 January 2002) | doi:10.1038/415514a; Received 2 October 2001; Accepted 17 December 2001

Increasing risk of great floods in a changing climate

P. C. D. Milly1, R. T. Wetherald2, K. A. Dunne1 & T. L. Delworth2

  1. US Geological Survey, GFDL/NOAA, P.O. Box 308, Princeton, New Jersey 08542, USA
  2. Geophysical Fluid Dynamics Laboratory/NOAA, P.O. Box 308, Princeton, New Jersey 08542, USA

Correspondence to: P. C. D. Milly1 Correspondence and requests for materials should be addressed to P.C.D.M. (e-mail: Email: cmilly@usgs.gov).

Radiative effects of anthropogenic changes in atmospheric composition are expected to cause climate changes, in particular an intensification of the global water cycle1 with a consequent increase in flood risk2. But the detection of anthropogenically forced changes in flooding is difficult because of the substantial natural variability3; the dependence of streamflow trends on flow regime4, 5 further complicates the issue. Here we investigate the changes in risk of great floods—that is, floods with discharges exceeding 100-year levels from basins larger than 200,000 km2—using both streamflow measurements and numerical simulations of the anthropogenic climate change associated with greenhouse gases and direct radiative effects of sulphate aerosols6. We find that the frequency of great floods increased substantially during the twentieth century. The recent emergence of a statistically significant positive trend in risk of great floods is consistent with results from the climate model, and the model suggests that the trend will continue.