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Increasing risk of great floods in a changing climate


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.

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Figure 1: Estimates of 100-yr flood discharges based on model (control experiment years 201–900) and observations (full period of record).
Figure 2: Map showing the gauged drainage areas and flood-risk sensitivities of the 29 river basins in this study.
Figure 3: Decadal extratropical flood frequencies for observations and for scenario experiments.
Figure 4: Normalized trend Z(t)/Z95(t) of extratropical flood frequency for observations and for scenario experiments.

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Streamflow data were collected and/or provided to us by the Global Runoff Data Centre (Germany), the US Geological Survey, Environment Canada, EarthInfo, Manaus Harbor Ltd/Portobras and Departmento Nacional de Aguas e Energia Electrica (Brazil), National Institute of Meteorology and Hydrology (Romania), J. Cordova, N. Garcia and J. Richey. We thank K. L. Findell, D. P. Lettenmaier and R. J. Stouffer for comments and suggestions.

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Correspondence to P. C. D. Milly.

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Milly, P., Wetherald, R., Dunne, K. et al. Increasing risk of great floods in a changing climate. Nature 415, 514–517 (2002).

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