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Presenter: Yukiko Hirabayashi

Seminar Title: Global projections of changing risks of flood under the global warming simulated by MIROC GCM

Simulated daily discharge derived from a relatively high-resolution (T106; about 1.1-degree) general circulation model (GCM) was used to investigate future projections of population changes under risks of more frequent flooding. A statistically significant increase or decrease in flood frequency was not predicted in the 20th century, and first appear around the middle of the 21st century in 21 of 30 large global rivers.

Statistics of world disasters indicate that the mean population impacted by flood disasters from 1991 to 2000 was 1.45 billion (2.5% of total population). The simulated global mean population which has affected by daily discharge higher than the 20^th century (20C) 100-year flood (defined using daily discharges from 1901 to 2000) was about 0.51 billion (0.9% of total population) for 1991-2000, while for 2091-2100 this increases to 6.96 billion (9.5% of total population). The minimum estimate of the flood-affected population after the middle of the 21st century becomes higher than the maximum of the flood-affected population in the 20th century.

Annual fluctuations in the recorded flood-affected population were higher than that estimated using the modeled 20C 100-year flood. It is therefore important to show ranges of the flood-affected population in the model simulation using a statistical method, taking into account years when floods are concentrated in highly-populated regions and years when most floods occurred in low-populated regions.

Statistical analysis using a Monte Carlo approach revealed that the population experiencing daily discharge higher than the 20C 100-year flood from 2091 to 2100 is in the top 35% of the ranges of possible population sizes (sum of the population in regions that were randomly selected from global land of the same area as the regions with flood discharge), while that in 1991-2000 was below 30% of the probable population sets. This result indicates that future flood increases will be more common in regions with high population densities.

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