Floods and droughts pose some of the greatest risks to water supply and flood protection, and they often occur together or in quick succession, as when a severe drought ends in damaging floods. Their frequency and severity are governed by large-scale climate processes, including storm tracks, moisture transport and ocean–atmosphere modes such as the El Niño–Southern Oscillation. A warmer atmosphere holds about 7% more water vapour for each degree Celsius of warming and draws more water from the land through higher evaporative demand, so warming tends to intensify both wet and dry extremes. Because these controls are shifting, risk estimates drawn only from the historical record can understate the extremes to come.
My current work focuses on atmospheric rivers, which deliver a large share of California’s precipitation and cause many of its largest floods, and on how the El Niño–Southern Oscillation and the Pacific Decadal Oscillation together shape their frequency, their intensity and the extreme precipitation they bring. Because these climate modes evolve slowly and can be monitored months in advance, they offer a physical basis for seasonal outlooks of flood risk. The impact of an atmospheric river also depends on conditions on the ground. Intense rain on recently burned hillslopes can trigger debris flows and carry large amounts of sediment into streams and reservoirs, while rain on wet soils or on snow can amplify runoff. Combining atmospheric-river forecasts with information on burn scars, soil moisture and snow could therefore give earlier warning of debris flows, erosion and floods.
My earlier work on floods, droughts and heatwaves in Indian river basins led to a broader question: how extremes interact, and how human systems shape their impact. Extremes rarely act in isolation. Droughts coincide with heatwaves, wet seasons follow dry ones, and reservoir operations and groundwater use change the impact of each extreme. Understanding these interactions, and how they are changing with warming, is central to water security worldwide. My aim is a physically based, global understanding of compound water risks, grounded in detailed studies of individual basins.