Accelerated lagged compound floods and droughts in northwest North America under 1.5 °C-4 °C global warming levels

The recent upsurge in the occurrence of hydroclimatic extremes and their temporal swings has led to severe consequences in many regions around the world. In this study, we characterize the swings between flood and drought events and assess their spatiotemporal frequency and magnitude under climate change. Different scenarios for nonstationary flood and drought swings are investigated over three major river basins in northwest North America. In addition, two novel indices are developed to quantify the severity of the compound flood and drought events in a changing climate. The frequency of flood-to-drought events is projected to increase, with occurrences at the 1.5 degrees C global warming level happening almost twice than that of the base period. Such transitions are expected to occur more frequently if the world progressively warms. Additionally, the transition time of flood-to-drought events is projected to decrease across all three basins in the study area. Even though drought-to-flood events are expected to become less frequent in a changing climate, the southwest Columbia basin stands out as a hotspot for such transitions. Furthermore, floods are expected to occur earlier in the season, while droughts are projected to occur later, highlighting the increasing variability of floods and droughts of compound hydroclimatic events in a warming world. Additionally, there is a possibility of increased severity for such compound events if global warming is not limited. Our findings assert the necessity of integrating mitigation measures targeting lagged compound flood and drought events into disaster risk reduction strategies.