The risk of concurrent heatwaves and extreme sea levels along the global coastline is increasing

Concurrent heatwaves and extreme sea levels could pose a serious threat to coastal communities under climate change; however, the spatiotemporal characteristics and dynamic evolution of them along global coastline remain poorly understood. Here, we use reanalysis datasets and model projections to assess historical and future changes in global concurrent heatwaves and extreme sea levels. We find that 87.73% of coastlines experienced such concurrent extremes during 1979-2017. There is an average increase of 3.72 days in the occurrence during 1998-2017 compared to 1979-1998. A one-percentile increase in heatwave intensity is associated with a 2.07% increase in the likelihood of concurrent extremes. Global coastlines are projected to experience 38 days of concurrent extremes each year during 2025-2049 under the highest emission scenario. The weakening of geopotential height associated with a surface low-pressure system may serve as an important indicator for the occurrence of extreme sea levels during heatwaves. The simultaneous occurrence of coastal heat waves and extreme sea levels increased during the period 1979-2017, particularly in the tropical region, and its likelihood is expected to increase 5-fold between 2025 and 2049 under a high emission scenario, according to climate reanalysis and future model projections.