Future population exposure to heatwaves in 83 global megacities.

Global warming leads to more frequent and intense heatwaves, putting urban populations at greater risk. Previous related studies considered only surface air temperature or one or two Shared Socioeconomic Pathways (SSPs) and were limited to specific regions. Moreover, no research focused on heatwave exposure in highly-populated global megacities facing severe threats. This study is the first to project future population exposure to heatwaves in 83 global megacities by 2100 using fine-resolution data, suitable indices reflecting human comfort in heatwaves by incorporating temperature and humidity, and a future population exposure projection and analysis framework. The results show that (1) the global frequency of extreme heatwave events and average change rate in each megacity sequentially increase from SSP1-2.6 to SSP5-8.5, and the change rate is generally larger in megacities in the Southern Hemisphere; (2) the increases in heatwave exposure are greatest under SSP370, and the change rates are generally larger for megacities in Southern Asia; (3) there is a high degree of inequality (Gini of 0.6 to 0.63) in future heatwave exposure globally, with the highest inequality under SSP5-8.5 and the lowest under SSP3-7.0; (4) the average exposure, increase rate, and change are highest in low-income megacities and lowest in high-income megacities. The distribution of exposure is the most balanced in middle-income megacities and the least balanced in high-income megacities; and (5) population growth contributes more to the change in exposure than total warming in high-income megacities under SSP1-2.6, and total urban warming contributes much more than population growth in all other cases. Every effort should be made to avoid the SSP3-7.0 scenario and pursue sustainable and rational urban economic development. Mumbai, Manila, Kolkata, and Jakarta warrant particular attention due to their rapid exposure growth. Additionally, policymakers and urban planners must focus on improving sustainable development planning for megacities in southern Asia and low-income megacities.