Human Thermal Indices and the Risk of Abrupt Population Disruption

This study investigates the timing of abrupt disruptions to human populations resulting from extreme heat stress on a global scale. Utilizing data from 38 Global Climate Models (GCMs) under multiple Shared Socioeconomic Pathways (SSPs: 2.6, 4.5, and 8.5) within the CMIP6 framework, we project a suite of human thermal indices, including apparent temperature (indoor & outdoor), discomfort index, effective temperature, heat index, humidex, modified discomfort index, net effective temperature, simplified wet globe temperature, wet bulb globe temperature, and wind chill temperature, for both historical (1850-2024) and future (2024-2100) periods. Abrupt disruption is defined as a continuous period exceeding historical thresholds for at least five consecutive years.

Our analysis, conducted within Köppen-Geiger climate regions, reveals a concerning trend: the onset of abrupt disruption is projected to occur significantly earlier than previously anticipated across all SSPs. Even under the most optimistic mitigation scenario (SSP2.6), millions of people are projected to experience abrupt disruptions before 2050. By 2100, over 5% of the global population could be affected by these abrupt changes, with substantial regional variations.

Furthermore, our analysis incorporates population projections from SSPs to estimate the number of individuals impacted by these disruptions in each decade. Results indicate a substantial increase in the number of people exposed to extreme heat stress, with significant implications for human health, livelihoods, and societal stability.

These findings underscore the urgency of implementing robust adaptation strategies to mitigate the severe impacts of extreme heat on human populations. Such strategies should include investments in early warning systems, improved urban planning, and the development of heat-resilient infrastructure.