Global urban population exposure to extreme heat

Study key findings & significance

• Exposure to potentially deadly extreme heat between 1983 and 2016 tripled worldwide due to population growth, climate change, and cities getting hotter as roads and other hard surfaces absorb heat from the sun.
• Most of the increased exposure to deadly heat is being felt in South Asia and sub-Saharan Africa—places that are already very hot and where urban populations are growing rapidly.
• This study is the first to examine in fine detail global trends in extreme heat exposure across urban areas. The study spanned more than 13,000 settlements.

Author quotes

“The risk for mortality from heat going forward is a daunting challenge."

Cascade Tuholske, lead author and postdoctoral researcher at the Center for International Earth Science Information Network at Columbia University

“Our study reveals that exposure to extreme heat in urban areas is much more widespread — and increasing in many more areas — than we had previously realized. Almost one in five people on Earth experienced increases in exposure to urban heat over the past 30 years.”

Kelly Caylor, co-author and director of the university’s Earth Research Institute and Tuholske’s doctoral advisor.

Abstract

Increased exposure to extreme heat from both climate change and the urban heat island effect—total urban warming—threatens the sustainability of rapidly growing urban settlements worldwide. Extreme heat exposure is highly unequal and severely impacts the urban poor. While previous studies have quantified global exposure to extreme heat, the lack of a globally accurate, fine-resolution temporal analysis of urban exposure crucially limits our ability to deploy adaptations. Here, we estimate daily urban population exposure to extreme heat for 13,115 urban settlements from 1983 to 2016. We harmonize global, fine-resolution (0.05°), daily temperature maxima and relative humidity estimates with geolocated and longitudinal global urban population data. We measure the average annual rate of increase in exposure (person-days/year⁻¹) at the global, regional, national, and municipality levels, separating the contribution to exposure trajectories from urban population growth versus total urban warming. Using a daily maximum wet bulb globe temperature threshold of 30 °C, global exposure increased nearly 200% from 1983 to 2016. Total urban warming elevated the annual increase in exposure by 52% compared to urban population growth alone. Exposure trajectories increased for 46% of urban settlements, which together in 2016 comprised 23% of the planet’s population (1.7 billion people). However, how total urban warming and population growth drove exposure trajectories is spatially heterogeneous. This study reinforces the importance of employing multiple extreme heat exposure metrics to identify local patterns and compare exposure trends across geographies. Our results suggest that previous research underestimates extreme heat exposure, highlighting the urgency for targeted adaptations and early warning systems to reduce harm from urban extreme heat exposure.