Urban heat is an escalating global challenge driven by climate change and urban development. This research explores satellite-based land surface temperature (LST) and population exposure across 1400 cities worldwide over a 20-year period. It addresses a critical gap in global understanding regarding where heat extremes coincide with population growth, providing necessary insights for developing targeted adaptation strategies across diverse urban environments.

Results and discussion

The study highlights that urban heat intensity varies significantly based on temperature metrics, timing, and regional characteristics. While canopy urban heat is evaluated using air temperature measurements, which are often sparse, this research utilises satellite-derived LST to provide consistent global coverage. This approach allows for trend detection in data-sparse regions, enabling informed decisions in rapidly urbanising areas. The findings indicate that both daytime and nighttime heat exposure must be examined together, as physiological responses and adaptation capabilities vary between these periods.

Long-term trends in LST

From 2002 to 2021, urban surface temperatures rose globally by 0.03-0.2 °C/year during the day and 0.01-0.15 °C/year at night. The most significant increases were observed in cold-climate cities and during winter months, specifically in Eastern Europe and Western Asia. In these regions, large land masses and high-pressure systems, combined with Arctic amplification, make continental regions particularly susceptible to long-term climate change effects.

Statistical analysis reveals that 86% of cities experienced daytime warming, and 96% experienced nighttime warming. However, cooling trends were observed in specific regions like Iran, India, Afghanistan, and northeastern China. These trends are linked to increased aerosol concentrations that scatter solar radiation and extensive “greening” efforts, such as large-scale tree planting programmes in China and India, which enhance evapotranspiration and reduce heat storage.

Long-term changes in hot and cold extremes

Extreme temperature events have become more frequent globally, particularly hot nights. In arid cities, the frequency of hot nights increased by 47% over 19 years. Global hot extremes rose from an average of 9.56 nights in 2003 to 12.62 nights in 2021, while cold extremes declined from 13.56 to 9.23 nights over the same period. This increasing occurrence of warm nights is concerning as elevated nighttime temperatures limit physiological recovery and increase cardiovascular strain.

In winter, temperature increases are particularly pronounced in continental cities. Nighttime LST in these regions rose by 0.17 °C per year, while daytime LST showed a substantial increase from 0.7 to 5.2 °C over two decades. Such shifts have significant public health implications; for instance, research suggests that a 0.5 °C difference in warming could result in nearly 28,000 additional heat-related deaths annually across urban populations in China.

Urban population exposure to hot extremes

Global exposure to extreme daytime heat increased by 51% over the study period, reaching 8.4 billion person-days in 2020. Climate warming amplified this increase by 82% relative to what population growth would have caused alone. The relative importance of these drivers varies by climate zone: climate change dominates daytime exposure in continental regions (71%), whereas population growth is the primary driver in arid cities (69%).

Asia remains the continent with the highest annual increase in exposure, contributing 0.1 billion person-days during the day and 0.18 billion person-nights at night. In areas with slower population growth, such as Europe, North America, and Oceania, increased daytime exposure is driven more by climate change than by demographic shifts.

Limitations and future work

While satellite-derived LST provides valuable global data, it does not directly reflect the thermal conditions experienced by humans in the same way as air temperature does. The analysis is also limited to clear-sky conditions, potentially missing heat stress during cloudy or humid periods. The study emphasises that future assessments should integrate additional factors like wind speed, humidity, and building-level spatial variation to improve the accuracy of heat-related risk evaluations and support the development of locally adapted mitigation strategies, such as cooling-centre operations and heat-resilient urban design.