Daily mapping of global surface temperature reveals intensified local extremes of Surface Urban Heat Island

Urban temperatures are generally higher in cities than in their non-urbanized surroundings, both during day and night. This phenomenon, known as the Urban Heat Island effect, represents a hazard, as it exacerbates heat-related illnesses and mortality. Its intensity can be estimated from remote sensing retrievals of Land Surface Temperature (LST), and in such conditions it is usually referred to as Surface Urban Heat Island (SUHI). Past global studies analyzed this phenomenon in terms of urban and/or annual/seasonal means, but the impact on human health depend on short-term heat stress experienced locally. On the other hand, local studies are often performed on time-limited and not always representative empirical cases, employ different types of measurements and methodologies, making them difficult to intercompare. Moreover, they cover extensively a few developed areas, such as Northern America, Europe and Eastern Asia, leading to a knowledge gap with respect to less studied regions.

To fill this gap, here we developed a high resolution (1 km) dataset of observations of day and night SUHI based on 18 years of MODIS Aqua imagery, which offers an unprecedented insight into the short-time and short-range behavior of the Urban Heat Island. Our results show that 3-day SUHI extremes are on average more than twice as high as the warm-season median SUHI, with local exceedances up to 10 K, and with hotspots of intense heat and relatively cooler areas are clearly observable within the same city. Furthermore, over this period, SUHI extremes have increased more rapidly than warm-season medians, and averaged worldwide are now 1.04 K or 31% higher compared to 2003. This can be linked with increasing urbanization, more frequent heatwaves, and greening of the earth, processes that are all expected to continue in the coming decades.

These data provide clear evidence of the importance of high space-time resolution in studying the Urban Heat Island and the threat it poses. They can be used in a range of applications, from the day-by-day assessment of urban heat, to the calibration of models of the urban climate (Mentaschi et al., 2022).

 

References

Mentaschi, L., Duveiller, G., Zulian, G., Corbane, C., Pesaresi, M., Maes, J., Stocchino, A. and Feyen, L.: Global long-term mapping of surface temperature shows intensified intra-city urban heat island extremes, Glob. Environ. Chang., 72, 102441, doi:10.1016/j.gloenvcha.2021.102441, 2022.