Climate Change Trends for the Urban Heat Island Intensities in Two Major Portuguese Cities

Abstract

Urban Heat Island (UHI) intensities are analyzed for the metropolitan areas of the two major Portuguese cities, Lisbon and Porto, in the period 2008–2017. Projections for the UHI intensity averaged over 2008–2017 and a future period 2021–2050 are calculated under the Representative Concentration Pathway (RCP) 8.5. The spatiotemporal characteristics of the UHI intensity are assessed for daytime, nighttime, and average daily conditions. This analysis is carried out for the winter (Dec-Jan-Feb, DJF) and summer (Jun-Jul-Aug, JJA) meteorological seasons. Maximum UHI intensities of about 3.5 °C were reached in 2008–2017 in both metropolitan areas, but over a wider region during winter nighttime than during summer nighttime. Contrariwise, the most intense urban cool island effect reached −1.5 °C/−1 °C in Lisbon/Porto. These UHI intensities were depicted during summer daytime and in less urbanized areas. Overall, the UHI intensities were stronger during the winter than in the summer for both cities. Results show that the UHI intensity is closely related to underlying surfaces, as the strongest intensities are confined around the most urbanized areas in both cities. Until 2050, under RCP8.5, the highest statistically significant trends are projected for summer daytime, of about 0.25 °C (per year) for Lisbon and 0.3 °C (per year) for the UHI 99th percentile intensities in both metropolitan areas. Conversely, the lowest positive statistically significant trends (0.03 °C/0.02 °C per year) are found for the winter daytime UHI intensities in Lisbon and the winter nighttime and average UHI intensities in Porto, respectively. These statistically significant patterns (at a 5% significance level) are in line with the also statistically significant trends of summer mean and maximum temperatures in Portugal, under RCP8.5 until 2050. Scientists, urban planners, and policymakers face a significant challenge, as the contribution of urbanization and the forcing promoted by global warming should be duly understood to project more sustainable, go-green, carbon-neutral, and heat-resilient cities.