How has the urban development of Madrid affected the local climate? Analysis ofthe last 50 years

The proportion of the world’s population living in cities has increased from 37% to 56% over the last 50 years, and it is expected to continue rising further to 60% by 2030 (UN, 2022). As an essential part of this evolution, urban land cover has expanded rapidly. It is well established that urbanization reduces the vegetated cover and modifies surfaces properties altering the surface-atmosphere interactions compared to rural areas. Therefore, analyzing the impact of the past changes in urban land cover contributes to understand the potential risks that urban residents might face considering the future urban grown and the expected rising of air temperatures, as this has adverse impacts on human health, livelihoods, and key infrastructure.

Our objective is to examine the impact of Madrid's urban growth in the last 50 years (1970-2020) on local climate. A modeling study is carried out using the Weather Research and Forecasting (WRF) model, where land use and land cover have been modified according to each decade's urban expansion from 1970 to 2020. Two representative scenarios of extreme meteorological conditions are selected for this study: a period of intense heatwave during the summer season and a short period of strongly stable atmospheric conditions in winter. In areas where the urban fraction increased significantly, the model results reveal an increase of 1-2 ºC in 2 m air temperatures during the day and approximately 4-6 ºC at night in summer. In contrast, smaller differences are obtained at night for the winter period, probably due to the presence of thermally driven buoyancy flows generated during the night under stable conditions. The current results also indicate how these changes from non-urban to urban land cover over time have altered the surface energy balance system in Madrid over the last 50 years. And in turn, it reveals the relative contribution of urbanization and anthropogenic heat emissions to the warmer temperatures observed in cities compared to rural areas.