Greener or Greyer? Exploring the Trends of Sealed and Permeable Spaces Availability in Italian Built-Up Areas during the Last Three Decades

Increasing the availability of greenspaces in built-up areas (GSB) is one of the main challenges to improving sustainability and livability in urban landscapes. Concurrently, the availability of brownfields and permeable spaces offers the chance to increase sustainability through the implementation of Nature-Based Solutions. This work aims to evaluate how land use/cover changes influenced the availability of permeable spaces in Italian built-up areas over the last three decades. These spaces were classified according to population density, vegetation type, and average size, to better characterize recent dynamics (2008–2016) and offer remarks and tips concerning ongoing soil sealing dynamics. According to the findings, despite an overall increase of 41.5% in built-up areas with respect to their 1990 extension, permeable spaces increased only by 25.2% during the same time-span, moving from 49.8% coverage in 1990 to 44.7% today, in relation to the whole national built-up area. Moreover, our in-depth analysis for the 2008–2016 period shows that forested spaces increased by 0.4%, while permeable unforested ones decreased by 0.7%, especially in thinly and intermediately populated areas. Overall, the scarcity of these spaces should be carefully considered when assigning residual vacant lots to new buildings and grey infrastructure, especially in densely populated areas. The proposed methodology provides reliable estimates and represents a starting point to develop advanced monitoring tools supporting sustainable urban policies.