Population density, defined as the ratio between population and occupied area, is a key indicator to measure the efficiency of urbanization processes. Higher density means that less space is needed to accommodate the same population, hence more land can be saved for other uses, including agriculture and biodiversity conservation. Among the indicators selected to measure the progress towards Sustainable Development Goal 11, the ratio of land consumption rate to population growth rate clearly points to densification as a pathway towards more sustainable human settlements. 

Irrespective of international policy efforts, scientific research revealed that, between 1970 and 2010, population density decreased in most urban areas worldwide. Observed rates of urban land expansion were consistently higher than the rates of urban population growth in all regions, with the only exception of Latin America and Southwestern Asia. Despite being the least dense in the world, North-American and European cities were in the forefront of this de-densification trend, mostly driven by the expansion of low-density, car dependent residential suburbs. 

However, European cities showed in the last decades a variety of urban development pathways. Shrinking cities were quite common and, in some of them, urban expansion continued despite population loss, pushed by the demand from an increasing number of small households. On the contrary, a few other cities, mostly in former socialist countries, achieved densification through large scale demolitions of unused housing estates. These cases suggest that, to support policy-making, density indicators must be framed within a broader understanding of the underlying population and land take trends. 

Our research analyzed the density trends of more than 300 European cities in the two periods 2006-2012 and 2012-2018, and their underlying trends in population and residential area. We hypothesized that different types of population change could have different impacts on urban development. Hence, we broke down the total change into natural change, i.e. the difference between births and deaths, and net migration, i.e., the difference between immigrants and emigrants. The analysis combined land use and statistical data from two EU-wide databases (Urban Atlas and Eurostat Urban Audit), complemented by national demographic statistics. 

New density trends in European cities

The results for the first period are in line with previous findings, with residential density decreasing in 60% of the sample. De-densification prevailed everywhere except in northern cities, but it was particularly intense in the eastern countries and in southern Italy and Spain. The pattern changed between 2012 and 2018. Around a quarter of the cities shifted from de-densification to densification, making the latter the most common trend. Densification was on average more intense than in the first period, and particularly in Italy, Germany, and the Netherlands, where density had decreased in the previous period. De-densification continued to prevail only in the most peripheral areas of Europe, both in the Iberian Peninsula to the West and in the former socialist countries to the East. 

The first driver of this shift was a more diffuse population growth. The share of growing cities increased from 60% to 75% of the sample between the two periods. While between 2006 and 2012 shrinkage was common in large parts of Europe, although especially prevalent in the eastern region, between 2012 and 2018 it was confined almost exclusively to the eastern countries and the Iberian Peninsula. At the same time, the share of cities characterized by a net positive migration increased from 42% to 56%, while those with a positive natural growth diminished from 67% to 51%. Cases of population growth driven by positive migration in a context of negative natural change became more frequent. 

The second driver of the shift was a relevant slowdown of land take for residential use between the two periods. Although residential area continued to expand in almost all cities, the speed of this expansion was much lower between 2012 and 2018 than before. The slowdown was particularly evident in southern cities, characterized by the highest shares of land take in the first period. Interestingly, we found that the expansion of residential areas was significantly correlated with natural population change, but not with net migration. 

We also observed that density trajectories were linked to changes in population growth rates. Cities with small variations between the two periods mostly continued on the same density trend. On the contrary, shifts from de-densification to densification were frequently associated to an acceleration in population growth rates, often due to increasing immigration. Similarly, a slowdown in population growth rates was often linked to a shift from densification to de-densification. 

Risks and opportunities behind the new densification wave 

The predominant tendency of European cities to (re-)densify in the most recent years is a peculiarity in a worldwide panorama of de-densifying and sprawling cities, and a positive news for the environment. The ‘no net land take’ strategy launched in 2011 by the European Commission might have partly contributed to this trend, but the economic recession after the global financial crisis of 2007-2008 was most probably the main driver of the reduced residential expansion. At the same time, population trends reveal a growing attractivity of city cores. 

The fact that many cities could face a great and fast population increase with limited land take suggests that a “buffer” capacity of unused housing stock is often available. This reserve can play a key role in achieving and sustaining urban densification in the future. Our scale of analysis also shed light on the role of national-level policies in complementing and supporting local urban development strategies. The results reflect successful policies to promote densification in the Nordic countries, the UK, and The Netherlands, but also the indirect effects of social and family policies in France, a unique case where some cities shifted to densification due to an acceleration in fertility-driven population growth. 

But our findings also reveal some risks associated to the observed trends. The drivers of this new densification wave might be hard to sustain in the long term. The pandemic has already weakened the attractiveness of city cores, while policies to counteract the effects of economic crises often favor new construction activities. Moreover, if achieved at the expense of green areas and in an imbalanced way, densification could undermine both social cohesion and quality of life. Future policies must be aware of these risks to promote a truly sustainable urban densification.