Global urbanisation is now a relentless phenomenon, meaning that increasing numbers of cities are having to face up to multiple climate change-related challenges. These challenges materialize in the form of problems such as heat stress and air pollution, and most importantly, in serious consequences for the health of city inhabitants. In response, a growing number of policy-makers, technical experts and scientists are claiming that greener cities are also more sustainable, more liveable, healthier and safer. Thus, urban green spaces (e.g. city parks, street trees, green roofs and walls, etc.) are increasingly being considered, planned and managed as a “green infrastructure” with the ultimate aim of ensuring people’s wellbeing and health. However, the matter of how far urban green infrastructure can really address these challenges is rarely assessed, which means that political decision-makers lack solid information on which to base policies.

This article concludes that the contribution of this green infrastructure is often uncertain or less than expected, especially when considering an entire city or metropolitan area. It summarises the results of various scientific assessments undertaken in Barcelona and other European cities, aimed at determining to what extent urban green infrastructure helps compensate carbon emissions, reduce heat stress and reduce air pollution in the city. The results indicate that, although it is true that recognizing green spaces as a key urban infrastructure may support municipal initiatives, it is also important to take into account and investigate in depth the limitations involved.

Our planet is increasingly urbanised: over half of the world’s population now lives in cities. By 2050 that fraction will have increased to 66% according to United Nations (UN) forecasts, adding 2.5 billion people to the world’s urban inhabitants (UN, 2015). However, the majority of cities and metropolitan areas are facing multiple challenges caused or exacerbated by climate change, such as heat stress, inland and coastal flooding, droughts, increased aridity and air pollution, often with dramatic consequences for the health of their urban populations (Revi et al., 2014).

Thus, for example, the World Health Organization estimates that ambient air pollution caused the premature death of 3.7 million people worldwide in 2012. So making cities and human settlements more resilient, sustainable, liveable and safe should be a fundamental priority on every local government’s agenda, as reflected in the eleventh UN Sustainable Development Goal.

Within this context, there are growing numbers of policy-makers, technical experts and scientists who consider that priority must be given to planning and managing urban and periurban green spaces (city parks and gardens, street trees, green roofs and walls, periurban forests, etc.) as a cost-effective infrastructure to cope with this growing number of climate threats. The European Union describes the concept of green infrastructure as “a strategically planned network of natural and semi-natural areas with other environmental features, designed and managed to deliver a wide range of ecosystem services” (EC, 2013). In turn, these “ecosystem services” are generally defined as “the direct and indirect contributions of ecosystems to human wellbeing and health” (TEEB, 2011) and hence they enable nature-society interactions to be examined in depth.

In urban areas, the ecosystem services usually given priority are those that contribute to improving environmental quality and the wellbeing and health of the urban population (figure 1). They are oriented, for example, towards air purification, noise reduction, urban temperature regulation or mitigation of runoff (excess rainwater or water from other sources running over non-absorbent surfaces). However, how far urban green infrastructure can provide these ecosystem services effectively depends on multiple structural, functional and environmental conditions. Thus, for example, urban vegetation can reduce local temperatures (by providing shade and transporting water from the soil to the air), hence mitigating heatwave risks for human health. Obviously, urban trees play a much more prominent role in both processes than other types of vegetation such as shrubs or grass.

However, the true scope of these green interventions is unknown to local authorities and other urban stakeholders, since its real or potential contribution is often overlooked in evaluations of this type. This article helps bridge this knowledge gap and assess the potential of green infrastructure to cope with different urban challenges related with climate change across five European cities. The research focuses on air pollution abatement, climate change mitigation and heat stress reduction, with a spotlight on Barcelona.

This research encompasses an original cross-analysis of five European cities: Rotterdam, Berlin, Salzburg, Stockholm and Barcelona. Indicators on air purification, carbon sequestration (capacity for removal of CO2 from the atmosphere) and urban temperature regulation were estimated, based on mathematical models produced via widely applied tools. The services that underwent evaluation contribute to tackling various urban challenges such as improvement in air quality, climate change mitigation and reduction of the heat stress suffered by cities. Air pollution and greenhouse gas emissions, indicators that measure the “environmental pressure” being placed on cities, were also estimated in order to assess the relative benefits of green infrastructure.

The research results indicate that average air quality improvements due to air purification provision by green infrastructure are relatively low at the municipal scale for the three air pollutants analysed in all the cities in the study: particulate matter, nitrogen dioxide and greenhouse gases (table 1).

Therefore, the average green infrastructure contribution towards compliance with air quality standards is considered modest at city-wide level in all the cases analysed. This suggests that increasing these infrastructures (for example, through tree-planting programmes) has only limited effectiveness against air pollution problems, unless levels are moderate at the outset.

Similarly, the contribution of green infrastructure to carbon sequestration (to offset city greenhouse emissions) is somewhat weak. Of the total greenhouse gases emitted in Rotterdam (representing the worst case), 0.12% were neutralised thanks to these infrastructures. In the best of cases (Salzburg), the percentage reaches only 2.75%. These results also reveal that the contribution to greenhouse gas reduction targets in each case-study city is very modest (less than 15%: in the best of cases, Salzburg, it reaches 13.77%).

Finally, this assessment and other empirical studies (Bowler et al., 2010) make it clear that vegetation may be somewhat relevant in regulating urban temperature and mitigating heat stress at the local site scale; however, its impact at the wider city scale is uncertain.

Barcelona’s urban area is a complex socioecological system that makes an excellent testing ground for the purposes of this research and one where local and regional authorities are evaluating the strategic possibility of implementing the green infrastructure approach (for example, with the Barcelona Green Infrastructure and Biodiversity Plan 2020). Firstly, because Barcelona is one of Europe’s most densely populated urban areas (especially at the city scale, which poses great pressures and challenges in relation to urban green infrastructure policies) and secondly, because the city still contains a rich variety of natural and agricultural habitats of great relevance in terms of ecosystem services provision at the metropolitan level.

To complement the analysis carried out in the city of Barcelona, an assessment was conducted at the scale of the Barcelona metropolitan area, which encompasses 164 municipalities across seven counties. Here, several ecosystem services indicators were estimated using tools that include models developed to support environmental policies on a European scale (Zulian et al., 2018). As would be expected, the highest levels of pollutants and carbon appeared basically in the municipality of Barcelona and the adjacent middle-sized cities (figures 2b and 3b).  The carbon-offsetting impact of urban green infrastructure is small on average (less than 5% of the total of emissions are absorbed) across the Barcelona metropolitan region (figure 3c). In only five out of 164 municipalities are the estimated carbon emissions completely offset by carbon sequestration from local ecosystems. However, these municipalities are characterized by a very low population (fewer than 500 inhabitants) and a predominance of forest land cover.

Air purification (measured through NO2 removal) and global climate regulation (measured through carbon sequestration) when transposed onto the map of Barcelona’s metropolitan region, show similar spatial distribution patterns (figures 2a and 3a). The impact of these ecosystem services is especially significant in forest areas on the city outskirts, such as the Collserola mountain range and other natural areas located further inland in the region. However, effective NO2 removal in some of these areas (e.g., the Montseny massif) is relatively low because they are not adjacent to the main cities and so cannot contribute towards offsetting their emissions (figure 2b). In urban and agricultural land, the provision capacity of ecosystem services is the lowest of those studied.

As observed in the local scale assessment, Barcelona’s urban core is characterized by a compact form, a very high population density and a relatively small share of inner green areas, which explains these results. The other middle-sized municipalities, located both along the coastline and in the interior, suffer, in the majority of cases, lower environmental pressure.

The NO2 concentration map also reveals that high-capacity roads (motorways and dual carriageways) are major sources of NO2 pollution. The resulting map (figure 2b) shows places where ecosystem service provision cannot sustain a good air quality level in line with the NO2 annual limit value of 40 micrograms per cubic metre, as set by the EU Air Quality Directive.

The carbon-offsetting impact of urban green infrastructure is small on average (less than 5% of the total of emissions are absorbed) across the Barcelona metropolitan region (figure 3c). In only five out of 164 municipalities are the estimated carbon emissions completely offset by carbon sequestration from local ecosystems. However, these municipalities are characterized by a very low population (fewer than 500 inhabitants) and a predominance of forest land cover.

Urban green infrastructure’s potential for offsetting carbon emissions, air pollution and heat stress is often limited and/or uncertain, especially in compact cities such as Barcelona. This suggests that, as a general rule, the magnitude of these environmental problems is too high at the city scale as compared to the real or potential contribution of urban ecosystem services in mitigating their impacts.

On the metropolitan scale, the proportion of urban green infrastructure versus built-up or urbanized land is generally substantially higher than at the core city level. However, ecosystem services assessments on this scale also show marginal impacts on the overall carbon balance, i.e. on the ratio between carbon sequestration and carbon emissions (in the case of Barcelona, less than 1% of emissions are sequestered). Furthermore, the high capacity for contributing to a potential improvement in air quality and a reduction in heat stress, estimated in large areas of metropolitan green infrastructures (such as protected natural spaces) generally does not materialize. This is due to their distance from demand sites, such as residential areas most affected by air pollution or the urban heat island effect.

This result indicates that the relevant scale data for applying these strategies is probably limited to city level or even smaller scales. In fact, results from other studies largely corroborate the fact that urban green infrastructure, especially urban trees, can improve air quality, offset carbon emissions and reduce heat stress at the site level (especially within and around green spaces). However, several factors, such as species selection and management practices, can have a critical impact on the performance of urban ecosystem services.

Table 2 summarizes, on three different scales (metropolitan, city and site), the scientific evidence associated with the potential of the three regulating ecosystem services considered in this article as nature-based solutions. The latter scale is divided into two categories (green space and street) as these are generally the two most relevant urban sites in terms of green infrastructure.

Based on these results, the following urban policy and research implications can be drawn:

Air pollution problems and local targets for the reduction of greenhouse gases should be tackled through emission reduction policies such as road traffic management or energy efficiency measures. In other words, urban policies on air pollution and climate change mitigation should focus primarily on contamination sources (built infrastructure and transport systems) rather than on so-called “sinks” (urban vegetation that absorbs carbon and pollutants). Green infrastructure strategies play a complementary, not alternative role to these policies. Additionally, carbon offsets associated with green infrastructure should be fostered by local and metropolitan authorities beyond urban boundaries because the scale of the challenge is global.

Urban green infrastructure can contribute to site-scale strategies to improve air quality and thermal comfort, and hence human health. Thus, for example, urban parks, street trees or green roofs or walls (vegetation on buildings) can act as clean air or cool areas and corridors within cities. The potential of green roofs, green walls and street trees is particularly relevant due to the lack of available land in urban cores.

The benefits and limitations related to urban green infrastructure should be considered in planning and management in order to estimate net contributions to environmental quality. Even if most urban green infrastructure elements are multi-functional in relation to the three ecosystem services considered in this analysis, some potential problems have been also identified. Thus, for example, street trees provide a high shading effect, but they are also associated with a potential “barrier effect” by which they prevent the dispersion of pollutants into the air.

Although the scope of this analysis is limited to three regulating ecosystem services (air quality, local climate and carbon sequestration), the urban green infrastructure can obviously also provide additional services and benefits. These include runoff control and subsequent opportunities for recreational activities (Demuzere et al., 2014). Unlike traditional urban infrastructures (such as road infrastructure) normally designed as single-purpose, the added value of urban green infrastructure resides in its multi-functionality. 

Therefore, planning and managing urban green infrastructure within the context of its contribution to climate change mitigation and adaptation requires a holistic approach. Planners and policy-makers must take into consideration the whole range of ecosystem services provided by different types of urban green infrastructure and the interactions between them, together with the different spatial scales at which these ecosystem services can be significant. This approach calls for strong multi-scale institutional coordination between all the authorities dealing with urban and environmental policies and for the harmonization of planning and management instruments across different sectors.

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