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.