Phasing out fossil energy from transport, heating and cooling and other major emitting processes is key to achieving carbon-neutral cities^1,2. However, an issue often overlooked in energy systems and environmental engineering is how to spatially organize and use nature-based solutions (NBS), which can play a critical role in addressing the causes and consequences of climate change^3,4. In terms of reducing carbon emissions, most attention has been paid to the direct effects, that is, carbon sequestration in vegetation, soil⁵ and wetlands⁶. However, it is estimated that carbon sequestration can offset only a limited proportion of total anthropogenic carbon emissions, especially in urban settings⁷. To understand the full climate neutrality potential of NBS, comprehensive impacts should be estimated and quantified, including direct and indirect impacts on social and economic systems⁸.

Carbon emissions mitigation through NBS involves ecosystem services and green infrastructure (GI) approaches that support human wellbeing, saving resources and costs, and sequestering carbon emitted from human activities^4,9. For example, urban agriculture in combination with the greening of streetscapes can promote pro-environmental behaviours, for example, nudging local recreational bicycle trips instead of long-distance driving, while also providing educational and participatory opportunities that promote consumer preferences for foods and products of lower environmental impacts and carbon emissions^10,11. Additionally, microclimate regulation, combined with the aesthetic ecosystem services of NBS, can promote cycling and walking, prevent urban sprawl, alleviate dependence on automobiles, and reduce heating and cooling load^12,13,14. When all benefits are considered, NBS can contribute much more to urban climate neutrality goals than mere carbon sequestration effects.

Cities are ideal experiment and innovation hubs for technologies, instruments and policies for carbon neutrality¹⁵. For example, the European Union (EU) has pledged to decrease net emissions by 57% by 2030 (compared with 1990 levels), with the inclusion of land-use and carbon sequestration goals¹⁶. This climate action is to be led by 100 cities in EU member states that have pledged to achieve climate neutrality by 2030 (ref. ¹⁷). At the same time, the use of NBS is in line with EU climate policy and goals for addressing the challenges of climate change¹⁸. However, the current policy programme does not address or identify opportunities for NBS to mitigate carbon emissions beyond direct sequestration. Moreover, due to the limited land resources in cities, the spatial allocation and configuration of urban NBS need to be optimized to achieve maximum carbon emissions reductions.

In this Analysis, we assessed and quantified five potential carbon emissions reduction mechanisms for different types of NBS in EU cities. On the basis of sector-wise carbon emissions and the local context of 54 major European cities, we spatially allocated these five categories of NBS to each city and estimated the emissions reduction potential for each sector and city. One NBS implementation was allocated on each land use grid (30 m × 30 m) but could be functional in different categories (that is, GI that saves energy consumption while also functioning as carbon sequestration). We then compared the estimated emissions reduction potential against the 2030 climate neutrality targets of the 54 cities, to assess the potential contributions of prioritized NBS to these targets.

We first identified types of NBS linked to the effects and mechanisms of carbon emissions reduction. From established definitions of NBS in the literature^19,20,21, we identified major mechanisms by which NBS can mitigate carbon emissions, namely saving resources and costs, reducing urban sprawl, promoting pro-environment behaviour, microclimate regulation and carbon sequestration. Based on how recognized NBS implementations are linked to each of these carbon emissions mitigation mechanisms, we selected five types of NBS (GI, street trees and green pavements, urban green spaces and agriculture, habitat preservation and remediation, and green buildings), as they were assessed as having the closest links to the five selected carbon emissions mitigation mechanisms (connections between NBS implementations and the mechanisms are shown in Supplementary Material 2 Table A2.3). Carbon sequestration and saving resources and costs are more direct carbon emissions reduction mechanisms, for example, via carbon sequestration from urban trees and energy saving from using GI for stormwater treatment. Promoting pro-environmental behaviour generally reduces carbon emissions through indirect pathways, which are enabled by ecosystem services such as microclimate regulation, recreational opportunities and aesthetic nature experiences during daily routines. For example, microclimate regulation by GI can reduce heating and cooling demand, and hence energy use, in residential and industrial buildings²². This may simultaneously result in more walking, cycling and other pro-environmental habits that replace automobile driving, owing to the improved outdoor recreational opportunities and aesthetic nature experiences.

High potential for mitigation was offered by the various NBS implementations considered (Fig. 1). In all sectors, our estimates showed that NBS implementation can reduce urban carbon emissions from the spatial unit of its implementation (30 m × 30 m land use grid) by up to 25%. In the transport sector, carbon emissions can be reduced by up to 1.4% by improving streetscape design (that is, improving street greening design, narrower roads), as well as improving accessibility to urban parks and agriculture to create more agreeable environments for walking and cycling in urban centres²³, and thus reducing the need for automobile travel, and associated carbon emissions from fuel combustion²⁴. The potential for carbon sequestration can sometimes be quite substantial in some cases such as the Swedish capital Stockholm²⁵. In the residential sector, urban parks and agriculture can have the strongest effects in promoting pro-environmental habits, including less driving, sustainable consumption, and reduced heating and cooling demand, thus reducing carbon emissions by up to 6.2% (ref. ²⁶). In rural and suburban areas, strict land use policy that includes habitat preservation and remediation and GI as instruments can limit large-scale, single-family residential development and save 6% of carbon emissions²⁷. In industrial areas, NBS regulating microclimate and direct sequestration are the most effective emissions mitigation strategies. Green building measures can achieve both effects and reduce industrial carbon emissions by up to 18% (ref. ²⁸). When the co-benefits of mitigating carbon emissions in different sectors and spatial locations are taken into account, the effectiveness of each urban NBS implementation can be even higher, with examples presented in Supplementary Material 4.

a, Potential pathways of NBS to reduce emissions. b, Key NBS approaches and their contribution to carbon neutrality. n is the number of articles reviewed. c, Summary of NBS impact evaluations.

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We selected 54 cities across Europe to model prioritized spatial allocation of NBS implementations in order to maximize the carbon emissions reductions. The cities are all EU member states (as of 2022) and were selected because large amounts of consistent data are available for their home countries through the European Commission and other EU bodies (Fig. 2). The selection criteria included having a diversity of nations represented and a range of socioeconomic importance (that is, being capital cities or with high population and land area). Details of the selection criteria are provided in Supplementary Material 1.

a, Locations of the cities (the numbers representing cities are listed in Supplementary Material 1 Fig. A1.1). b, Residential emission distribution. c, Transport emission distribution. d, Industrial carbon distribution. e, Regional carbon emissions across different parts of the EU.

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The NBS implementations were spatially allocated on the basis of two major factors. The first of these was sectoral carbon emission sources identified within each city’s land use grid. Based on the estimated potential of each NBS, the implementations were allocated to the cell with the strongest identified emission sources in each sector. The second factor was the local context of each city and each location, including socioeconomic characteristics (population density, economic activities and location in the city centre), built environment (such as current access to local parks, road density and presence of manufacturing industries) and land use structure (type, mix and intensity). For example, street trees and green pavements as an NBS to promote walking and cycling should ideally be located along city roads in high-density urban areas, while preserved habitats should be located at the urban fringe where new urban developments are likely to occur. NBS types found to be most effective in reducing emissions in the respective sector during the systematic review were prioritized for allocation in the land-use grid according to the intensity of emissions from each sector. Although the allocation of prioritized NBS was mainly based on narratives of how different NBS fit with different socioeconomic contexts and their estimated emissions mitigation effects from the literature, some basic principles needed to be applied so that allocations for the 54 cities could be performed systematically. Details of these principles, the underlying criteria and narratives of the NBS allocation process can be found in Supplementary Material 2 Table A2.4.

In prioritized NBS (NBS^P), we varied the spatial allocation of different NBS types to match emissions differences between the cities (Fig. 3). In cities with large industrial areas and also ample natural areas (mainly central and eastern European cities, for example, Zagreb, Bucharest and Stuttgart), urban forests through habitat preservation were mostly allocated to the natural areas. Another type of habitat preservation, greenbelts, which have stronger effects in limiting urban sprawl, was allocated to the urban fringe of many major and growing cities across Europe, including Warsaw, Barcelona, Cologne, Helsinki and Marseille. Road greening through street trees was allocated to the urban fringe of cities that are currently characterized by low-density development and high level of car-based commuting, including Rome, Athens, Naples, Valletta, Copenhagen, Helsinki and Stockholm. Urban parks and agriculture can simultaneously reduce carbon emissions and improve the wellbeing of urban residents, so these NBS^P were prioritized in southern and eastern European cities. The spatial allocations included improving green access through urban parks in urban centres in Barcelona and Lasi, and improving green access through urban parks and agriculture in the urban fringe in Zagreb, Naples, Lodz, Valletta and Włocławek. Many southern and eastern European cities with high residential emissions and high population density were co-allocated green access and green buildings. Co-allocated green buildings and road greening were considered the most effective implementations in denser and more developed cities, mainly western European cities. Six typical cities are selected for presentation of different prioritizations of NBS allocations based on their emission patterns (Fig. 4).

a, The allocation process of NBS implementations. b, Residential emission source identification for Paris. c, Transport emission source identification for Gothenburg. d, Industrial emission source identification for Berlin.

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Allocations for all 54 EU cities are shown in Supplementary Material 1 Fig. A1.2. The coordinate grid is shown alongside each map, with x axis and y axis showing the longitude and latitude, respectively, of World Geodetic System 1984 (WGS84).

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If NBS^P implementations were to be introduced in practice, they could prevent large proportions of carbon emissions from different sectors. For all cities, NBS^P could reduce total carbon emissions by on average 17.4%, with 8.1% in the residential sector, 14.0% in the industrial sector and 9.6% in the transport sector. Of the remaining carbon emissions, 5.6% could be compensated for by carbon sequestration. The largest emissions reductions would occur in the industrial sector, with GI contributing most to the effect as it can save resources (water, energy and building materials) and reduce maintenance costs associated with industrial buildings. The residential sector has the lowest potential for NBS-related emissions reductions, as green buildings are usually limited in size and carbon sequestration potential is limited by space constraints in residential blocks. Moreover, green buildings are less effective in certain parts of the EU with lower cooling potential (that is, northern EU). Note that some of the carbon sequestration effects of some NBS^Pimplementations (mainly urban forests and parks) allocated on existing natural areas may already have been captured by existing vegetation, while for NBS^P that reduce anthropogenic carbon emissions through indirect pathways, our estimated emission reductions are almost additional even though some vegetation is already in place. Detailed explanations of the spatial allocation of NBS^P can be found in Methods.

The carbon emissions saving of NBS^P was found to vary between the different cities and regions, with the results for 15 typical cities presented in Table 1. The highest carbon emissions reduction potential was found for eastern EU cities, where NBS^P could reduce total carbon emissions by 20.3%, closely followed by northern EU (18.2%). The lowest carbon emissions reduction potential identified was in western European cities, closely followed by central EU cities, where NBS^P could reduce total carbon emissions by 13.0% and 13.2%, respectively. The differences between cities in emissions reduction potential were largely attributable to differences in carbon sequestration potential. Among the remaining emissions reduction potential following NBS^P implementation in the three sectors, carbon sequestration could offset 47.6% and 10.7% of the carbon emissions in northern and eastern EU cities, but only 6.7% in central European cities. As consistent with previous studies⁷, Stockholm has a very high carbon sequestration rate (55%) for the remaining emissions, partly due to very low industrial emission density in the region, as well as large natural areas within the urban boundary.

The emissions saving potential identified for different sectors highlights the importance of considering the socioeconomic and ecosystem context when prioritizing NBS implementations to maximize emissions reductions. In terms of residential emissions, northern EU cities were found to have the lowest residential emissions saving potential through NBS^P (4.5%), while central European cities had the highest (6.8%). Northern EU cities also had the lowest industrial emissions saving potential (6.7%), while western EU cities had the highest (11.0%). This highlights the diverse pathways towards carbon neutrality in the two most developed regions in the EU. Western EU cities will need to rely on green–blue infrastructure to reduce industrial emissions, while northern EU cities will have to depend more on natural ecosystems for carbon sequestration and transport emissions reduction, as they were found to have the highest transportation emissions-saving potential (11.8%). More details of sector- and city-specific NBS^P contributions are provided in Supplementary Material 4.

Next, we compared the carbon emissions reductions of the different NBS strategies and the carbon sequestration potential under Shared Socioeconomic Pathways (SSPs) differing in terms of climate actions and developmental paths. NBS^P was most effective in SSP1 (Fig. 5), in which maximizing NBS implementations on all available land parcels in the Representative Concentration Pathway (RCP) 1.9 scenario would reduce total carbon emissions by on average 57.3% (95% confidence interval (CI) 47.9–66.7%) compared with the SSP1 baseline, with 22% marginal reduction by indirect pathways including human behavioural interventions by NBS compared with the RCP 1.9 scenario. NBS was least effective in SSP5, in which maximizing NBS implementations on all available land parcels in the RCP 8.5 scenario would reduce total carbon emissions by on average 16.2% (95% CI 2.7–29.7%) compared with the SSP5 baseline, with only 1.7% marginal reduction compared with the RCP 8.5 scenario. Taking carbon sequestration into consideration, maximizing NBS implementations and carbon sequestration on all available land parcels in the RCP 1.9 scenario would reduce total carbon emissions by on average 62.5% (95% CI 49.4–108.6%) compared with the SSP1 baseline, by capturing a further 13.3% of carbon emissions. Maximizing NBS implementations on all available land parcels in the RCP 8.5 scenario would reduce total carbon emissions by on average 19.9% (95% CI 0.0–65.3%) compared with the SSP5 baseline, by achieving a further 3.6% marginal reduction compared with the RCP 8.5 scenario.

a, SSP1–RCP 1.9. b, SSP2–RCP 2.6. c, SSP3–RCP 4.5. d, SSP4–RCP 6.0. e, SSP5–RCP 8.5. MTCO₂, metric tons of CO₂.

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The effects of targeted policies to reduce carbon emissions in the RCP scenarios were stronger for EU12 countries (original 12 EU member states), while NBS and carbon sequestration effects were stronger for EU15 countries (member states that joined before 2004). Three cities were projected to achieve carbon neutrality before 2030 with the maximized implementation of NBS and carbon sequestration. These were Nicosia (in all scenarios), Zaragoza (RCP 1.9 and RCP 2.6) and Plovdiv (RCP 1.9, RCP 2.6 and RCP 4.5). Other cities, including some pioneering cities in climate action such as Amsterdam, Copenhagen, Helsinki and Stockholm, were projected to have less than 0.5 tons of carbon emissions per capita and year by 2030, almost achieving the carbon neutrality goal.

Compared with carbon sequestration, the indirect effects of NBS in carbon emissions reduction through human behaviour interventions were shown to play a much larger role in reducing urban carbon emissions. In comparison with prioritized NBS implementations, carbon sequestration was found to have a lower overall potential for meeting the goal of carbon neutrality in the European cities studied. Previous studies have estimated that in the most favourable scenario, the terrestrial biosphere in the EU can sequester 6.5–8% of projected anthropogenic emissions by 2030 (ref. ²⁹). Our novel analysis extended previous research by identifying the carbon emissions reduction potential of NBS through broader, more effective direct and indirect mechanisms (human behaviour interventions and resource and cost savings), rather than simply the carbon sequestration effects reported in other studies^30,31,32.

The findings in this study have important policy implications in terms of the allocation and implementation of NBS for the goal of climate change mitigation. Successful implementation will require a much better understanding of the behavioural aspects of NBS and of the socioeconomic, industrial and cultural context of each city, and the interactions with biophysical factors. Importantly, co-benefits and hybrids to reduce emissions from multiple sectors (transport, residential and industrial) should be prioritized and targeted, through integrating fossil-free energy systems and (grey) technologies with NBS such as green access, greening of streetscapes and roads, and green buildings and GI penetrating into residential high-density urban areas. In sparser but fast-developing parts of cities and in industrial urban areas, exploiting the multi-functions of urban forests, including greenbelt and GI protection, should be prioritized.

Another policy implication relates to the 2030 policy goal of carbon-neutral cities, where more ambitious climate policies phasing out fossil fuels can seek to maximize NBS potential at the same time. Our comparisons of different RCP scenarios revealed that policy pathways with more ambitious emissions reduction measures and targets (for example, RCP 1.9 and RCP 2.6) were positively correlated with greater NBS implementation. More ambitious climate policies preserve more natural land, protect ecosystem quality, and promote high-density and compact urban development, which all present opportunities for spatial allocation of more implementations to maximize NBS potential. Thus, pioneering cities that have created the best opportunities for NBS to be effective should extend their climate action plans to fully incorporate NBS implementations, which would maximize their chances of achieving city carbon neutrality. Inspired by how other environmental pollutants have been successfully handled in the past²⁵, we end by suggesting that the EU Commission should implement NBS-based spatial urban planning as a key strategy to achieve carbon-neutral cities.