Cities are amongst the biggest contributors to carbon emissions because of widescale urbanisation, while also being highly vulnerable to resultant impacts of these emissions due to climate change impacts. Solutions that reduce or capture more carbon emissions than they release are called carbon sinks. Urban carbon sinks refer to a combination of solutions focused on maximising carbon capture and sequestration through the plantation of particular trees in urban and peri-urban green spaces, alongside existing or planned infrastructure. The solution can be combined with other solutions such as biochar production from green areas and garden waste to enhance carbon sequestration. Selecting tree species and managing the sink across the tree lifecycle is key to achieving the end objective of carbon sequestration. The focus should be on native species, which need minimal resources and have high survival rates.
Urban carbon sink can be well integrated as a strategic approach while planning and developing urban green spaces. Cities may introduce a policy framework outlining appropriate tree species, financing and incentive models, and operating model for development of urban green spaces (e.g. parks, parklets, urban forests, private green areas). Apart from maximising carbon sequestration, urban carbon sinks have numerous direct benefits such as reducing effect of urban heat island effect by increasing shadow surface and improving air quality through the removal of harmful gases and particulate matter from atmosphere. Additionally, these green spaces also provide many ecosystem benefits such as improving human health and well-being, regulating storm water infiltration, enhancing biodiversity in the area, improving economic value and creating livelihood opportunities for local community, recreation and nature education among others.
- Improved air quality: Plants are a great resouce for removing air pollutants and smog from the atmosphere, and protecting from harmful UV rays;
- Improved human health and enhanced citizen participation, connectivity, and sense of community: these spaces create a green space network and support busy city life, improveing health, prosperity, and happiness of citizens;
- Enhanced biodiversity and pollinator increase: with enhanced green areas, an instant improvement in biodiversity is observed. This contributes to soil moisture content enhancement and improved nutrient cycle, thereby impacting the surrounding flora and fauna;
- Enhanced green mobility: urban sinks offer aesthetically green spaces to move around in the built concrete spaces;
- Increased carbon sequestration: green areas increase the absorption of carbon dioxide and increase the supply of oxygen;
- Reduced hotspots and urban heat island impact: trees and plants reduce the impact of urban heat island effect as they create a cooling effect around them;
- Enhanced attractiveness of the cities: green areas create aesthetically beautiful environment in the cities which further beautifies the city. Urban carbon sinks can create an interconnected network of green spaces that sparks interest in citizens and harmonizes the negative impacts of urban expansion;
- Green awareness: improved awareness amongst citizen of native tree species and the value of trees;
- Reduced noise pollution: trees can reduce noise level by human ear by more than 50%; and
- Economic opportunities: by leading to enhanced land prices for the properties in proximity.
Urban carbon sink is linked to solutions such as cycle and pedestrian green route, cooling trees, other singular green infrastructure solutions, water intervention, environmental awareness, etc.
Urban carbon sink is an integrated approach towards developing urban nature-based solutions, with an objective to store carbon emissions as well as provide a slew of ecosystem services.
Climate and Geography:
Urban forests and urban green spaces can be planted anywhere adapted to the local tree species, which should be selected according to hardiness zone, soil type, sunlight and rain data, frost schedules and other factors that affect the success of trees and vegetation, as well as carbon sequestration potential. In terms of soil conditions, sandy and loamy soils provide better conditions for new forests given their strong infiltration capacity and aeration of the soil. In addition, different sizes and their interaction with each other will be considered in order to favour the planting framework for optimum growth and new ecosystem formation,
Urban form and layout:
Urban green cover can be planned along infrastructure networks, residential streets and large-scale infrastructure – highways and rail tracks are suitable for linear urban green cover. Urban forests can be planned in degraded natural forest areas, alluvial sites along water bodies, steep slopes at risk of soil erosion and landslides, non- productive agriculture and industrial wood plantation sites in peri-urban areas.
The planning of urban carbon sinks should be holistic and integrated with urban planning and city climate action plans. These green areas should become an integral part of conservation zones and in the development of green areas and public parks. The following key technical aspects should be taken into consideration.
- Slope: direction of slope is a key factor during the incubation period of trees.
- Site preparation: may be required in terms of removal of weeds, augmentation of soil structure, or irrigation provision.
- Species selection: given the objective of urban carbon sinks is to sequester carbon alongside ecosystem benefits, the planting and growing strategy should be prepared in consideration of the most suitable local tree species. Also, the focused-on C fixation capacity, easy management, aesthetics, health and ecological coherence and integrity criteria
- Seedling and tree production: the best strategy should be selected depending on local context – in some cases, planting of large sampling instead of conventional seedling can be more preferable. The availability of a nursery is crucial for quality samplings.
- Planting and growing strategy: A sustainable strategy is required to consider human and land management practices in urban contexts. In case of plantation in degraded land, well-watered sturdy plants from nurseries are required. Urban forests or green cover planned alongside gray infrastructure often require additional considerations for seedlings, tree size, pre-planting, formative pruning and root management.
- Time of planting: seedlings should be planted at the right time of year, in consideration of the local weather considerations related to precipitation.
- Green waste management: including timely pruning and conversion of waste from pruning into compost or biochar. Cities should develop an urban carbon sink strategy and action plan with a localised approach which considers the factors outlined above, along with a robust institutional and governance framework to ensure on-ground implementation.
Policy and regulatory/legal framework:
The urban carbon sink should be well-integrated into urban planning principles and should be implemented through a holistic strategy – supported by a policy framework, spanning across different departments from parks and recreation to utilities. Policies along with guidelines can outline the creation and management of urban carbon sinks (including the appropriate species and techniques) as a critical element to be included in planning grey infrastructure, defining land uses, as well as planning resilience in cities. Regulations mandating private sector support for greening activities in public infrastructure – as well as private sector developments in form NBS and green infrastructure ordinances in local development and building by-laws – can create an enabling environment for holistic development of urban carbon sinks in the city.
Funding and Financing:
Urban carbon sinks in public places can be primarily financed through a city’s own resources. These resources can be blended with private sector funding initiatives such as CSR funding, advertisement/ naming rights and ecosystem benefit charges. Monetisation of the environmental attribute can also be a potential funding model. Cities might introduce schemes such as gift-a-tree to enhance community participation. Access to EU/public funding and innovation procurement can further incentivise the development of urban carbon sinks. For instance, the Interreg project URBforDAN, where more information on funding and best practices was obtained through participation in multi-stakeholder dialogues at EU-level.
Economic and social context:
It is very important that citizens and the local community are aware of the public value of urban carbon sinks and greening. Urban citizens and elected officials must understand the full range of services trees and green space provide to properly plan and manage urban carbon sinks, including urban forests and urban green spaces. Therefore, it is important to spread awareness of a Nature-based approach that will reduce emissions emitted due to the degradation of forests. Implementing NBS for a carbon sink would aid in resolving social issues and provide ecosystem services that are beneficial to human well-being and biodiversity. This can be done through educational activities on NBS, city coaching in NBS, monitoring impact and reporting the ecosystem benefits of NBS (refer instruments below for more details).
Project Governance and Implementation modalities:
It is critical projects be implemented and monitored at a central-level in the city as a part of climate action and resilient planning. The implementation may follow three sub steps.
- Forest/ Green cover assessments: Type mapping of natural area, stand assessment, protective modelling and vision setting
- Planning and policy: development of city-scale green policy, planting and management goals and sub-area specific goals
- Management actions: Implementation plan and strategy development for conservation and new plantation assigned to a specific agency or a third-party. Monitoring and Verification through mapping of trees and biannual monitoring of growth.
Climate and geography:
Afforestation must be implemented with due diligence in dry climates to avoid any impact on water tables. In areas where water is critical to tree growth, artificial irrigation may be required to sustain the forests. Soils with a high proportion of clay and compact soils are much less favourable to the establishment of new roots. In semi-arid and arid climates, special considerations regarding water needs, irrigation, and the impacts of establishing green corridors on water tables and soil recharge rates should be evaluated prior to implementation.
Urban form and layout:
The urban environment results in challenging conditions for tree growth due to exposure to pollutants, high temperatures, limited space above and below ground and increased susceptibility to insects and diseases.
Given the challenging condition outlined above, proper planning and strategy development is key towards successful implementation. Maintenance is key to realise the benefits of urban carbon sink, particularly in the first 5 years. Tree seedlings may require watering or irrigation and need to be protected from weeds competing for light, moisture, and nutrients, and from grazing wild and domestic animals. Pruning and cutting may also be required. As maintenance is a manpower- and resource-intensive exercise – and dependent on ownership and responsibilities of departments – it can become a key constraint. Innovative maintenance models, including communities, incentive mechanisms and regular monitoring with contractual obligation, can be used to overcome this barrier. Innovative procurement techniques for development and maintenance of urban carbon sinks can be used, involving local communities and the private sector for implementation.
Policy and regulatory/legal framework:
Some policies may have negative impacts on urban carbon sinks. For instance, policies related to design of roads and sidewalks that fail to provide adequate space for roots and tree canopy. National and regional policies that guide urban expansion planning and development should include urban carbon sink as a key design element.
Economic and social context:
The component of urban carbon sinks i.e. urban trees and urban forest are often considered a financial burden or risk, as ecosystem benefits are poorly understood and may not be fairly valued by citizens, communities and decision-makers.
Climate and Geography:
In cities where seasonally, dry climate forest fire poses a significant risk, timely rational maintenance pruning and an effective fire prevention strategy is essential. In absence of these, the solution may result in an increase in overall emissions. Dead seedlings should be replaced early in the following rainy season, ideally with seedlings of a similar size to those surviving nearby.
Economic and social context:
The plantation and maintenance activities are resource- and capital-intensive, and the success rate is largely dependent on monitoring and active maintenance. Hence, investments made by the city in urban carbon sinks can only be justified if the value of the associated ecosystem services is captured and communicated.
Given that urban carbon sinks are generally dependent on regional precipitation and water balance, they are more vulnerable to extreme climate events. The strategy and action plan for urban carbon sinks should therefore take into consideration their vulnerability to extreme climate events.
- NBS Catalogue [pages 101-102]
- Urban Carbon Sink in URBAN GreenUP catalogue [Pages 58-60]
- Choosing the right nature-based solutions to meet diverse urban challenges
- Compendium of Nature-based and ‘grey’ solutions to address climate- and water-related problems in European cities (GrowGreen project)
- Green Resting Areas (URBAN GreenUP project)
- NBS Handbook (Think-Nature project)
- Addressing climate change in cities, NBS catalogue
- Nature and net zero: Why investing in nature is important (World Economic Forum)
- Approaches to financing nature-based solutions in cities (GrowGreen project)
- Designing for biodiversity in low carbon and net zero buildings (UK)
Emissions Carbon Storage and Sequestration:
Urban forestry can offset 18.57% of the carbon emitted by urban industries and store 1.75 times the annual carbon emitted by city energy use. (Urban Forestry | Climate Technology Centre & Network | Tue, 11/08/2016)
Trees can cut 40% of residential and commercial building energy use due to their cooling impact.
Urban carbon sinks purify the air by removing NO2, O3, SO2, and PM10 particles, resulting in improved lung health of citizens.
Flood risk reduction:
Well-managed urban carbon sinks can retain stormwater and decrease the risk of urban flooding.
Capital cost for afforestation (planting, establishment and financing): EUR 15,000–19,000/ ha (Cambridge Econometrics 2020).
- Forest fire resulting from extreme heat and climate change events can pose a significant risk on urban forests, resulting in impacts on the following environment objective:
- Climate Change Mitigation: Release of carbon back to atmosphere
- Pollution prevention and control: Release of harmful gases can impact the local air quality significantly
- Educational activities on NBS: https://netzerocities.app/resource-1518
- Supporting municipalities to monitor resource flows in line with impact targets and measurement processes: https://netzerocities.app/resource-1528
- Engagement, co-creation and co-design of NBS and Green Infrastructure plans and interventions: https://netzerocities.app/resource-1608
- City coaching in NBS: https://netzerocities.app/resource-1618
- Platform for Enhancing Multi Stakeholder Dialogue to Implement NBS across EU: https://netzerocities.app/resource-1628
- Integrated climate plans for cities (i.e. SECAPs): https://netzerocities.app/resource-1698
- Public procurement for innovative NBS and Green Infrastructure interventions: https://netzerocities.app/resource-588
- NBS and Green Infrastructure regulation and ordinances: https://netzerocities.app/resource-1813
- NBS and Green Infrastructure plans and strategy design and governance: https://netzerocities.app/resource-1823
- Helsinki, Finland, Case Study
Forest Carbon Sink Resource Asset Evaluation with Case Study of Fujian Province in China
Carbon sink asset evaluation methods must mature as the forest carbon sink market matures and trading information opens. In this paper, asset appraisal basic, market method, cost method, and income method were used to study forest carbon sink value. Asset appraisal related economic and technical means were used to confirm forest carbon sink savings, which is the basis of its economic value.
Co-Designing Urban Carbon Sink Parks: Case Carbon Lane in Helsinki
Municipalities worldwide must adopt negative emission technologies to achieve carbon neutrality in 20 years. This paper examined the main principles of urban demonstration areas using trees and biochar for carbon sequestration and found that demonstration sites of urban carbon sinks in public parks must be safe, visible, and scientifically-sound for reliable and cost-effective verification of carbon sequestration.
The City of Toronto recognises the value of urban forests and aims to increase its tree canopy cover to 40%. The City's focus is on maximising the potential ecological, social, and economic benefits of urban trees. The Urban Forestry branch of the Parks, Forestry and Recreation division maintains over four million trees on public property and works with local groups and residents to expand and improve the urban forest throughout the city. Since 2013, the city has been planting approximately 100,000 trees on public lands – parks, streets, ravines – per year, with ambitions to increase that to 300,000 trees per year through new private–public partnerships with private landowners.
Urban Food Forest Rijnvliet, 2017–ongoing
Residents of the Rijksstraatweg and the Metaalkathedraal areas proposed the concept of a food forest in the new urban development of Rijnvliet in 2017. The municipality developed a public space for this purpose – the edible residential area. All plantings were chosen for their value to nature, with strong preferences for edible plants and trees, even in the private residential gardens. The municipality has also accorded Rijnvlie a central food forest of 15,000 m2, dedicated space built on seven multiple layers that form an integrated ecosystem. A neighborhood orchard for recreational activities and play areas for children is also in planning. Residents, the school, and the municipality regularly discuss fresh ideas to implement.
The Green Belt of Vitoria-Gasteiz Spain, 1990s–2008
The Green Belt is a group of peri-urban parks of high ecological and landscape value, strategically linked by eco-recreational corridors. It is a result of an ambitious environmental restoration project initiated in the early 1990s around the outlying areas of Vitoria-Gasteiz with the objective of creating a large, green area for recreational use around the city. It offers many different environments with a wealth of natural features. Woods, rivers, wetlands, meadows, fields, groves, and hedgerows are some of the varied ecosystems that coexist. Some of these ecosystems, such as the restored wetlands of Salburuaor and the River Zadorra ecosystem, have won recognition at international level for their high environmental value.