Every green space counts as carbon sink
The potential of urban vegetation and soils as carbon sinks should be better taken into account as cities seek to reach their climate goals.
Urban nature, green spaces, courtyards, and street planting have a number of positive effects on our environment, contributing to biodiversity, regulating stormwater, adapting to heat waves, and supporting human health and wellbeing. The vegetation and soil in green spaces also sequester carbon dioxide, thereby mitigating climate change. The most significant share of carbon is stored in the soil and the rest in the biomass of the vegetation. Due to its numerous benefits, the European Union sees urban nature as a cost-effective climate solution. According to a recent calculation, urban green spaces in Helsinki captured 5% of fossil fuel emissions in 2019. This share can be significantly increased through carbon-wise design, construction, and maintenance.
Strengthening of carbon sinks is included in the climate targets and calculations of many cities. However, discussion has focused on forests, largely neglecting urban green spaces.
Similarly in Finland, the newly reformed Climate Change Act and the pending Planning and Building Act stress carbon sinks in general but bypass the potential of urban carbon sinks of green spaces. For example, land use categories of the Climate Change Act do not comprehensively identify urban nature in the classification. Moreover, the draft regulation on the climate assessment of a construction project recognizes trees but excludes other vegetation types and the soil as a carbon sink. The role of vegetation and soil in carbon sequestration is also missing from the national guidelines for the construction and maintenance of green areas.
Urban nature should be more strongly integrated into the climate debate at all levels — from land use legislation to guidelines for the design and management of green spaces.
A key message is to increase the amount of green infrastructure within the urban fabric and especially to preserve existing green areas, their vegetation and soils, which already act as carbon sinks. Secondly, long life span of urban trees and favourable growing conditions must be ensured, such as sufficient space for root systems and canopy cover. In addition, growing media solutions, such as the addition of biochar, can have a significant impact on the soil’s efficiency in storing carbon.
Thirdly, the maintenance of green spaces plays a role in carbon sequestration and should be considered. Although the removal of leaves and grass clippings is part of current management practices and represents an ideal of a well-maintained park, it reduces the soil carbon storage. Fourthly, the construction of green spaces should also take into account emissions — for example, earthworks and asphalt and concrete pavements significantly increase the carbon footprint of green spaces. Furthermore, podium courtyards with parking underground commonly used in dense urban areas are particularly problematic in terms of life cycle, as the renovation of moisture insulation often destroys the vegetation and its carbon storage.
Increasing the carbon sink potential of urban nature requires clear indicators and tools for decision-making concerning green spaces. In urban planning, carbon calculations need to be supported by methods that better consider urban green spaces, especially soil. Carbon sequestration should also be a key element in national guidelines for the construction and management of green spaces. Moreover, home gardeners would benefit from guidelines for carbon-smart gardening.
However, urban nature cannot be designed solely with climate objectives in mind but must also take into account the promotion of biodiversity and the impact on well-being. The aim is to find synergies and solutions that deliver multiple benefits at the same time. Supporting the multifunctionality of urban nature requires cross-sectoral and cross-governmental decision-making as well as integration of, for example, the currently separate climate and biodiversity targets. As cities become more compact, urban nature will become even more important. A climate-resilient city takes care of its green spaces — because green spaces take care of all of us.
Ranja Hautamäki
Ranja Hautamäki is associate professor in landscape architecture at Aalto University and the vice-leader of CO-CARBON consortium. She works in the interface of planning practice and research to tackle societal challenges related to climate change, biodiversity, and well-being.
Leena Järvi
Leena Järvi is professor in Urban Meteorology at University of Helsinki. She works with observations and modelling of urban greenhouse gas emissions and sinks and leads the CO-CARBON consortium.
