CO2 uptake of urban vegetation in a warming Nordic city

Many cities are committed to reducing greenhouse gas emissions, improving air quality, and reducing air temperatures through climate actions. Maintaining or increasing carbon sinks in urban green areas is relevant, as the sinks compensate a part of the emissions in addition to reducing emissions themselves. This research assesses the magnitude of carbon dioxide (CO2) net uptake (i.e. respiration and CO2 uptake by photosynthesis) and local 2-m air temperature in Helsinki, Finland, using an urban ecosystem model SUEWS (The Surface Urban Energy and Water Balance Scheme), and examines their potential future changes due to climate change. The model was run at an hourly resolution within the entire city at a spatial resolution of 250 × 250 m2. Two separate simulations were considered: the present climate by simulating years 2014–2019 and the future climate in the 2050s following the climate scenario RCP8.5. Each modelled grid was further divided into natural and built surfaces using Local Climate Zones (LCZs) to determine how vegetation in forests and various urban vegetation types contributes to cooling and carbon sequestration. According to our simulations, the urban green space in Helsinki annually sequestered 36.3 ± 7.7 kt C in 2015–2019, offsetting circa 7% of the city's anthropogenic emissions. The mean annual temperatures varied by 2.1∘C between natural and built areas within the city. Although urban forests were the strongest sinks (0.3 kg C m−2 year−1), urban neighbourhoods contributed 47% of Helsinki's net carbon sinks. Local temperatures were expected to increase with the RCP8.5 climate scenario on average by 1.3∘C within the simulated area and CO2 net uptake by 11%, without altering existing green spaces. Overall, this research highlights the significance of urban green as carbon sinks and how climate change may influence their role in mitigating greenhouse gas emissions and local climate conditions in urban environments.