Flux and Radiocarbon Evidence of Urban Carbon Emission Reductions under Climate Mitigation Policies

Abstract. Urban areas account for the majority of global CO₂ emissions, yet direct observational evidence of emission reductions under climate mitigation policies remains limited. Here we combine eddy covariance measurements with tree-ring radiocarbon (¹⁴C) to quantify changes in urban CO₂ emissions and attribute their sources in two megacities, Beijing and Xi’an, from 2020 to 2024. We find that all urban sites remained net carbon sources but exhibited consistent interannual declines in CO₂ fluxes, with the strongest reductions during heating season and traffic peak hours. Compared to 2021, the CO₂ fluxes in Beijing and Xi’an in 2024 reduced by 21.6% (1.1 kg C m⁻² yr⁻¹) and 41.2% (0.7 kg C m⁻² yr⁻¹), respectively. Independent constraints from tree-ring ¹⁴C reveal a concurrent decrease in fossil fuel-derived CO₂ (CO_2ff), demonstrating that the observed emission reductions are primarily driven by declining fossil fuel combustion. Analysis of energy consumption data indicates that these reductions are associated with substitution of coal and petroleum with electricity and natural gas, as well as rapid adoption of new energy vehicles. Our results provide direct observational evidence that urban carbon emissions can decline under climate mitigation policies and highlight the central role of energy system transformation in achieving sustained emission reductions.