Reduction in Anthropogenic CO2 Emissions Detected Through Two Decades of Observation in the Tokyo Metropolitan Area

Reducing CO2 emissions is a global goal aimed at mitigating climate change, but such reductions must be scientifically tracked and verified based on long-term observational data. We analyzed the long-term trend in CO2 concentration observed for a period of 19 years from 2002 to 2020 at two stations in the vicinity of Tokyo, one near a mountain summit and the other suburban. The CO2 concentration was higher at the suburban station than at the mountain station, while the annual rate of increase was lower at the suburban station than at the mountain station. The difference between the CO2 concentrations at the suburban and mountain stations (ΔCO2*) showed a significant decreasing trend over the two decades. The long-term trends (−1.39 ± 0.24% yr−1) of winter-nighttime ΔCO2* closely matched the trends (−1.54 ± 0.11% yr−1) of anthropogenic CO2 emissions in the region around the two stations. Based on this similarity, we conclude that the decreasing trend in ΔCO2* corresponds to a reduction in anthropogenic CO2 emissions around the Tokyo Metropolitan Area. This is the first evidence of two-decade-scale reductions in urban CO2 emissions from long-term continuous CO2 concentration monitoring.