Reductions in California's urban fossil fuel CO2 emissions during the
COVID-19 pandemic
Abstract
Fossil fuel CO2 emissions (ffCO2) constitute the majority of greenhouse
gas emissions and are the main determinant of global climate change. The
COVID-19 pandemic caused wide-scale disruption to human activity and
provided an opportunity to evaluate our capability to detect ffCO2
emission reductions. Quantifying changes in ffCO2 levels is especially
challenging in cities, where climate mitigation policies are being
implemented but local emissions lead to spatially and temporally complex
atmospheric mixing ratios. Here, we used direct observations of on-road
CO2 mole fractions with analyses of the radiocarbon (14C) content of
annual grasses collected by community scientists in Los Angeles and
California, USA to assess reductions in ffCO2 emissions during the first
two years of the COVID-19 pandemic. With COVID-19 mobility restrictions
in place in 2020, we observed a significant reduction in ffCO2 levels
across California, especially in urban centers. In Los Angeles, CO2
enhancements on freeways were 60 ± 16% lower and ffCO2 levels were
43-55% lower than in pre-pandemic years. By 2021, California’s ffCO2
levels rebounded to pre-pandemic levels, albeit with substantial spatial
heterogeneity related to local and regional pandemic measures. Taken
together, our results indicate that a reduction in traffic emissions by
~60% (or 10-24% of Los Angeles’ total ffCO2 emissions)
can be robustly detected by plant 14C analysis and pave the way for
mobile- and plant-based monitoring of ffCO2 in cities without CO2
monitoring infrastructure such as those in the Global South.