CO2 surface variability: from the stratosphere or not?

Fluctuations in atmospheric CO2 can be measured with great precision and are used to identify human-driven sources as well as natural cycles of ocean and land carbon. One source of variability is the stratosphere, where the influx of aged CO2-depleted air can produce fluctuations at the surface. This process has been speculated to be a potential source of interannual variability (IAV) in CO2 that might obscure the quantification of other sources of IAV. Given the recent success in demonstrating that the stratospheric influx of N2O- and chlorofluorocarbon-depleted air is a dominant source of their surface IAV in the Southern Hemisphere, I apply the same model and measurement analysis here to CO2. Using chemistry-transport modeling or scaling of the observed N2O variability, I find that the stratosphere-driven surface variability in CO2 is at most 10 % of the observed IAV and is not an important source. Diagnosing the amplitude of the CO2 annual cycle and its increase from 1985 to 2021 through the annual variance gives rates similar to traditional methods in the Northern Hemisphere (BRW, MLO) but can identify the emergence of small trends (0.08 ppm per decade) in the Southern Hemisphere (SMO, CGO).

Automated summary

Fluctuations of atmospheric CO2 are mainly caused by human-driven sources and natural cycles of ocean and land carbon. The influx of aged CO2-depleted air in the stratosphere can lead to surface CO2 fluctuations. However, the stratosphere-driven surface variability in CO2 is found to be at most 10% of the observed interannual variability and is not a significant source. Analysis of the annual variance can identify the amplitude and increase of the CO2 annual cycle over time.