Surface CO2 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 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, we here apply the same model and measurement
analysis to CO2.  Using chemistry-transport modeling or scaling
of the observed N2O variability, we 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).