Attribution of stratospheric and tropospheric ozone changes between 1850
and 2014 in CMIP6 models
Abstract
We quantify the impacts of halogenated ozone-depleting substances
(ODSs), methane, N2O, CO2, and short-lived ozone precursors on total and
partial ozone column changes between 1850 and 2014 using CMIP6 Aerosol
and Chemistry Model Intercomparison Project (AerChemMIP) simulations. We
find that whilst substantial ODS-induced ozone loss dominates the
stratospheric ozone changes since the 1970s, the increases in
short-lived ozone precursors and methane lead to increases in
tropospheric ozone since the 1950s that make increasingly important
contributions to total column ozone (TCO) changes. Our results show that
methane impacts stratospheric ozone changes through its reaction with
atomic chlorine leading to ozone increases, but this impact will
decrease with declining ODSs. The N2O increases mainly impact ozone
through NOx-induced ozone destruction in the stratosphere, having an
overall small negative impact on TCO. CO2 increases lead to increased
global stratospheric ozone due to stratospheric cooling. However,
importantly CO2 increases cause TCO to decrease in the tropics. Large
interannual variability obscures the responses of stratospheric ozone to
N2O and CO2 changes. Substantial inter-model differences originate in
the models’ representations of ODS-induced ozone depletion. We find
that, although the tropospheric ozone trend is driven by the increase in
its precursors, the stratospheric changes significantly impact the upper
tropospheric ozone trend through modified stratospheric circulation and
stratospheric ozone depletion. The speed-up of stratospheric overturning
(i.e. decreasing age of air) is driven mainly by ODS and CO2; increases.
Changes in methane and ozone precursors also modulate the
cross-tropopause ozone flux.