Global sensitivity analysis of chemistry–climate model budgets of tropospheric ozone and OH: exploring model diversity

Projections of future atmospheric composition
change and its impacts on air quality and climate depend
heavily on chemistry–climate models that allow us to investigate
the effects of changing emissions and meteorology.
These models are imperfect as they rely on our understanding
of the chemical, physical and dynamical processes governing
atmospheric composition, on the approximations needed
to represent these numerically, and on the limitations of the
observations required to constrain them. Model intercomparison
studies show substantial diversity in results that reflect
underlying uncertainties, but little progress has been made
in explaining the causes of this or in identifying the weaknesses
in process understanding or representation that could
lead to improved models and to better scientific understanding.
Global sensitivity analysis provides a valuable method
of identifying and quantifying the main causes of diversity in
current models. For the first time, we apply Gaussian process
emulation with three independent global chemistry-transport
models to quantify the sensitivity of ozone and hydroxyl radicals
(OH) to important climate-relevant variables, poorly
characterised processes and uncertain emissions. We show a
clear sensitivity of tropospheric ozone to atmospheric humidity
and precursor emissions which is similar for the models,
but find large differences between models for methane lifetime,
highlighting substantial differences in the sensitivity of
OH to primary and secondary production. This approach allows
us to identify key areas where model improvements are
required while providing valuable new insight into the processes
driving tropospheric composition change.