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), greenhouse gases (GHGs), and short-lived ozone precursors on ozone changes between 1850 and 2014 using single-forcing perturbation simulations from several Earth system models with interactive chemistry participating in the Coupled Model Intercomparison Project Aerosol and Chemistry Model Intercomparison Project. We present the responses of ozone to individual forcings and an attribution of changes in ozone columns and vertically resolved stratospheric and tropospheric ozone to these forcings. We find that whilst substantial ODS-induced ozone loss dominates the stratospheric ozone changes since the 1970s, in agreement with previous studies, increases in tropospheric ozone due to increases in short-lived ozone precursors and methane since the 1950s make increasingly important contributions to total column ozone (TCO) changes. Increases in methane also lead to substantial extra-tropical stratospheric ozone increases. Impacts of nitrous oxide and carbon dioxide on stratospheric ozone are significant but their impacts on TCO are small overall due to several opposing factors and are also associated with large dynamical variability. The multi-model mean (MMM) results show a clear change in the stratospheric ozone trends after 2000 due to now declining ODSs, but the trends are generally not significantly positive, except in the extra-tropical upper stratosphere, due to relatively small changes in forcing over this period combined with large model uncertainty. Although the MMM ozone compares well with the observations, the inter-model differences are large primarily due to the large differences in the models' representation of ODS-induced ozone depletion.Plain Language Summary Overhead ozone absorbs harmful solar ultraviolet light, protecting life on Earth. Due to human activities since the nineteenth century, emissions of greenhouse gases (GHGs) and ozone-depleting substances (ODSs) containing chlorine and bromine have profoundly affected stratospheric ozone. Near the Earth' surface, ozone has increased substantially leading to worsening air quality. In this study, we use Earth system models to interactively assess the roles of ODSs, ozone-forming pollutants, and GHGs including methane, carbon dioxide (CO 2 ), and nitrous oxide (N 2 O) on ozone changes from the surface to the upper stratosphere. Whilst substantial reductions in stratospheric ozone due to ODSs occurred since the 1970s, the lower-atmospheric ozone increases due to anthropogenic pollution have counteracted this decrease. Increases in GHGs lead to various positive and negative effects on stratospheric ozone in different regions, and their impacts vary with ODS levels in the atmosphere. Amongst the GHGs assessed here, the increase in methane leads to overwhelming positive trends in both stratospheric and tropospheric ozone through mainly chemical effects. The impact of changes in N 2 O and CO 2 on total column ozone is more uncertain due to large inter-model differences, although their...

Evolution of total column ozone prior to the era of ozone depletion

“…Trends were calculated for each latitude band. Then, a linear regression approach, largely following Brönnimann (2022), was used to account for the effects of atmospheric circulation and solar UV. The following predictors were used: zonal mean 200 hPa geopotential height (Slivinski et al, 2019), the QBO at 50 hPa (Brönnimann et al, 2007) from May to October, solar UV radiation (Lean, 2018), a measure of the strength of the polar vortex at 200 hPa in December to April defined as the difference between geopotential height at 75-90 °N and 45-55 °N (taken from Brönnimann, 2022), and a June-to-February average of the El Niño index NINO4.…”

“…A very similar regression model (but using local rather than zonal mean 200 hPa height as a proxy for local tropopause heights and using the additional variable equivalent stratospheric chlorine and methane; see Brönnimann (2022) for a full description of the model) was applied to the long, homogenized series of Arosa, Oxford, and Tromsø. I then analyzed and filtered the residual with a 10-year moving average as a visual check for inhomogeneities (Supplementary Figure S1).…”

Arctic Tropospheric Ozone Trends

“…Evidence for declining O 3 precursor trends is supported by decreases in observed CO in the Arctic during autumn and winter (Figure S10 in Supporting Information S1). At the same time, CH 4 continues to increase globally contributing to rising O 3 in the NH (Zeng et al, 2022) (see also Text S4 in Supporting Information S1 on Arctic O 3 precursor trends). Another intriguing finding is springtime surface O 3 increases at Utqiaġvik (especially over 1999-2019, Figure S4 in Supporting Information S1), but no discernible trends at Alert and Villum.…”

“…Evidence for declining O 3 precursor trends is supported by decreases in observed CO in the Arctic during autumn and winter (Figure S10 in Supporting Information ). At the same time, CH 4 continues to increase globally contributing to rising O 3 in the NH (Zeng et al., 2022) (see also Text S4 in Supporting Information on Arctic O 3 precursor trends).…”

Fertilization strategies for abating N pollution at the scale of a highly vulnerable and diverse semi-arid agricultural region (Murcia, Spain)

“…However, only 22% of the new N entering the global agrifood system reaches our plates (Sutton et al 2021). The rest is largely released to the environment as reactive N (Nr) in the form of (i) ammonia (NH 3 ) (Bittman et al 2014) and oxides of N (NO x ) (Guardia et al 2018), which pollute the air, (ii) nitrate (NO 3 -), which pollutes drinking water and contributes to eutrophication (Quemada et al 2013), and (iii) nitrous oxide (N 2 O) (Thompson et al 2019), which promotes global warming (Smith et al 2021) and stratospheric ozone depletion (Zeng et al 2022). Overuse of N fertilizers in croplands, both synthetic and organic, is one of the main drivers of this alteration (Lassaletta et al 2016).…”