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Hydrogen is expected to play a key role in the global energy transition to net zero emissions in many scenarios. However, fugitive emissions of hydrogen into the atmosphere during its production, storage, distribution and use could reduce the climate benefit and also have implications for air quality. Here, we explore the atmospheric composition and climate impacts of increases in atmospheric hydrogen abundance using the UK Earth System Model (UKESM1) chemistry–climate model. Increases in hydrogen result in increases in methane, tropospheric ozone and stratospheric water vapour, resulting in a positive radiative forcing. However, some of the impacts of hydrogen leakage are partially offset by potential reductions in emissions of methane, carbon monoxide, nitrogen oxides and volatile organic compounds from the consumption of fossil fuels. We derive a refined methodology for determining indirect global warming potentials (GWPs) from parameters derived from steady-state simulations, which is applicable to both shorter-lived species and those with intermediate and longer lifetimes, such as hydrogen. Using this methodology, we determine a 100-year global warming potential for hydrogen of 12 ± 6. Based on this GWP and hydrogen leakage rates of 1 % and 10 %, we find that hydrogen leakage offsets approximately 0.4 % and 4 % respectively of total equivalent CO 2 emission reductions in our global hydrogen economy scenario. To maximise the benefit of hydrogen as an energy source, emissions associated with hydrogen leakage and emissions of the ozone precursor gases need to be minimised. [ABSTRACT FROM AUTHOR]

The year when total column ozone (TCO) returns to 1980 levels is commonly used to quantify recovery from disturbance caused by ozone‐depleting substances. This date is reasonable but somewhat arbitrary. Here we borrow an indicator from climate change research, the signal‐to‐noise (S/N) metric to investigate how TCO might return to statistically similar levels to those of the pre‐ozone hole era (1960–1970). We calculate S/N for ozone projections from the Chemistry‐Climate Model Initiative. In most regions, a return to 1980 levels doesn't represent TCO recovery to pre‐disturbance conditions because it does not account for internal variability or reflect when statistically significant TCO losses occurred. Future projections show that, in most regions, TCO "de‐emerges" (i.e., S/N becomes less than one standard deviation from the pre‐disturbance mean) before it returns to its 1980 value. We conclude that S/N is an appropriate, perhaps complementary metric for determining when TCO returns to a pre‐disturbance state. Plain Language Summary: The stratospheric ozone layer is expected to recover from the damaging effects of chlorofluorocarbons (CFCs) and other halogen‐containing source gases this century. Ozone recovery is traditionally but somewhat arbitrarily defined as the year when total column ozone (TCO) abundances return to 1980 levels. Here, we use a metric commonly used in the climate change emergence literature—signal‐to‐noise (S/N)—to quantify statistically significant changes in TCO in models and observations relative to a period before CFC‐induced ozone depletion began. The S/N framing ties recovery to unperturbed environmental conditions, rather than to the environment in a specific year. We argue that tying recovery to unperturbed local environmental conditions is at least as reasonable as tying it to the same global date. Our calculations, which account for natural variability, show that 1980 does not reflect the onset of statistically significant ozone loss. Using a return of TCO to 1980 levels underestimates the time of recovery in Antarctica, where losses have been most significant, and overestimates the time to recovery in other regions. We conclude that signal‐to‐noise is a useful metric to assess the return of TCO to its baseline state and evaluate the effectiveness of the Montreal Protocol. Key Points: We present signal‐to‐noise calculations of stratospheric ozone depletion in chemistry‐climate models relative to 1960–1970Statistically significant ozone depletion does not emerge in 1980, which is traditionally used as a benchmark for ozone recoveryIn most regions de‐emergence occurs before ozone returns to 1980 levels except in Antarctica where de‐emergence occurs 25 years later [ABSTRACT FROM AUTHOR]

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 (CO2), and nitrous oxide (N2O) 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 N2O and CO2 on total column ozone is more uncertain due to large inter‐model differences, although their overall impact is small during the simulation period. Key Points: New multi‐model results show significant positive effects of ozone precursors on near‐global ozone offsetting the negative effects of ozone‐depleting substances (ODSs)ODS and greenhouse gases dominate stratospheric ozone changes but with large inter‐model differences due to uncertainties in responses to ODS changesIncreases in carbon dioxide and nitrous oxide significantly impact stratospheric ozone, but their net effects on total columns are small due to cancellations [ABSTRACT FROM AUTHOR]

We present the first incorporation of the Common Representative Intermediates version 2.2 tropospheric chemistry mechanism, CRI v2.2, combined with stratospheric chemistry, into the global chemistry–climate United Kingdom Chemistry and Aerosols (UKCA) model to give the CRI-Strat 2 mechanism. A rigorous comparison of CRI-Strat 2 with the earlier version, CRI-Strat, is performed in UKCA in addition to an evaluation of three mechanisms, CRI-Strat 2, CRI-Strat and the standard UKCA chemical mechanism, StratTrop v1.0, against a wide array of surface and airborne chemical data. CRI-Strat 2 comprises a state-of-the-art isoprene scheme, optimized against the Master Chemical Mechanism v3.3.1, which includes isoprene peroxy radical isomerization, HO x recycling through the addition of photolabile hydroperoxy aldehydes (HPALDs), and isoprene epoxy diol (IEPOX) formation. CRI-Strat 2 also features updates to several rate constants for the inorganic chemistry, including the reactions of inorganic nitrogen and O(1D). The update to the isoprene chemistry in CRI-Strat 2 increases OH over the lowest 500 m in tropical forested regions by 30 % –50 % relative to CRI-Strat, leading to an improvement in model–observation comparisons for surface OH and isoprene relative to CRI-Strat and StratTrop. Enhanced oxidants also cause a 25 % reduction in isoprene burden and an increase in oxidation fluxes of isoprene and other biogenic volatile organic compounds (BVOCs) at low altitudes with likely impacts on subsequent aerosol formation, atmospheric lifetime, and climate. By contrast, updates to the rate constants of O(1D) with its main reactants relative to CRI-Strat reduces OH in much of the free troposphere, producing a 2 % increase in the methane lifetime, and increases the tropospheric ozone burden by 8 % , primarily from reduced loss via O(1D)+H2O. The changes to inorganic nitrogen reaction rate constants increase the NO x burden by 4 % and shift the distribution of nitrated species closer to that simulated by StratTrop. CRI-Strat 2 is suitable for multi-decadal model integrations and the improved representation of isoprene chemistry provides an opportunity to explore the consequences of HO x recycling in the United Kingdom Earth System Model (UKESM1). This new mechanism will enable a re-evaluation of the impact of BVOCs on the chemical composition of the atmosphere and further probe the feedback between the biosphere and the climate. [ABSTRACT FROM AUTHOR]

The evolution of tropospheric ozone from 1850 to 2100 has been studied using data from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We evaluate long-term changes using coupled atmosphere–ocean chemistry–climate models, focusing on the CMIP Historical and ScenarioMIP ssp370 experiments, for which detailed tropospheric-ozone diagnostics were archived. The model ensemble has been evaluated against a suite of surface, sonde and satellite observations of the past several decades and found to reproduce well the salient spatial, seasonal and decadal variability and trends. The multi-model mean tropospheric-ozone burden increases from 247 ± 36 Tg in 1850 to a mean value of 356 ± 31 Tg for the period 2005–2014, an increase of 44 %. Modelled present-day values agree well with previous determinations (ACCENT: 336 ± 27 Tg; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP: 337 ± 23 Tg; Tropospheric Ozone Assessment Report, TOAR: 340 ± 34 Tg). In the ssp370 experiments, the ozone burden increases to 416 ± 35 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (PO3) and loss (LO3) diagnostics. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (PO3 - LO3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere–troposphere transport reaches a minimum around the same time before recovering in the 21st century, while dry deposition increases steadily over the period 1850–2100. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden. [ABSTRACT FROM AUTHOR]

The temporal evolution of the abundance of long-lived, anthropogenic chlorofluorocarbons in the atmosphere is a major factor in determining the timing of total column ozone (TCO) recovery. Recent observations have shown that the atmospheric mixing ratio of CFC-11 is not declining as rapidly as expected under full compliance with the Montreal Protocol and indicate a new source of CFC-11 emissions. In this study, the impact of a number of potential future CFC-11 emissions scenarios on the timing of the TCO return to the 1960–1980 mean (an important milestone on the road to recovery) is investigated using the Met Office's Unified Model (Hewitt et al., 2011) coupled with the United Kingdom Chemistry and Aerosol scheme (UM-UKCA). Key uncertainties related to this new CFC-11 source and their impact on the timing of the TCO return date are explored, including the duration of new CFC-11 production and emissions; the impact of any newly created CFC-11 bank; and the effects of co-production of CFC-12. Scenario-independent relationships are identified between cumulative CFC emissions and the timing of the TCO return date, which can be used to establish the impact of future CFC emissions pathways on ozone recovery in the real world. It is found that, for every 200 Gg Cl (∼258 Gg CFC-11) emitted, the timing of the global TCO return to 1960–1980 averaged values is delayed by ∼0.56 years. However, a marked hemispheric asymmetry in the latitudinal impacts of cumulative Cl emissions on the timing of the TCO return date is identified, with longer delays in the Southern Hemisphere than the Northern Hemisphere for the same emission. Together, these results indicate that, if rapid action is taken to curb recently identified CFC-11 production, then no significant delay in the timing of the TCO return to the 1960–1980 mean is expected, highlighting the importance of ongoing, long-term measurement efforts to inform the accountability phase of the Montreal Protocol. However, if the emissions are allowed to continue into the future and are associated with the creation of large banks, then significant delays in the timing of the TCO return date may occur. [ABSTRACT FROM AUTHOR]

We study the evolution of tropospheric ozone over the period 1979–2010 using a chemistry‐climate model employing a stratosphere‐troposphere chemistry scheme. By running with specified dynamics, the key feedback of composition on meteorology is suppressed, isolating the chemical response. By using historical forcings and emissions, interactions between processes are realistically represented. We use the model to assess how the ozone responds over time and to investigate model responses and trends. We find that the chlorofluorocarbon (CFC)‐driven decrease in stratospheric ozone plays a significant role in the tropospheric ozone burden. Over the period 1979–1994, the decline in transport of ozone from the stratosphere, partially offsets an emissions‐driven increase in tropospheric ozone production. From 1994–2010, despite a leveling off in emissions, increased stratosphere‐to‐troposphere transport of ozone drives a small increase in the tropospheric ozone burden. These results have implications for the impact of future stratospheric ozone recovery on air quality and radiative forcing. Plain Language Summary: We use a modeling approach to study the effect of stratospheric ozone depletion on the composition of the troposphere. We focus on the period 1979–2010 and use a chemistry‐climate model employing historical emissions, climate forcing, and meteorology. Our model has a good description of both stratospheric and tropospheric ozone chemistry and allows us to calculate the effect of exchange between stratosphere and troposphere. We show that stratospheric ozone depletion over the period 1979–2010 has a critical effect on tropospheric composition – with less ozone in the lower stratosphere, there is less transport to the troposphere, and this offsets an emissions‐driven increase in ozone production in the troposphere. Such combined studies are important to quantify the future effects of stratospheric ozone recovery on the evolution of tropospheric composition. Key Points: We show that stratospheric ozone recovery has an affect on tropospheric ozone levelsStratospheric ozone loss over the period 1979–1994 offset an emissions‐driven increase in tropospheric ozoneFuture stratospheric ozone recovery and changes to the rate of stratosphere‐to‐troposphere transport will be important in the future [ABSTRACT FROM AUTHOR]

Here we present a description of the UKCA StratTrop chemical mechanism, which is used in the UKESM1 Earth system model for CMIP6. The StratTrop chemical mechanism is a merger of previously well-evaluated tropospheric and stratospheric mechanisms, and we provide results from a series of bespoke integrations to assess the overall performance of the model. We find that the StratTrop scheme performs well when compared to a wide array of observations. The analysis we present here focuses on key components of atmospheric composition, namely the performance of the model to simulate ozone in the stratosphere and troposphere and constituents that are important for ozone in these regions. We find that the results obtained for tropospheric ozone and its budget terms from the use of the StratTrop mechanism are sensitive to the host model; simulations with the same chemical mechanism run in an earlier version of the MetUM host model show a range of sensitivity to emissions that the current model does not fall within. Whilst the general model performance is suitable for use in the UKESM1 CMIP6 integrations, we note some shortcomings in the scheme that future targeted studies will address. [ABSTRACT FROM AUTHOR]

We analyse simulations performed for the Chemistry-Climate Model Initiative (CCMI) to estimate the return dates of the stratospheric ozone layer from depletion caused by anthropogenic stratospheric chlorine and bromine. We consider a total of 155 simulations from 20 models, including a range of sensitivity studies which examine the impact of climate change on ozone recovery. For the control simulations (unconstrained by nudging towards analysed meteorology) there is a large spread (±20 DU in the global average) in the predictions of the absolute ozone column. Therefore, the model results need to be adjusted for biases against historical data. Also, the interannual variability in the model results need to be smoothed in order to provide a reasonably narrow estimate of the range of ozone return dates. Consistent with previous studies, but here for a Representative Concentration Pathway (RCP) of 6.0, these new CCMI simulations project that global total column ozone will return to 1980 values in 2049 (with a 1σ uncertainty of 2043–2055). At Southern Hemisphere mid-latitudes column ozone is projected to return to 1980 values in 2045 (2039–2050), and at Northern Hemisphere mid-latitudes in 2032 (2020–2044). In the polar regions, the return dates are 2060 (2055–2066) in the Antarctic in October and 2034 (2025–2043) in the Arctic in March. The earlier return dates in the Northern Hemisphere reflect the larger sensitivity to dynamical changes. Our estimates of return dates are later than those presented in the 2014 Ozone Assessment by approximately 5–17 years, depending on the region, with the previous best estimates often falling outside of our uncertainty range. In the tropics only around half the models predict a return of ozone to 1980 values, around 2040, while the other half do not reach the 1980 value. All models show a negative trend in tropical total column ozone towards the end of the 21st century. The CCMI models generally agree in their simulation of the time evolution of stratospheric chlorine and bromine, which are the main drivers of ozone loss and recovery. However, there are a few outliers which show that the multi-model mean results for ozone recovery are not as tightly constrained as possible. Throughout the stratosphere the spread of ozone return dates to 1980 values between models tends to correlate with the spread of the return of inorganic chlorine to 1980 values. In the upper stratosphere, greenhouse gas-induced cooling speeds up the return by about 10–20 years. In the lower stratosphere, and for the column, there is a more direct link in the timing of the return dates of ozone and chlorine, especially for the large Antarctic depletion. Comparisons of total column ozone between the models is affected by different predictions of the evolution of tropospheric ozone within the same scenario, presumably due to differing treatment of tropospheric chemistry. Therefore, for many scenarios, clear conclusions can only be drawn for stratospheric ozone columns rather than the total column. As noted by previous studies, the timing of ozone recovery is affected by the evolution of N2O and CH4. However, quantifying the effect in the simulations analysed here is limited by the few realisations available for these experiments compared to internal model variability. The large increase in N2O given in RCP 6.0 extends the ozone return globally by ∼ 15 years relative to N2O fixed at 1960 abundances, mainly because it allows tropical column ozone to be depleted. The effect in extratropical latitudes is much smaller. The large increase in CH4 given in the RCP 8.5 scenario compared to RCP 6.0 also lengthens ozone return by ∼ 15 years, again mainly through its impact in the tropics. Overall, our estimates of ozone return dates are uncertain due to both uncertainties in future scenarios, in particular those of greenhouse gases, and uncertainties in models. The scenario uncertainty is small in the short term but increases with time, and becomes large by the end of the century. There are still some model–model differences related to well-known processes which affect ozone recovery. Efforts need to continue to ensure that models used for assessment purposes accurately represent stratospheric chemistry and the prescribed scenarios of ozone-depleting substances, and only those models are used to calculate return dates. For future assessments of single forcing or combined effects of CO2, CH4, and N2O on the stratospheric column ozone return dates, this work suggests that it is more important to have multi-member (at least three) ensembles for each scenario from every established participating model, rather than a large number of individual models. [ABSTRACT FROM AUTHOR]

We present an overview of state-of-the-art chemistry-climate and chemistry transport models that are used within phase 1 of the Chemistry-Climate Model Initiative (CCMI-1). The CCMI aims to conduct a detailed evaluation of participating models using process-oriented diagnostics derived from observations in order to gain confidence in the models' projections of the stratospheric ozone layer, tropospheric composition, air quality, where applicable global climate change, and the interactions between them. Interpretation of these diagnostics requires detailed knowledge of the radiative, chemical, dynamical, and physical processes incorporated in the models. Also an understanding of the degree to which CCMI-1 recommendations for simulations have been followed is necessary to understand model responses to anthropogenic and natural forcing and also to explain intermodel differences. This becomes even more important given the ongoing development and the ever-growing complexity of these models. This paper also provides an overview of the available CCMI-1 simulations with the aim of informing CCMI data users. [ABSTRACT FROM AUTHOR]

Alkyl nitrates (RONO2) are important reservoirs of tropospheric reactive nitrogen (NOx).They are produced from the oxidation of hydrocarbons in the presence of NOx and emittedfrom the ocean and biomass burning. Due to their relatively long lifetime they can bedestroyed far away from their sources and release NO2 back to the local atmosphere. Thatmight change ozone concentrations on regional levels and alter the oxidising capacity of theatmosphere. To investigate the impact of the most abundant, C1-C3 RONO2 on troposphericchemistry, we analysed the differences in HOx, NOx and NOy burden and distribution inlong-term perpetual year global chemistry climate model (UKCA) runs with andwithout RONO2. First, we updated the reaction rate coefficients of the inorganicand C1-C3 alkane (RH) chemistry of UKCA’s standard tropospheric chemistrymechanism. This was done to keep the model inline with the latest chemical kineticsrecommendations. Then we extended the mechanism to include C2-C3 RONO2photochemical production and loss. We tested the new mechanism in a steady state boxmodel in a range of NOx-RH conditions using the Master Chemical Mechanism as abenchmark. Finally, we implemented the new mechanism into UKCA and validated it with avariety of aircraft observations. It appears that UKCA (a) overestimates C1 RONO2during NH summer outside equatorial region due to a low C1 RONO2 loss and (b)underestimates C1 RONO2 within the equatorial region because of a lack of an oceanicsource. To explore the sensitivity of the model to RONO2 dry deposition and oceanicemissions and to calculate the contribution of these processes to the global RONO2budget, we ran the model with and without RONO2 dry deposition and C1 and C2RONO2 oceanic emissions. The latter were implemented using data from Fisher et al.(2018). Fisher, J. A., Atlas, E. L., Barletta, B., Meinardi, S., Blake, D. R., Thompson, C. R.,Ryerson, T. B., Peischl, J., Tzompa-Sosa, Z. A., and Murray, L. T. (2018). Methyl, ethyl,and propyl nitrates: global distribution and impacts on reactive nitrogen in remotemarine environments. Journal of Geophysical Research: Atmospheres, (x):1–23. [ABSTRACT FROM AUTHOR]