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.

Nitrate aerosol plays an important role in affecting regional air quality as well as Earth’s climate. However, it is not well represented or even neglected in many global climate models. In this study, we couple the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) module with the four-mode version of the Modal Aerosol Module (MAM4) in DOE’s Energy Exascale Earth System Model version 2 (E3SMv2) to treat nitrate aerosol and its radiative effects. We find that nitrate aerosol simulated by E3SMv2-MAM4-MOSAIC is sensitive to the treatment of gaseous HNO3 transfer to/from interstitial particles related to accommodation coefficients of HNO3 (αHNO3) on dust and non-dust particles. We compare three different treatments of HNO3 transfer: 1) a treatment (MTC_SLOW) that uses a low αHNO3 in the mass transfer coefficient (MTC) calculation; 2) a dust-weighted MTC treatment (MTC_WGT) that uses a high αHNO3 on non-dust particles; and 3) a dust-weighted MTC treatment that also splits coarse mode aerosols into the coarse dust and sea salt sub-modes in MOSAIC (MTC_SPLC). MTC_WGT and MTC_SPLC increase the global annual mean (2005-2014) nitrate burden from 0.096 (MTC_SLOW) to 0.237 and 0.185 Tg N, respectively, mostly in the coarse mode. They also produce stronger nitrate direct radiative forcing (–0.048 and –0.051 W m–2, respectively) and indirect forcing (–0.33 and –0.35 W m–2, respectively) than MTC_SLOW (–0.021 and –0.24 W m–2). All three treatments overestimate nitrate surface concentrations compared with ground-based observations. MTC_WGT and MTC_SPLC improve the vertical profiles of nitrate concentrations against aircraft measurements below 400 hPa.

Geoengineering, the deliberate modification of the climate system, is a proposed set of techniques to counter some of the effects of Anthropogenic Global Warming (AGW; Shepherd, 2009). Geoengineering includes Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM; Council, 2015). Global modelling studies of geoengineering have typically considered idealized scenarios, to understand the physical processes of interventions and their general impacts. These scenarios are not necessarily policy-relevant, and are often physically implausible (such as instantaneous quadrupling of CO2). The climatic and ecological impacts of politically-relevant and potentially plausible solutions have rarely been modeled and assessed. Nevertheless, commentators and policymakers often falsely assume that idealized or extreme scenarios are proposed as solutions to the AGW problem. This paper discusses various scenarios, which appear to be broadly plausible from both a political and Earth-system standpoint. These fall into the following categories: 1) Well-designed intervention strategies, including combined approaches, intended to meet specific climate targets 2) Global emergency response strategies, in case of unexpected natural or social events 3) Regional intervention strategies (with potential global consequences) 4) Miscellaneous and long-term intervention strategies We propose relevant, non-prioritized model experiments, corresponding to these scenarios. Some may be performed with existing setups of global climate models; others need to be first designed in detail. Developing and running these experiments, and assessing likely resulting impacts on society and ecosystems, would help to inform the public debate on the real-world feasibility of CDR and SRM geoengineering.

Wildfire emissions can vary substantially between different inventories due to the assumptions made in the emission creation process, including the defined vegetation type, fire detection, fuel loading, fraction of vegetation burned and emissions factors. Here, we focus on the uncertainty in emission factors and the resulting impact on modeled composition. We use the Community Atmosphere Model with chemistry (CAM-chem) to simulate 2014 atmospheric composition and focus on carbon monoxide (CO), a trace gas emitted from incomplete combustion and also produced from secondary oxidation of volatile organic compounds (VOCs). Fire is a major source of atmospheric CO and VOCs. Multiple simulations are compared, from an ensemble using four fire emission inventories (CMIP6/GFED4s, QFED2.5, GFAS1.2 and FINN1.5) and a range of sensitivity tests based on CO and VOC emission factor uncertainties. We compare model output and evaluate against CO observations from the Measurements of Pollution in the Troposphere (MOPITT) satellite-based instrument. For some regions, emission factor uncertainty spans the results found by using different inventories. Finally, we use modeled ozone (O3) to investigate how emission factor uncertainty influences the atmospheric oxidative environment. Overall, accounting for emission factor uncertainty lends a range of uncertainty to simulated results.