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The modelling of BVOC chemistry strongly affects how doubling of BVOC emissions affects climate. Lower oxidant depletion with state-of-science chemistry leads to 43% smaller positive forcing from smaller methane increases and cloud albedo decreases.

Abstract Near‐term climate forcers (NTCFs) can influence climate via interaction with the Earth's radiative balance and include both aerosols and trace gas constituents of the atmosphere (such as methane and ozone). Two of the principal NTCFs, aerosols (particulate matter) and tropospheric ozone (O3), can also affect local air quality when present in the lower levels of the atmosphere. Previous studies have shown that mitigation of NTCFs has the potential to improve air quality and reduce the rate of surface warming induced by long‐lived greenhouse gases. Here, we assess the combined air quality and climate impacts from changes in NTCFs under numerous different future mitigation scenarios, relative to a future reference scenario, that were conducted by a single Earth system model (UKESM1) as part of the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). Co‐benefits to both global air quality and climate are only achieved in the future scenario with strong mitigation measures applied to all NTCFs, particularly aerosols and methane, with penalties identified for inaction. When compared to the combined NTCF mitigation scenario, analysis of individual mitigation scenarios shows that there are important non‐linearities and interactions between NTCFs (e.g., aerosols and clouds). If only aerosol components are mitigated, there are still benefits to air quality but detrimental impacts on climate, particularly at the regional scale. In addition, other changes in future land‐use and climate could have important impacts on regional NTCFs, which should be considered when designing future mitigation measures to anthropogenic emissions.

Abstract We document the development of the first version of the U.K. Earth System Model UKESM1. The model represents a major advance on its predecessor HadGEM2‐ES, with enhancements to all component models and new feedback mechanisms. These include a new core physical model with a well‐resolved stratosphere; terrestrial biogeochemistry with coupled carbon and nitrogen cycles and enhanced land management; tropospheric‐stratospheric chemistry allowing the holistic simulation of radiative forcing from ozone, methane, and nitrous oxide; two‐moment, five‐species, modal aerosol; and ocean biogeochemistry with two‐way coupling to the carbon cycle and atmospheric aerosols. The complexity of coupling between the ocean, land, and atmosphere physical climate and biogeochemical cycles in UKESM1 is unprecedented for an Earth system model. We describe in detail the process by which the coupled model was developed and tuned to achieve acceptable performance in key physical and Earth system quantities and discuss the challenges involved in mitigating biases in a model with complex connections between its components. Overall, the model performs well, with a stable pre‐industrial state and good agreement with observations in the latter period of its historical simulations. However, global mean surface temperature exhibits stronger‐than‐observed cooling from 1950 to 1970, followed by rapid warming from 1980 to 2014. Metrics from idealized simulations show a high climate sensitivity relative to previous generations of models: Equilibrium climate sensitivity is 5.4 K, transient climate response ranges from 2.68 to 2.85 K, and transient climate response to cumulative emissions is 2.49 to 2.66 K TtC−1.

Abstract We describe the scientific and technical implementation of two models for a core set of experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The models used are the physical atmosphere‐land‐ocean‐sea ice model HadGEM3‐GC3.1 and the Earth system model UKESM1 which adds a carbon‐nitrogen cycle and atmospheric chemistry to HadGEM3‐GC3.1. The model results are constrained by the external boundary conditions (forcing data) and initial conditions. We outline the scientific rationale and assumptions made in specifying these. Notable details of the implementation include an ozone redistribution scheme for prescribed ozone simulations (HadGEM3‐GC3.1) to avoid inconsistencies with the model's thermal tropopause, and land use change in dynamic vegetation simulations (UKESM1) whose influence will be subject to potential biases in the simulation of background natural vegetation. We discuss the implications of these decisions for interpretation of the simulation results. These simulations are expensive in terms of human and CPU resources and will underpin many further experiments; we describe some of the technical steps taken to ensure their scientific robustness and reproducibility.

The configuration of alkyl substituents about carbon–carbon unsaturated bonds exerts a controlling influence on the rate of the ozonolysis reaction. Alkyl substituents can increase (via the inductive effect) and decrease (via the steric effect) the activity of unsaturated bonds, and an accurate description of this information ought to correlate with the ozonolysis rate coefficient. A strong linear relationship is observed (R2 = 0.94), providing the basis of our SAR method. SAR estimates were tested against literature measurements of ozonolysis rate coefficients for 48 aliphatic alkenes and dialkenes, and were found to be accurate to within a factor of 2.3 of the measured value for the entire dataset. This represents a significant improvement over methods reported in the literature, where quoted predictions are at best accurate to within a factor of 6.5. Rates of gas-phase ozonolysis of alkenes and dialkenes can now be predicted with unprecedented accuracy using a simple SAR. The SAR was then validated against new experimental data. Absolute rate coefficients for the gas-phase reaction of ozone with a series of alkenes were determined in a simulation chamber at 295 ± 2 K and atmospheric pressure by monitoring the loss of ozone in the presence of excess alkene. The rate coefficients (in units of 1 × 10−18 cm3 molecule−1 s−1) are: 5.12 ± 0.93 for 1-pentene, 2,3-dimethyl; 406 ± 49 for 2-pentene, 2-methyl; 151 ± 5 for (E)-2-hexene, 14.5 ± 1.0 for 1,5-hexadiene and 20.7 ± 3.1 for 1,5-hexadiene, 2-methyl. There is good agreement between the experimental and predicted values and the adjustable parameters of the SAR are shown to be insensitive to the inclusion of the new data. The use of the SAR in atmospheric chemical modelling is investigated. Ozonolysis and OH radical rate coefficients are estimated for each alkene and dialkene present in the MCM v3.1. The effects of error within predicted rate coefficients upon modelled concentrations of a number of key species, including O3, OH, HO2, NO and NO2 were rather small and is not in itself a major cause of uncertainty in modelled concentrations. [ABSTRACT FROM AUTHOR]