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Solar radiation management as a sustained deliberate source of SO2 into the stratosphere (strat-SRM) has been proposed as an option for climate intervention. Global interactive aerosol-chemistry-climate models are often used to investigate the potential cooling efficiencies and side effects of hypothesised strat-SRM scenarios. A recent strat-SRM model intercomparison study for composition-climate models with interactive stratospheric aerosol suggests that the modelled climate response to a particular assumed injection strategy, depends on the type of aerosol microphysical scheme used (e.g., modal or sectional representation), alongside also host model resolution and transport. Compared to short-duration volcanic SO2 emission, the continuous SO2 injections in strat-SRM scenarios may pose a greater challenge to the numerical implementation of of microphysical processes such as nucleation, condensation, and coagulation. This study explores how changing the timesteps and sequencing of microphysical processes in the sectional aerosol-chemistry-climate model SOCOL-AERv2 (40 size bins) affect model predicted climate and ozone layer impacts considering strat-SRM SO2 injections of of 5 and 25 Tg(S) yr-1 at 20 km altitude between 30° S and 30° N. The model experiments consider year 2040 boundary conditions for ozone depleting substances and green house gases. We focus on the length of the microphysical timestep and the call sequence of nucleation and condensation, the two competing sink processes for gaseous H2SO4. Under stratospheric background conditions, we find no effect of the microphysical setup on the simulated aerosol properties. However, at the high sulfur loadings reached in the scenarios injecting 25 Mt/yr of sulfur with a default microphysical timesetp of 6 min, changing the call sequence from the default 'condensation first' to 'nucleation first' leads to a massive increase in the number densities of particles in the nucleation mode (R < 0.01 μm) and a small decrease in coarse mode particles (R > 1 μm). As expected, the influence of the call sequence becomes negligible when the microphysical timestep is reduced to a few seconds, with the model solutions converging to a size distribution with a pronounced nucleation mode. While the main features and spatial patterns of climate forcing by SO2 injections are not strongly affected by the microphysical configuration, the absolute numbers vary considerably. For the extreme injection with 25 Tg(S) yr-1, the simulated net global radiative forcing ranges from -2.3 W m-2 to -5.3 W m-2, depending on the microphysical configuration. "Nucleation first" shifts the size distribution towards radii better suited for solar scattering (0.3 μm < R < 0.4 μm), enhancing the intervention efficiency. The size-distribution shift however generates more ultra-fine aerosol particles, increasing the surface area density, resulting in 10 DU less ozone (about 3 % of total column) in the northern midlatitudes and 20 DU less ozone (6 %) over the polar caps, compared to the 'condensation first' approach. Our results suggest that a reasonably short microphysical time step of 2 minutes or less must be applied to accurately capture the magnitude of the H2SO2 supersaturation resulting from SO2 injection scenarios or volcanic eruptions. Taken together these results underscore how structural aspects of model representation of aerosol microphysical processes become important under conditions of elevated stratospheric sulfur in determining atmospheric chemistry and climate impacts. [ABSTRACT FROM AUTHOR]

The global warming potential of Ozone‐depleting substances (ODS) has long been known to be thousands of times larger than the one of CO2, but their climate impacts as greenhouse gases, i.e. unmediated ozone depletion, has received relatively little attention. Focusing on the period 1955–2005, we here present results from offline radiative forcing (RF) calculations from a global chemistry climate model. Using realistic distributions of ODS and consistent stratospheric ozone, we show that ODS dominate the adjusted stratospheric warming of the lower stratosphere, where CO2 has little radiative impact. We also show that the global mean RF of stratospheric ozone only cancels a fraction of the RF of ODS, leaving an important ODS contribution to anthropogenic forcing. Finally we show that the RF of ODS opposes Arctic amplification, its equator‐to‐pole gradient being larger than the one of CO2. Plain Language Summary: In the decades following World War II, organic compounds of chlorine and bromine, widely used in refrigerators and spray cans, entered the atmosphere and led to the formation of the ozone hole. However, these compounds, are also powerful greenhouse gases and are important for global warming. Here we study their warming effects in the stratosphere and at the surface. We find that, over the period 1955–2005, they were the dominant atmospheric gas warming the tropical lower stratosphere, where CO2 is largely inactive; that their surface warming effect (their radiative forcing) amounted to approximately one third of the CO2 value, even after accounting for reductions due to the ozone depletion they induce; and, finally that, even more than CO2, their radiative effects warmed the equatorial more the polar regions and, as such, cannot be the reason why the Arctic is warming more than the rest of the planet. Key Points: The radiative effect of Ozone‐depleting substances on lower stratospheric temperatures is the largest of all well‐mixed greenhouse gasesTheir radiative forcing is only partially (∼25%) canceled by the stratospheric ozone depletion they induceTheir radiative forcing is weaker at the pole than at the equator opposing Arctic amplification even more than CO2 forcing [ABSTRACT FROM AUTHOR]

Studies of stratospheric solar geoengineering have tended to focus on modification of the sulfuric acid aerosol layer, and almost all climate model experiments that mechanistically increase the sulfuric acid aerosol burden assume injection of SO 2. A key finding from these model studies is that the radiative forcing would increase sublinearly with increasing SO 2 injection because most of the added sulfur increases the mass of existing particles, resulting in shorter aerosol residence times and aerosols that are above the optimal size for scattering. Injection of SO 3 or H 2 SO 4 from an aircraft in stratospheric flight is expected to produce particles predominantly in the accumulation-mode size range following microphysical processing within an expanding plume, and such injection may result in a smaller average stratospheric particle size, allowing a given injection of sulfur to produce more radiative forcing. We report the first multi-model intercomparison to evaluate this approach, which we label AM-H 2 SO 4 injection. A coordinated multi-model experiment designed to represent this SO 3 - or H 2 SO 4 -driven geoengineering scenario was carried out with three interactive stratospheric aerosol microphysics models: the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM2) with the Whole Atmosphere Community Climate Model (WACCM) atmospheric configuration, the Max-Planck Institute's middle atmosphere version of ECHAM5 with the HAM microphysical module (MAECHAM5-HAM) and ETH's SOlar Climate Ozone Links with AER microphysics (SOCOL-AER) coordinated as a test-bed experiment within the Geoengineering Model Intercomparison Project (GeoMIP). The intercomparison explores how the injection of new accumulation-mode particles changes the large-scale particle size distribution and thus the overall radiative and dynamical response to stratospheric sulfur injection. Each model used the same injection scenarios testing AM-H 2 SO 4 and SO 2 injections at 5 and 25 Tg(S) yr -1 to test linearity and climate response sensitivity. All three models find that AM-H 2 SO 4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injected, relative to SO 2 injection. Increased radiative efficacy means that when compared to the use of SO 2 to produce the same radiative forcing, AM-H 2 SO 4 emissions would reduce side effects of sulfuric acid aerosol geoengineering that are proportional to mass burden. The model studies were carried out with two different idealized geographical distributions of injection mass representing deployment scenarios with different objectives, one designed to force mainly the midlatitudes by injecting into two grid points at 30 ∘ N and 30 ∘ S, and the other designed to maximize aerosol residence time by injecting uniformly in the region between 30 ∘ S and 30 ∘ N. Analysis of aerosol size distributions in the perturbed stratosphere of the models shows that particle sizes evolve differently in response to concentrated versus dispersed injections depending on the form of the injected sulfur (SO 2 gas or AM-H 2 SO 4 particulate) and suggests that prior model results for concentrated injection of SO 2 may be strongly dependent on model resolution. Differences among models arise from differences in aerosol formulation and differences in model dynamics, factors whose interplay cannot be easily untangled by this intercomparison. [ABSTRACT FROM AUTHOR]

Arctic amplification (AA) of surface warming is a prominent feature of anthropogenic climate change with important implications for human and natural systems. Despite its importance, the underlying causes of AA are not fully understood. Here, analyzing coupled climate model simulations, we show that AA develops rapidly (within the first few months) following an instantaneous quadrupling of atmospheric CO2. This rapid AA response—which occurs before any significant loss of Arctic sea ice—is produced by a positive lapse rate feedback over the Arctic. Sea ice loss is therefore not needed to produce polar‐amplified warming, although it contributes significantly to this warming after the first few months. Our results provide new and compelling evidence that AA owes its existence, fundamentally, to fast atmospheric processes. Plain Language Summary: Climate warming is greater in the Arctic than at lower latitudes, a phenomenon known as Arctic amplification. Despite its importance for humans and ecosystems, the causes of Arctic amplification are not fully understood. Sea ice loss has long been thought to be a primary cause. However, we show here that Arctic amplification happens faster than sea ice loss in climate models when atmospheric CO2 is increased. This indicates that atmospheric processes alone are capable of causing Arctic amplification. Key Points: Arctic amplification develops rapidly (within a few months) in climate models forced with abrupt CO2 quadruplingThis rapid response is produced by a positive lapse rate feedback over the ArcticThe response occurs before Arctic sea ice loss becomes significant [ABSTRACT FROM AUTHOR]

The quantification of the climate impacts exerted by stratospheric ozone changes in abrupt 4 × CO2 forcing experiments is an important step in assessing the role of the ozone layer in the climate system. Here, we build on our previous work on the change of the ozone layer under 4 × CO2 and examine the effects of ozone changes on the climate response to 4 × CO2, using the Whole Atmosphere Community Climate Model. We show that the global-mean radiative perturbation induced by the ozone changes under 4 × CO2 is small, due to nearly total cancellation between high and low latitudes, and between longwave and shortwave fluxes. Consistent with the small global-mean radiative perturbation, the effect of ozone changes on the global-mean surface temperature response to 4 × CO2 is negligible. However, changes in the ozone layer due to 4 × CO2 have a considerable impact on the tropospheric circulation. During boreal winter, we find significant ozone-induced tropospheric circulation responses in both hemispheres. In particular, ozone changes cause an equatorward shift of the North Atlantic jet, cooling over Eurasia, and drying over northern Europe. The ozone signals generally oppose the direct effects of increased CO2 levels and are robust across the range of ozone changes imposed in this study. Our results demonstrate that stratospheric ozone changes play a considerable role in shaping the atmospheric circulation response to CO2 forcing in both hemispheres and should be accounted for in climate sensitivity studies. [ABSTRACT FROM AUTHOR]