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Sulphate aerosol in the stratosphere recently became an interactive part of many global climate models and its uncertainties are not yet well constrained. Stratospheric sulphate aerosol is modulated by natural emissions of several sulphur-containing species, including volcanic eruptions, as well as anthropogenic emissions. If not directly injected into the stratosphere by large volcanic eruptions, sulphate aerosols and their precursors are transported into the stratosphere via the tropical tropopause. While there have been some model intercomparison activity for the large volcanic eruptions, the background conditions of sulphur species and in particular the stratospheric aerosol layer have thus far not been addressed at all. Evaluating the background conditions in global models allow to identify modelling issues that are usually masked by larger perturbations such as volcanic eruptions, yet may still be of importance after such a perturbation. Some key factors causing differences between models include different microphysical schemes, chemical schemes, as well as transport and cross-tropopause fluxes. In this work, we use 8 models and available observational data to quantify the full background atmospheric sulphur cycle (burdens, fluxes, and their variability) and investigate its uncertainties within the framework of the Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP). We focus on stratospheric aerosol and its transport with the Brewer-Dobson-Circulation, as well as the influence of the polar vortices. We find significant inter-model variations in the background burden of the major sulphur species. Seasonal cycles agree well in the southern hemisphere, whereas the northern hemisphere shows more inter-model differences due to the individual representations of the northern polar vortex.

The evolution of the size distribution of stratospheric aerosols after volcanic eruptions is still not understood very well, due to the temporal sparsity of in situ measurements, the low spatial coverage by ground based observations and the difficulties to derive aerosol size information from satellite measurements. To contribute to this ongoing research, we show data from our aerosol size retrieval using SAGE III/ISS solar occultation measurements. Using a three wavelength extinction approach the parameters of assumed to be monomodal lognormal particle size distributions are retrieved.Surprisingly we find that some volcanic eruptions can lead to a decrease in average stratospheric aerosol size, in this case the eruptions of Ambae in 2018, Ulawun in 2019 and La Soufrière in 2021, while other eruptions have a more expected increasing effect on the average particle size, like the 2019 Raikoke eruption. We show how different parameters like the median radius, the absolute mode width and the number density evolve after the mentioned eruptions.Additionally, as a part of our ongoing research to understand the underlying mechanisms controlling the observed aerosol size reduction, we show simulations of the aforementioned volcanic eruptions using the aerosol-climate model MAECHAM5-HAM. Although the initial conditions in the model simulations are different from observations due to missing smaller emissions in the time before the eruptions, a good agreement in the perturbations of the extinction coefficient was achieved.

The stratospheric aerosol layer plays an important role in the radiative balance of earth primarily through scattering of solar radiation. The magnitude of this effect depends critically on the size distribution of the aerosols. The aerosol layer is in large part fed by volcanic eruptions strong enough to inject gaseous sulfur species into the stratosphere. The evolution of the stratospheric aerosol size after volcanic eruptions is currently one of the biggest uncertainties in stratospheric aerosol science. We retrieved aerosol particle size information from satellite solar occultation measurements from the Stratospheric Aerosol and Gas Experiment III mounted on the International Space Station (SAGE III/ISS) using a robust spectral method. We show that, surprisingly, some volcanic eruptions can lead to a decrease in average aerosol size, like the 2018 Ambae and the 2021 La Soufrière eruptions. In 2019 an intriguing contrast is observed, where the Raikoke eruption (48° N, 153° E) in 2019 led to the more expected stratospheric aerosol size increase, while the Ulawun eruptions (5° S, 151° E), which followed shortly after, again resulted in a reduction of the median radius and absolute mode width values in the lowermost stratosphere. In addition, the Raikoke and Ulawun eruptions were simulated with the aerosol climate model MAECHAM5-HAM. In these model runs, the evolution of the extinction coefficient as well as of the effective radius could be reproduced well for the first 3 months of volcanic activity. However, the long lifetime of the very small aerosol sizes of many months observed in the satellite retrieval data could not be reproduced.

This study assesses the impacts of sulfate aerosol intervention (SAI) and solar dimming on stratospheric ozone based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) experiments, called G6sulfur and G6solar. For G6sulfur the stratospheric sulfate aerosol burden is increased to reflect some of the incoming solar radiation back into space in order to cool the surface climate, while for G6solar the global solar constant is reduced to achieve the same goal. The high emissions scenario SSP5-8.5 is used as the baseline experiment and surface temperature from the medium emission scenario SSP2-4.5 is the target. Based on three out of six Earth System Models (ESMs) that include interactive stratospheric chemistry, we find significant differences in the ozone distribution between G6solar and G6sulfur experiments compared to SSP5-8.5 and SSP2-4.5, which differ by both region and season. Both SAI and solar dimming methods reduce incoming solar insolation and result in tropospheric temperatures comparable to SSP2-4.5 conditions. G6sulfur increases the concentration of absorbing sulfate aerosols in the stratosphere, which increases lower tropical stratospheric temperatures by between 5 to 13 K for six different ESMs, leading to changes in stratospheric transport. The increase of the aerosol burden also increases aerosol surface area density, which is important for heterogeneous chemical reactions. The resulting changes in ozone include a significant reduction of total column ozone (TCO) in the Southern Hemisphere polar region in October of 10 DU at the onset and up to 20 DU by the end of the century. The relatively small reduction in TCO for the multi-model mean in the first two decades results from variations in the required sulfur injections in the models and differences in the complexity of the chemistry schemes, with no significant ozone loss for 2 out of 3 models. The decrease in the second half of the 21st century counters increasing TCO between SSP2-4.5 and SSP5-8.5 due to the super-recovery resulting from increasing greenhouse gases. In contrast, in the Northern Hemisphere (NH) high latitudes, only a small initial decline in TCO is simulated, with little change in TCO by the end of the century compared to SSP5-8.5. All models consistently simulate an increase in TCO in the NH mid-latitudes up to 20 DU compared to SSP5-8.5, in addition to 20 DU increase resulting from increasing greenhouse gases between SSP2-4.5 and SSP5-8.5. G6solar counters zonal wind and tropical upwelling changes between SSP2-4.5 and SSP5-8.5 but does not change stratospheric temperatures. Solar dimming results in little change in TCO compared to SSP5-8.5 and does not counter the effects of the ozone super-recovery. Only in the tropics, G6solar results in an increase of TCO of up to 8 DU compared to SSP2-4.5, which may counter the projected reduction due to climate change in the high forcing future scenario. This work identifies differences in the response of SAI and solar dimming on ozone, which are at least partly due to differences and shortcomings in the complexity of aerosol microphysics, chemistry, and the description of ozone photolysis in the models. It also identifies that solar dimming, if viewed as an analog to SAI using a predominantly scattering aerosol, would, for the most part, not counter the potential harmful increase in TCO beyond historical values induced by increasing greenhouse gases.

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 SO2. A key finding from these model studies is that the radiative forcing would increase sublinearly with increasing SO2 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 SO3 or H2SO4 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-H2SO4 injection. A coordinated multi-model experiment designed to represent this SO3- or H2SO4-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-H2SO4 and SO2 injections at 5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injected, relative to SO2 injection. Increased radiative efficacy means that when compared to the use of SO2 to produce the same radiative forcing, AM-H2SO4 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 (SO2 gas or AM-H2SO4 particulate) and suggests that prior model results for concentrated injection of SO2 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.

Injecting sulfur dioxide into the stratosphere with the intent to create an artificial reflective aerosol layer is one of the most studied options for solar radiation management. Previous modelling studies have shown that stratospheric sulfur injections have the potential to compensate for the greenhouse-gas-induced warming at the global scale. However, there is significant diversity in the modelled radiative forcing from stratospheric aerosols depending on the model and on which strategy is used to inject sulfur into the stratosphere. Until now, it has not been clear how the evolution of the aerosols and their resulting radiative forcing depends on the aerosol microphysical scheme used – that is, if aerosols are represented by a modal or sectional distribution. Here, we have studied different spatio-temporal injection strategies with different injection magnitudes using the aerosol–climate model ECHAM-HAMMOZ with two aerosol microphysical modules: the sectional module SALSA (Sectional Aerosol module for Large Scale Applications) and the modal module M7. We found significant differences in the model responses depending on the aerosol microphysical module used. In a case where SO2 was injected continuously in the equatorial stratosphere, simulations with SALSA produced an 88 %–154 % higher all-sky net radiative forcing than simulations with M7 for injection rates from 1 to 100 Tg (S) yr−1. These large differences are identified to be caused by two main factors. First, the competition between nucleation and condensation: while injected sulfur tends to produce new particles at the expense of gaseous sulfuric acid condensing on pre-existing particles in the SALSA module, most of the gaseous sulfuric acid partitions to particles via condensation at the expense of new particle formation in the M7 module. Thus, the effective radii of stratospheric aerosols were 10 %–52 % larger in M7 than in SALSA, depending on the injection rate and strategy. Second, the treatment of the modal size distribution in M7 limits the growth of the accumulation mode which results in a local minimum in the aerosol number size distribution between the accumulation and coarse modes. This local minimum is in the size range where the scattering of solar radiation is most efficient. We also found that different spatial-temporal injection strategies have a significant impact on the magnitude and zonal distribution of radiative forcing. Based on simulations with various injection rates using SALSA, the most efficient studied injection strategy produced a 33 %–42 % radiative forcing compared with the least efficient strategy, whereas simulations with M7 showed an even larger difference of 48 %–116 %. Differences in zonal mean radiative forcing were even larger than that. We also show that a consequent stratospheric heating and its impact on the quasi-biennial oscillation depend on both the injection strategy and the aerosol microphysical model. Overall, these results highlight the crucial impact of aerosol microphysics on the physical properties of stratospheric aerosol which, in turn, causes significant uncertainties in estimating the climate impacts of stratospheric sulfur injections.

A previous model intercomparison of the Tambora aerosol cloud has highlighted substantial differences among simulated volcanic aerosol properties in the pre-industrial stratosphere and has led to questions about the applicability of global aerosol models for large-magnitude explosive eruptions prior to the observational period. Here, we compare the evolution of the stratospheric aerosol cloud following the well-observed June 1991 Mt. Pinatubo eruption simulated with six interactive stratospheric aerosol microphysics models to a range of observational data sets. Our primary focus is on the uncertainties regarding initial SO2 emission following the Pinatubo eruption, as prescribed in the Historical Eruptions SO2 Emission Assessment experiments (HErSEA), in the framework of the Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP). Six global models with interactive aerosol microphysics took part in this study: ECHAM6-SALSA, EMAC, ECHAM5-HAM, SOCOL-AERv2, ULAQ-CCM, and UM-UKCA. Model simulations are performed by varying the SO2 injection amount (ranging between 5 and 10TgS) and the altitude of injection (between 18-25km). The comparisons show that all models consistently demonstrate faster reduction from the peak in sulfate mass burden in the tropical stratosphere. Most models also show a stronger transport towards the extratropics in the Northern Hemisphere, at the expense of the observed tropical confinement, suggesting a much weaker subtropical barrier in all the models, which results in a shorter e-folding time compared to the observations. Furthermore, simulations in which more than 5TgS in the form of SO2 is injected show an initial overestimation of the sulfate burden in the tropics and, in some models, in the Northern Hemisphere and a large surface area density a few months after the eruption compared to the values measured in the tropics and the in situ measurements over Laramie. This draws attention to the importance of including processes such as the ash injection for the removal of the initial SO2 and aerosol lofting through local heating., Atmospheric Chemistry and Physics, 23 (2), ISSN:1680-7375, ISSN:1680-7367

The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinating framework, started in 2010, that includes a series of standardized climate model experiments aimed at understanding the physical processes and projected impacts of solar geoengineering. Numerous experiments have been conducted, and numerous more have been proposed as “test-bed” experiments, spanning a variety of geoengineering techniques aimed at modifying the planetary radiation budget: stratospheric aerosol injection, marine cloud brightening, surface albedo modification, cirrus cloud thinning, and sunshade mirrors. To date, more than 100 studies have been published that used results from GeoMIP simulations. Here we provide a critical assessment of GeoMIP and its experiments. We discuss its successes and missed opportunities, for instance in terms of which experiments elicited more interest from the scientific community and which did not, and the potential reasons why that happened. We also discuss the knowledge that GeoMIP has contributed to the field of geoengineering research and climate science as a whole: what have we learned in terms of intermodel differences, robustness of the projected outcomes for specific geoengineering methods, and future areas of model development that would be necessary in the future? We also offer multiple examples of cases where GeoMIP experiments were fundamental for international assessments of climate change. Finally, we provide a series of recommendations, regarding both future experiments and more general activities, with the goal of continuously deepening our understanding of the effects of potential geoengineering approaches and reducing uncertainties in climate outcomes, important for assessing wider impacts on societies and ecosystems. In doing so, we refine the purpose of GeoMIP and outline a series of criteria whereby GeoMIP can best serve its participants, stakeholders, and the broader science community., Atmospheric Chemistry and Physics, 23 (9), ISSN:1680-7375, ISSN:1680-7367

Recent model inter-comparison studies highlighted model discrepancies in reproducing the climatic impacts of large explosive volcanic eruptions, calling into question the reliability of global aerosol model simulations for future scenarios. Here, we analyse the simulated evolution of the stratospheric aerosol plume following the well observed June 1991 Mt. Pinatubo eruption by six interactive stratospheric aerosol microphysics models in comparison to a range of observational data sets. Our primary focus is on the uncertainties regarding initial SO2 emission following the Pinatubo eruption in 1991, as prescribed in the Historical Eruptions SO2 Emission Assessment experiments (HErSEA), in the framework of the model intercomparison project ISA-MIP. Six global models with interactive aerosol microphysics took part in this study: ECHAM6-SALSA, EMAC, ECHAM5-HAM, SOCOL-AERv2, ULAQ-CCM and UM-UKCA. Model simulations are performed by varying SO2 injection amount (ranging between 5 and 10 Tg-S), and the altitude of injection (between 18–25 km). We find that the common and main weakness among all the models is that they can not reproduce the persistence of the sulfate aerosols in the stratosphere. Most models show a stronger transport towards the extratropics in the northern hemisphere, at the expense of the observed tropical confinement, suggesting a much weaker subtropical barrier in all the models, that results in a shorter e-folding time compared to the observations. Moreover, the simulations in which more than 5 Tg-S of SO2 are injected show a large surface area density a few months after the eruption compared to the values measured in the tropics and the in-situ measurements over Laramie. This results in an overestimation of the number of particles globally during the build-up phase, and an underestimation in the Southern Hemisphere, which draws attention to the importance of including processes as the ash injection and the eruption of Cerro Hudson.

We present here results from the Geoengineering Model Intercomparison Project (GeoMIP) simulations for the experiments G6sulfur and G6solar for six Earth system models participating in the Climate Model Intercomparison Project (CMIP) Phase 6. The aim of the experiments is to reduce the warming that results from a high-tier emission scenario (Shared Socioeconomic Pathways SSP5-8.5) to that resulting from a medium-tier emission scenario (SSP2-4.5). These simulations aim to analyze the response of climate models to a reduction in incoming surface radiation as a means to reduce global surface temperatures, and they do so either by simulating a stratospheric sulfate aerosol layer or, in a more idealized way, through a uniform reduction in the solar constant in the model. We find that over the final two decades of this century there are considerable inter-model spreads in the needed injection amounts of sulfate (29 ± 9 Tg-SO2/yr between 2081 and 2100), in the latitudinal distribution of the aerosol cloud and in the stratospheric temperature changes resulting from the added aerosol layer. Even in the simpler G6solar experiment, there is a spread in the needed solar dimming to achieve the same global temperature target (1.91 ± 0.44 %). The analyzed models already show significant differences in the response to the increasing CO2 concentrations for global mean temperatures and global mean precipitation (2.05 K ± 0.42 K and 2.28 ± 0.80 %, respectively, for SSP5-8.5 minus SSP2-4.5 averaged over 2081–2100). With aerosol injection, the differences in how the aerosols spread further change some of the underlying uncertainties, such as the global mean precipitation response (−3.79 ± 0.76 % for G6sulfur compared to −2.07 ± 0.40 % for G6solar against SSP2-4.5 between 2081 and 2100). These differences in the behavior of the aerosols also result in a larger uncertainty in the regional surface temperature response among models in the case of the G6sulfur simulations, suggesting the need to devise various, more specific experiments to single out and resolve particular sources of uncertainty. The spread in the modeled response suggests that a degree of caution is necessary when using these results for assessing specific impacts of geoengineering in various aspects of the Earth system. However, all models agree that compared to a scenario with unmitigated warming, stratospheric aerosol geoengineering has the potential to both globally and locally reduce the increase in surface temperatures.

The northern high-latitude permafrost contains almost twice the carbon content of the atmosphere, and it is widely considered as a non-linear and tipping element in the Earth's climate system under global warming. Solar geoengineering is a means of mitigating temperature rise and reduce some of the associated climate impacts by increasing the planetary albedo, including permafrost thaw. We analyze the permafrost response as simulated by five earth system models (ESMs) under four future scenarios; two solar geoengineering scenarios (G6solar and G6sulfur) restore the global temperature from the high emission scenario (ssp585) levels to the moderate mitigation scenario (ssp245) levels via solar dimming and stratospheric aerosol injection. G6solar and G6sulfur nearly restore the northern high-latitude permafrost area from ssp585 levels to those under ssp245. But deeper active layer thickness and more exposed unfrozen soil organic carbon are produced due to robust residual high-latitude warming, especially over Eurasia. However, G6solar and G6sulfur accumulate more soil carbon over the northern high-latitude permafrost region due to enhanced CO2 fertilization effects relative to ssp245 and weakened heterotrophic respiration relative to ssp585. The asynchronous changes in soil carbon inputs and soil carbon decomposition directly result from decoupling of temperature and atmospheric CO2 concentration under solar geoengineering. The permafrost ecosystem remains a carbon sink throughout this century under all four scenarios, and solar geoengineering can delay the transition of northern high-latitude permafrost ecosystem from carbon sink to carbon source.

As part of the Geoengineering Model Intercomparison Project a numerical experiment known as G6sulfur has been designed in which temperatures under a high-forcing future scenario (SSP5-8.5) are reduced to those under a medium-forcing scenario (SSP2-4.5) using the proposed geoengineering technique of stratospheric aerosol intervention (SAI). G6sulfur involves introducing sulphate aerosol into the tropical stratosphere where it reflects incoming sunlight back to space, thus cooling the planet. Here we compare the results from six Earth-system models which have performed the G6sulfur experiment and examine how SAI affects two important modes of natural variability, the northern wintertime North Atlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO). Although all models show that SAI is successful in reducing global-mean temperature as designed, they are also consistent in showing that it forces an increasingly positive phase of the NAO as the injection rate increases over the course of the 21st century, exacerbating precipitation reductions over parts of southern Europe compared with SSP5-8.5. In contrast to the robust result for the NAO there is less consistency for the impact on the QBO, but the results nevertheless indicate a risk that equatorial SAI could cause the QBO to stall and become locked in a phase with permanent westerly winds in the lower stratosphere.

Artificial injections of sulfur dioxide (SO2) into the stratosphere show in several model studies an impact on stratospheric dynamics. The quasi-biennial oscillation (QBO) has been shown to slow down or even vanish under higher SO2 injections in the equatorial region. But the impact is only qualitatively but not quantitatively consistent across the different studies using different numerical models. The aim of this study is to understand the reasons behind the differences in the QBO response to SO2 injections between two general circulation models, the Whole Atmosphere Community Climate Model (WACCM-110L) and MAECHAM5-HAM. We show that the response of the QBO to injections with the same SO2 injection rate is very different in the two models, but similar when a similar stratospheric heating rate is induced by SO2 injections of different amounts. The reason for the different response of the QBO corresponding to the same injection rate is very different vertical advection in the two models, even in the control simulation. The stronger vertical advection in WACCM results in a higher aerosol burden and stronger heating of the aerosols and, consequently, in a vanishing QBO at lower injection rate than in simulations with MAECHAM5-HAM. The vertical velocity increases slightly in MAECHAM5-HAM when increasing the horizontal resolution. This study highlights the crucial role of dynamical processes and helps to understand the large uncertainties in the response of different models to artificial SO2 injections in climate engineering studies.

Wir stellen den zeitlichen Verlauf von Größenverteilungsparametern stratosphärischer Aerosole im Zeitraum von 2002 bis 2005 basierend auf den Messungen des Stratospheric Aerosol and Gas Experiment (SAGE) III on Mетеор-3M (M3M) vor. Die Okkultationsmessungen von SAGE III-M3M decken in der Nordhemisphäre etwa einen Bereich zwischen 40°N und 80°N und in der Südhemisphäre zwischen 30°S und 60°S ab. Die Retrievalmethode, welche in Wrana et al. (2021, Atm. Meas. Tech.) ausführlich beschrieben wurde, macht sich die spektrale Abhängigkeit der im SAGE III-M3M-Datensatz gelieferten Extinktionskoeffizienten zunutze um den Medianradius und die Verteilungsbreite einer monomodalen Lognormalverteilung zu bestimmen. Durch die Verwendung von drei Wellenlängenkanälen des breiten von SAGE III-M3M abgedeckten spektralen Bereiches ist das gleichzeitige Retrieval beider Parameter möglich. Basierend auf den Ergebnissen wurden weitere Parameter, wie der effektive Radius und die Anzahldichte, bestimmt. Wir zeigen im Vortrag die im Datensatz zu beobachtende jahreszeitliche Variation der Größenverteilungsparameter in der nördlichen und südlichen Hemisphäre. Des Weiteren diskutieren wir das mögliche Auftreten von polaren Stratosphärenwolken (PSC), sowie die Manam-Eruption im Jahr 2004 als mögliche Ursache einer Verringerung der mittleren Größe der stratosphärischen Aerosole in den von SAGE III-M3M beobachteten mittleren Breiten beider Hemisphären in 2005. Zur Validierung zeigen wir außerdem einen Vergleich der Größenverteilungsparameter unseres Datensatzes mit Kollokationen von OPC-Messungen der Universität Wyoming in Kiruna, Schweden.

Analyses of stratospheric solar geoengineering have focused on sulfate aerosol, and almost all climate model experiments on sulfate aerosol have assumed injection of SO2. Yet continuous injection of SO2 may produce overly large aerosols. Injection of SO3 or H2SO4 from an aircraft in stratospheric flight is expected to produce new accumulation-mode particles (AM-H2SO4), and such injection may allow the sulfate aerosol size distribution to be nudged towards higher radiative efficacy. We report the first multi-model intercomparison of AM-H2SO4 injection. We compare three models: CESM2(WACCM), MAECHAM5-HAM, and SOCOL-AER coordinated as a testbed 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 sulfate aerosol injection. Each model used the same injection scenarios testing AM-H2SO4 and SO2 injections at 5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injection, relative to SO2 injection. Increased radiative efficacy means that when compared to the use of SO2 to produce the same radiative forcing, AM-H2SO4 emissions could reduce some side-effects of sulfate aerosol geoengineering such as stratospheric heating. We explore the sensitivity to injection pattern by comparing injection at two points at 30° N and 30° S to injection in a belt along the equator between 30° S and 30° N, and find opposite impacts on radiative efficacy for AM-H2SO4 and SO2, suggesting that prior model results for concentrated injection of SO2 may be strongly dependent on model resolution. Model differences arise from differences in aerosol formulation and differences in model transport and resolution, factors whose interplay cannot be easily untangled by this intercomparison.

A known adverse side effect of stratospheric aerosol modification (SAM) is the modification of the quasi-biennial oscillation (QBO), which is caused by the stratospheric heating associated with an artificial aerosol layer. Multiple studies found the QBO to slow down or even completely vanish for point-like injections of SO2 at the equator. The cause for this was found to be a modification of the thermal wind balance and a stronger tropical upwelling. For other injection strategies, different responses of the QBO have been observed. It has not yet been presented a theory which is able to explain those differences in a comprehensive manner, which is further complicated by the fact that the simulated QBO response is highly sensitive to the used model even under identical boundary conditions. Therefore, within this study we investigate the response of the QBO to SAM for three different injection strategies (point-like injection at the equator, point-like injection at 30° N and 30° S simultaneously, and areal injection into a 60° wide belt along the equator). Our simulations confirm that the QBO response significantly depends on the injection location. Based on the thermal wind balance, we demonstrate that this dependency is explained by differences in the meridional structure of the aerosol-induced stratospheric warming, i.e. the location and meridional extension of the maximum warming. Additionally, we also tested two different injection species (SO2 and H2SO4). The QBO response is qualitatively similar for both investigated injection species. Comparing the results to corresponding results of a second model, we further demonstrate the generality of our theory as well as the importance of an interactive treatment of stratospheric ozone for the simulated QBO response.

In 1963 a series of eruptions of Mt. Agung, Indonesia, resulted in the third largest eruption of the 20th century and claimed about 1900 lives. Two eruptions of this series injected SO2 into the stratosphere, which can create a long-lasting stratospheric sulfate layer. The estimated mass flux of the first eruption was about twice as large as the mass flux of the second eruption. We followed the estimated emission profiles and assumed for the first eruption on 17 March an injection rate of 4.7 Tg SO2 and 2.3 Tg SO2 for the second eruption on 16 May. The injected sulfur forms a sulfate layer in the stratosphere. The evolution of sulfur is nonlinear and depends on the injection rate and aerosol background conditions. We performed ensembles of two model experiments, one with a single eruption and a second one with two eruptions. The two smaller eruptions result in a lower sulfur burden, smaller aerosol particles, and 0.1 to 0.3 Wm−2 (10 %–20 %) lower radiative forcing in monthly mean global average compared to the individual eruption experiment. The differences are the consequence of slightly stronger meridional transport due to different seasons of the eruptions, lower injection height of the second eruption, and the resulting different aerosol evolution. Overall, the evolution of the volcanic clouds is different in case of two eruptions than with a single eruption only. The differences between the two experiments are significant. We conclude that there is no justification to use one eruption only and both climatic eruptions should be taken into account in future emission datasets.

Climate litigation has attracted renewed interest as a governance tool. A key challenge in climate litigation is to assess the factual basis of causation. Extreme weather attribution, specifically the Fraction of Attributable Risk (FAR), has been proposed as a way to tackle this challenge. What remains unclear is how attribution science interacts with the legal admissibility of evidence based on climate models. While evidence has to be legally admissible in order to be considered in a trial, it has to be reliable in order for the court to arrive at a legally correct conclusion. Since parties to the trial have incentives to produce evidence favorable to their case, admissibility requirements and the reliability of the evidence brought forward are linked. We provide a specific proposal for how to accommodate FAR estimates in admissibility standards by modifying an existing set of admissibility criteria, the Daubert criteria. We argue that two of the five Daubert criteria are unsuitable for dealing with such evidence and that replacing those criteria with ones directly addressing the reliability of FAR estimates is adequate. Lastly, we highlight the dependence of courts on both the existence and accessibility of a framework to determine the reliability of FAR estimates in executing such criteria.

The Los Chocoyos (14.6°N, 91.2°W) supereruption happened ?75,000 years ago in Guatemala and was one of the largest eruptions of the past 100,000 years. It emitted enormous amounts of sulfur, chlorine, and bromine, with multi-decadal consequences for the global climate and environment. Here, we simulate the impact of a Los Chocoyos-like eruption on the quasi-biennial oscillation (QBO), an oscillation of zonal winds in the tropical stratosphere, with a comprehensive aerosol chemistry Earth System Model. We find a ?10-year disruption of the QBO starting 4 months post eruption, with anomalous easterly winds lasting ?5 years, followed by westerlies, before returning to QBO conditions with a slightly prolonged periodicity. Volcanic aerosol heating and ozone depletion cooling leads to the QBO disruption and anomalous wind regimes through radiative changes and wave-mean flow interactions. Different model ensembles, volcanic forcing scenarios and results of a second model back up the robustness of our results.

Large magnitude tropical volcanic eruptions emit sulphur dioxide and other gases directly into the stratosphere, creating a long-lived volcanic aerosol cloud which scatter incoming solar radiation, absorbs outgoing terrestrial radiation, and can strongly affect the composition of the stratosphere.Such major volcanic enhancements of the stratospheric aerosol layer have strong “direct effects” on climate via these influences on radiative transfer, primarily surface cooling via the reduced insolation, but also have a range of indirect effects, due to the volcanic aerosol cloud’s effects on stratospheric circulation, dynamics and chemistry.In this study, we investigate the 3 largest volcanic enhancements to the stratospheric aerosol layer in the last 100 years (Mt Agung 1963; Mt El Chichón 1982; Mt Pinatubo 1991), comparing co-ordinated simulations within the so-called HErSEA experiments (Historical Eruptions SO2 Emission Assessment) several national climate modelling centres carried out for the model intercomparison project ISA-MIP.The HErSEA experiment saw participating models performing interactive stratospheric aerosol simulations of each of the volcanic aerosol clouds with common upper-, mid- and lower-estimate amounts and injection heights of sulfur dioxide, in order to better understand known differences among modelling studies for which initial emission gives best agreement with observations. First, we compare results of several models HErSEA simulations with a range of observations, with the aim to find where there is agreement between the models and where there are differences, at the different initial sulfur injection amount and altitude distribution.In this way, we could understand the differences and limitations in the mechanisms that controls the dynamical, microphysical and chemical processes of stratospheric aerosol layer.

A known adverse side effect of stratospheric aerosol modification (SAM) by artificial aerosol injections is the alteration of the quasi-biennial oscillation (QBO), which is caused by the stratospheric heating associated with an artificial aerosol layer. Multiple studies found the QBO to slow down or even completely vanish for point-like injections of SO2 at the equator. The reason for this was found to be a modification of the thermal wind balance and an acceleration of the residual circulation leading to a stronger tropical upwelling. For other injection strategies, different responses of the QBO have been observed in model simulations. It has not yet been presented a theory which is able to explain those differences in a comprehensive manner. This is further complicated by the fact that the simulated QBO response is highly sensitive to the used model even under identical boundary conditions.Therefore, within our study we investigated the response of the QBO to continuous artificial aerosol injections for three different injection strategies (point-like injection at the equator, point-like injection at 30°N and 30°S simultaneously, and areal injection into a 60° wide belt along the equator), and 3 different injection rates (5, 10, 25 Tg(S) yr -1). For each injection scenario we ran 10-year AMIP-style simulations with the general circulation model MAECHAM5, which was coupled interactively to the aerosol microphysical model HAM.Our simulations show that the QBO response significantly depends on the injection location. Based on thermal wind balance, we demonstrate that this dependency is explained by differences in the meridional structure of the aerosol-induced stratospheric warming, i.e. the location and meridional extension of the maximum warming, rather than its absolute magnitude. Additionally, we tested two different injection species, SO2and H2SO4, since the injection of H2SO4has been recently proposed as an alternative to an injection of SO2 as first studies indicate that an injection of H2SO4 may be more efficient than an injection of SO2. Our simulations indicate that the QBO response is qualitatively similar for both investigated injection species, but quantitatively stronger for an injection of H2SO4.

Extratropical volcanic eruptions are commonly thought to be less effective at driving large-scale surface cooling than tropical eruptions, and only the latter are commonly thought to be able to distribute sulfate globally. Here, we test both of these assumptions using a network of ice cores from the polar regions of Antarctica and Greenland covering the past 15’000 years and climate-aerosol modeling. We employ state-of-the-art analyses of trace elements, cryptoptephra and sulphur isotopes (Burke et al., 2019) to gain new insights into the timing of past eruptions, their stratospheric sulphur mass injections and subsequent sulphate aerosol lifecycle. We use this information to estimate the climate impact potential due to negative radiative forcing caused by Earth’s largest volcanic eruptions since the last Glacial. Our analysis encompasses over 1’000 eruptions and include the caldera-forming eruptions of Okmok II (Alaska, 43 BCE, VEI=6, 53°N; McConnell et al., 2020), Aniakchak II (Alaska, 1600s BCE, VEI=6, 57°N), Crater Lake (Mazama, Oregon, 5600s BCE, VEI=7, 43°N) and Laacher See (Germany, c. 13 ka BP, VEI=6, 50°N).We use our reconstructed radiative forcing and the coupled earth system models MPI-ESM1.2 and CESM (version 1.2.2) to analyze the climatic impact caused by these eruptions and compare the simulated temperature response with temperature reconstructions based on ultra-long tree-ring chronologies. Finally, based on these comparisons, we propose a number of stratigraphic age tie-points to anchor ice-core chronologies from Greenland (GICC05) and Antarctica (WD2014) to the absolute dated tree-ring chronology. We thereby aim to improve proxy synchronization throughout the Holocene -- a prerequisite for detection and attribution studies -- and invite the paleo-climate community to update climate proxy records based on ice cores to the latest chronologies. The European Research Council Grant 820047 under the European Union’s Horizon 2020 research and innovation program funded the research project THERA - Timing of Holocene Volcanic eruptions and their radiative aerosol forcing. References:Burke, A., Moore, K. A., Sigl, M., Nita, D. C., McConnell, J. R., and Adkins, J. F.: Stratospheric eruptions from tropical and extra-tropical volcanoes constrained using high-resolution sulfur isotopes in ice cores, Earth Planet Sc Lett, 521, 113-119, 2019.McConnell, J. R., Sigl, M., Plunkett, G., Burke, A., Kim, W., Raible, C. C., Wilson, A. I., Manning, J. G., Ludlow, F. M., Chellman, N. J., Innes, H. M., Yang, Z., Larsen, J. F., Schaefer, J. R., Kipfstuhl, S., Mojtabavi, S., Wilhelms, F., Opel, T., Meyer, H., and Steffensen, J. P.: Extreme climate after massive eruption of Alaska’s Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom, Proceedings of the National Academy of Sciences, 117, 15443-15449, 2020.

Stratospheric aerosols play an important role in the climate system and the atmospheric chemistry. They alter the radiative budget of the Earth affecting the global temperature and interact with stratospheric trace gases leading to ozone depletion. Effects are most noticeable after vulcanic eruptions enhancing the amount of aerosols in the stratosphere. Thus, vertically and spatially resolved knowledge about stratospheric aerosols, such as the particle size distribution and extinction coefficient, is crucial for the initialization of climate models, investigation of geoengineering, validation of aerosol micro-physical models, and improvement of trace gas retrievals. We present an algorithm to retrieve aerosol particle size distribution parameters (mode radius and distribution width, number density) from limb observations of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric ChartograpHY) operated aboard Envisat between 2002 and 2012. SCIAMACHY retrieved particle size distribution profiles are compared with in-situ balloon-borne measurements from Laramie, Wyoming. Both data-sets show good agreement. The stratospheric plume evolution after the eruption of Sarychev in the Kuril Islands, Russia, in June 2009 is investigated and compared to the output from the aerosol-climate modelling system ECHAM5-HAM.

Climatic warming from the last glacial maximum to the current interglacial period was punctuated by a similar to 1300 years long cold period, commonly referred to as the Younger Dryas (YD). Several hypotheses have been proposed for the mechanism triggering the abrupt inception of the YD, including freshwater forcing, an extra-terrestrial impact, and aerosols from volcanic eruptions. Here, we use synchronised sulphate and sulphur records from both Greenland and Antarctic ice cores to reconstruct volcanic forcing between 13,200-12,800 a BPGICC05 (years before 1950 CE on the Greenland Ice Core Chronology 2005; GICC05). This continuous reconstruction of stratospheric sulphur injections highlights a -110-year cluster of four major bipolar volcanic signals alongside several smaller events just prior to the YD inception. The cumulative Northern Hemisphere aerosol burden and radiative forcing from this cluster exceeds the most volcanically active periods during the Common Era, which experienced notable multidecadal scale cooling commonly attributed to volcanic effects. The Laacher See eruption (LSE), recently redated to 13,006 +/- 9 cal a BP, falls within our time window of study and has been proposed as a trigger for the YD but a direct volcanic imprint for the LSE in the Greenland ice cores has thus far proved elusive. Comparison of simulated sulphate deposition for mid- and high-sulphur LSE-type emission scenarios to the ice-core estimated sulphate deposition and interhemispheric asymmetry ratios allows several signals between 13,025 and 12,975 a BPGICC05 to be proposed as plausible candidates for the LSE. The magnitude and persistence of volcanic forcing directly preceding the YD inception highlights the need to consider stratospheric sulphur injections and their radiative forcing in future analyses and climate model experiments used to explore the mechanisms that triggered this or similar abrupt cooling events. (C) 2021 The Authors. Published by Elsevier Ltd.

Stratospheric aerosols are an important component of the climate system. They not only change the radiative budget of the Earth but also play an essential role in ozone depletion. These impacts are particularly noticeable after volcanic eruptions when SO2 injected with the eruption reaches the stratosphere, oxidizes, and forms stratospheric aerosol. There have been several studies in which a volcanic eruption plume and the associated radiative forcing were analyzed using climate models and/or data from satellite measurements. However, few have compared vertically and temporally resolved volcanic plumes using both measured and modeled data. In this paper, we compared changes in the stratospheric aerosol loading after the 2018 Ambae eruption observed by satellite remote sensing measurements and simulated by a global aerosol model. We use vertical profiles of the aerosol extinction coefficient at 869 nm retrieved at the Institute of Environmental Physics (IUP) in Bremen from OMPS-LP (Ozone Mapping and Profiling Suite – Limb Profiler) observations. Here, we present the retrieval algorithm and a comparison of the obtained profiles with those from SAGE III/ISS (Stratospheric Aerosol and Gas Experiment III on board the International Space Station). The observed differences are within 25 % for most latitude bins, which indicates a reasonable quality of the retrieved limb aerosol extinction product. The volcanic plume evolution is investigated using both monthly mean aerosol extinction coefficients and 10 d averaged data. The measurement results were compared with the model output from MAECHAM5-HAM (ECHAM for short). In order to simulate the eruption accurately, we use SO2 injection estimates from OMPS and OMI (Ozone Monitoring Instrument) for the first phase of eruption and the TROPOspheric Monitoring Instrument (TROPOMI) for the second phase. Generally, the agreement between the vertical and geographical distribution of the aerosol extinction coefficient from OMPS-LP and ECHAM is quite remarkable, in particular, for the second phase. We attribute the good consistency between the model and the measurements to the precise estimation of injected SO2 mass and height, as well as to the nudging to ECMWF ERA5 reanalysis data. Additionally, we compared the radiative forcing (RF) caused by the increase in the aerosol loading in the stratosphere after the eruption. After accounting for the uncertainties from different RF calculation methods, the RFs from ECHAM and OMPS-LP agree quite well. We estimate the tropical (20∘ N to 20∘ S) RF from the second Ambae eruption to be about −0.13 W m−2

Artificial injections of sulfur dioxide (SO2) into the stratosphere show in several model studies an impact on stratospheric dynamics. The quasi-biennial oscillation (QBO) has been shown to slow down or even vanish, under higher SO2 injections in the equatorial region. But the impact is only qualitatively, but not quantitatively consistent across the different studies using different numerical models. The aim of this study is to understand the reasons behind the differences in the QBO response to SO2 injections between two general circulation models, the Whole Atmosphere Community Climate Model (WACCM-110L) and MAECHAM5-HAM. We show that the response of the QBO to injections with the same SO2 injection rate is very different in the two models, but similar when a similar stratospheric heating rate is induced by SO2 injections of different amounts. The reason for the different response of the QBO corresponding to the same injection rate is very different vertical advection in the two models, even in the control simulation. The stronger vertical advection in WACCM results in a higher aerosol burden and stronger heating of the aerosols, and, consequently in a vanishing QBO at lower injection rate than in simulations with MAECHAM5-HAM.

The large VEI= 6 explosive eruption of the Laacher See volcano dated to c. 13,000 yrs BP (Reinig et al., 2020) marks the end of explosive volcanism in the East Eifel volcanic zone (Germany). It has previously been argued that this eruption temporarily impacted Northern Hemisphere climate (Graf and Timmreck, 2001), environments (Baales et al., 2002) and human communities (e.g. Blong et al., 2018). It has also recently been suggested again that the eruption may in fact be implicated in the onset of the Younger Dryas. Recent advances in the modelling of volcanically-induced climatic forcing warrant renewed attention to the eruption’s potential influence on Northern Hemisphere climate. Detailed reconstructions of its eruption dynamics have been proposed. The eruption might have lasted several weeks, most likely with a short (~10h) intense initial phase. A bipartite NE- and S- plume deposited tephra to the north-east the volcano towards the Baltic Sea and to the south towards Italy (Riede et al., 2011). In revisiting the eruption’s potential influence on Northern Hemisphere climate, we here present revised model simulations of the radiative impacts of the LSE using a global stratospheric aerosol model and new sulphur dioxide (SO2) emission estimates. The simulations were performed with the general circulation model MAECHAM5-HAM, which is coupled to an aerosol microphysical model. This allows us to simulate the evolution of the volcanic sulfur cloud and the transport of the ash cloud. The position of the observed deposits of the LSE depend on the weather and the wind direction during the eruption, demanding specific weather conditions to simulate similar locations of the observed deposits. Our models provide significantly improved insights into the meteorological situation during the eruption event as well as its impacts on Northern Hemisphere climate, with attendant implications for ecological and cultural impacts.ReferencesBaales, M., Jöris, O., Street, M., Bittmann, F., Weninger, B. and Wiethold, J.: Impact of the Late Glacial Eruption of the Laacher See Volcano, Central Rhineland, Germany, Quaternary Research, 58(3), 273–288, doi:10.1006/qres.2002.2379, 2002.Blong, R. J., Riede, F. and Chen, Q.: A fuzzy logic methodology for assessing the resilience of past communities to tephra fall: a Laacher See eruption 13,000 year BP case, Volcanica, 1(1), 63–84, doi:https://doi.org/10.30909/vol.01.01.6384, 2018.Graf, H.-F. and Timmreck, C.: A general climate model simulation of the aerosol radiative effects of the Laacher See eruption (10,900 B.C.), Journal of Geophysical Research, 106(14), 14747–14756, doi:0148-0227/01/2001JD900152, 2001.Reinig, F., Cherubini, P., Engels, S., Esper, J., Guidobaldi, G., Jöris, O., Lane, C., Nievergelt, D., Oppenheimer, C., Park, C., Pfanz, H., Riede, F., Schmincke, H.-U., Street, M., Wacker, L. and Büntgen, U.: Towards a dendrochronologically refined date of the Laacher See eruption around 13,000 years ago, Quaternary Science Reviews, 229, 106128, doi:10.1016/j.quascirev.2019.106128, 2020.Riede, F., Bazely, O., Newton, A. J. and Lane, C. S.: A Laacher See-eruption supplement to Tephrabase: Investigating distal tephra fallout dynamics, Quaternary International, 246(1–2), 134–144, doi:doi: 10.1016/j.quaint.2011.06.029, 2011.

Geoengineering, or climate intervention, describes methods of deliberately altering the climate system to offset anthropogenic climate change. As an idealized representation of near-surface solar geoengineering over the ocean, such as marine cloud brightening, this paper discusses experiment G1ocean-albedo of the Geoengineering Model Intercomparison Project (GeoMIP), involving an abrupt quadrupling of the CO2 concentration and an instantaneous increase in ocean albedo to maintain approximate net top-of-atmosphere radiative flux balance. A total of 11 Earth system models are relatively consistent in their temperature, radiative flux, and hydrological cycle responses to this experiment. Due to the imposed forcing, air over the land surface warms by a model average of 1.14 K, while air over most of the ocean cools. Some parts of the near-surface air temperature over ocean warm due to heat transport from land to ocean. These changes generally resolve within a few years, indicating that changes in ocean heat content play at most a small role in the warming over the oceans. The hydrological cycle response is a general slowing down, with high heterogeneity in the response, particularly in the tropics. While idealized, these results have important implications for marine cloud brightening, or other methods of geoengineering involving spatially heterogeneous forcing, or other general forcings with a strong land–ocean contrast. It also reinforces previous findings that keeping top-of-atmosphere net radiative flux constant is not sufficient for preventing changes in global mean temperature. © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License.

To contribute to a quantitative comparison of climate engineering (CE) methods, we assess atmosphere-, ocean-, and land-based CE measures with respect to Earth system effects consistently within one comprehensive model. We use the Max Planck Institute Earth System Model (MPI-ESM) with prognostic carbon cycle to compare solar radiation management (SRM) by stratospheric sulfur injection and two carbon dioxide removal methods: afforestation and ocean alkalinization. The CE model experiments are designed to offset the effect of fossil-fuel burning on global mean surface air temperature under the RCP8.5 scenario to follow or get closer to the RCP4.5 scenario. Our results show the importance of feedbacks in the CE effects. For example, as a response to SRM the land carbon uptake is enhanced by 92 Gt by the year 2100 compared to the reference RCP8.5 scenario due to reduced soil respiration thus reducing atmospheric CO2. Furthermore, we show that normalizations allow for a better comparability of different CE methods. For example, we find that due to compensating processes such as biogeophysical effects of afforestation more carbon needs to be removed from the atmosphere by afforestation than by alkalinization to reach the same global warming reduction. Overall, we illustrate how different CE methods affect the components of the Earth system; we identify challenges arising in a CE comparison, and thereby contribute to developing a framework for a comparative assessment of CE.

The enhancement of the stratospheric aerosol layer by volcanic eruptions induces a complex set of responses causing global and regional climate effects on a broad range of timescales. Uncertainties exist regarding the climatic response to strong volcanic forcing identified in coupled climate simulations that contributed to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). In order to better understand the sources of these model diversities, the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) has defined a coordinated set of idealized volcanic perturbation experiments to be carried out in alignment with the CMIP6 protocol. VolMIP provides a common stratospheric aerosol data set for each experiment to minimize differences in the applied volcanic forcing. It defines a set of initial conditions to assess how internal climate variability contributes to determining the response. VolMIP will assess to what extent volcanically forced responses of the coupled ocean–atmosphere system are robustly simulated by state-of-the-art coupled climate models and identify the causes that limit robust simulated behavior, especially differences in the treatment of physical processes. This paper illustrates the design of the idealized volcanic perturbation experiments in the VolMIP protocol and describes the common aerosol forcing input data sets to be used.

Economic evaluations of solar radiation management (SRM) usually assume that the temperature will be stabilized, with no economic impacts of climate change, but with possible side-effects. We know from experiments with climate models, however, that unlike emission control the spatial and temporal distributions of temperature, precipitation and wind conditions will change. Hence, SRM may have economic consequences under a stabilization of global mean temperature even if side-effects other than those related to the climatic responses are disregarded. This paper addresses the economic impacts of implementing two SRM technologies; stratospheric sulfur injection and marine cloud brightening. By the use of a computable general equilibrium model, we estimate the economic impacts of climatic responses based on the results from two earth system models, MPI-ESM and NorESM. We find that under a moderately increasing greenhouse-gas concentration path, RCP4.5, the economic benefits of implementing climate engineering are small, and may become negative. Global GDP increases in three of the four experiments and all experiments include regions where the benefits from climate engineering are negative.

We present a suite of new climate model experiment designs for the Geoengineering Model Intercomparison Project (GeoMIP). This set of experiments, named GeoMIP6 (to be consistent with the Coupled Model Intercomparison Project Phase 6), builds on the previous GeoMIP project simulations, and has been expanded to address several further important topics, including key uncertainties in extreme events, the use of geoengineering as part of a portfolio of responses to climate change, and the relatively new idea of cirrus cloud thinning to allow more longwave radiation to escape to space. We discuss experiment designs, as well as the rationale for those designs, showing preliminary results from individual models when available. We also introduce a new feature, called the GeoMIP Testbed, which provides a platform for simulations that will be performed with a few models and subsequently assessed to determine whether the proposed experiment designs will be adopted as core (Tier 1) GeoMIP experiments. This is meant to encourage various stakeholders to propose new targeted experiments that address their key open science questions, with the goal of making GeoMIP more relevant to a broader set of communities.

Considering the ambitious climate targets of the Paris Agreement to limit global warming to 2°C, with aspirations of even 1.5°C, questions arise on how to achieve this. Climate geoengineering has been proposed as a potential tool to minimize global harm from anthropogenic climate change. Here, an Earth system model is used to evaluate the climate response when transferring from a high CO2 forcing scenario, RCP8.5, to a middle-of-the-road forcing scenario, like RCP4.5, using aerosol geoengineering. Three different techniques are considered: stratospheric aerosol injections (SAI), marine sky brightening (MSB), and cirrus cloud thinning (CCT). The climate states appearing in the climate geoengineering cases are found to be closer to RCP4.5 than RCP8.5 and many anthropogenic global warming symptoms are alleviated. All three techniques result in comparable global mean temperature evolutions. However, there are some notable differences in other climate variables due to the nature of the forcings applied. CCT acts mainly on the longwave part of the radiation budget, as opposed to MSB and SAI acting in the shortwave. This yields a difference in the response, particularly in the hydrological cycle. The responses in sea ice, sea level, ocean heat, and circulation, as well as the carbon cycle, are furthermore compared. Sudden termination of the aerosol injection geoengineering shows that the climate very rapidly (within two decades) reverts to the path of RCP8.5, questioning the sustainable nature of such climate geoengineering, and simultaneous mitigation during any such form of climate geoengineering would be needed to limit termination risks. © 2018 American Meteorological Society. Locked until 11.1.2019 due to copyright restrictions.

Simulations from a multi-model ensemble for the RCP4.5 climate change scenario for the 21st century, and for two solar radiation management (SRM) schemes (stratospheric sulfate injection (G3), SULF and marine cloud brightening by sea salt emission SALT) have been analysed in terms of changes in the mean and extremes of surface air temperature and precipitation. The climate engineering and termination periods are investigated. During the climate engineering period, both schemes, as intended, offset temperature increases by about 60 % globally, but are more effective in the low latitudes and exhibit some residual warming in the Arctic (especially in the case of SALT which is only applied in the low latitudes). In both climate engineering scenarios, extreme temperature changes are similar to the mean temperature changes over much of the globe. The exceptions are the mid- and high latitudes in the Northern Hemisphere, where high temperatures (90th percentile of the distribution) of the climate engineering period compared to RCP4.5 control period rise less than the mean, and cold temperatures (10th percentile), much more than the mean. This aspect of the SRM schemes is also reflected in simulated reduction in the frost day frequency of occurrence for both schemes. However, summer day frequency of occurrence increases less in the SALT experiment than the SULF experiment, especially over the tropics. Precipitation extremes in the two SRM scenarios act differently – the SULF experiment more effectively mitigates extreme precipitation increases over land compared to the SALT experiment. A reduction in dry spell occurrence over land is observed in the SALT experiment. The SULF experiment has a slight increase in the length of dry spells. A strong termination effect is found for the two climate engineering schemes, with large temperature increases especially in the Arctic. Globally, SULF is more effective in reducing extreme temperature increases over land than SALT. Extreme precipitation increases over land is also more reduced in SULF than the SALT experiment. However, globally SALT decreases the frequency of dry spell length and reduces the occurrence of hot days compared to SULF.

Tropical forests represent a major atmospheric carbon dioxide sink. Here the gross primary productivity (GPP) response of tropical rainforests to climate engineering via marine sky brightening under a future scenario is investigated in three Earth system models. The model response is diverse, and in two of the three models, the tropical GPP shows a decrease from the marine sky brightening climate engineering. Partial correlation analysis indicates precipitation to be important in one of those models, while precipitation and temperature are limiting factors in the other. One model experiences a reversal of its Amazon dieback under marine sky brightening. There, the strongest partial correlation of GPP is to temperature and incoming solar radiation at the surface. Carbon fertilization provides a higher future tropical rainforest GPP overall, both with and without climate engineering. Salt damage to plants and soils could be an important aspect of marine sky brightening.

The eruption of Mount Pinatubo in 1991 injected a large amount of SO2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH4 in the years following the eruption. We use the three-dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH4 is quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first 2 years after the eruption. SO2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH4 is dominated by the effect of aerosol extinction, while SO2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH4 growth rate of 4 and 2 ppb/yr in the first and second years after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH4 burden change in late 1991 and early 1992. Key Points We modeled the effect of Pinatubo sulfur on tropospheric photochemistry SO2 absorption and aerosol extinction reduce tropospheric UV levels The tropospheric OH sink of CH4 decreased by 17.8 Tg during June 1991-June 1993

A new Geoengineering Model Intercomparison Project (GeoMIP) experiment "G4 specified stratospheric aerosols" (short name: G4SSA) is proposed to investigate the impact of stratospheric aerosol geoengineering on atmosphere, chemistry, dynamics, climate, and the environment. In contrast to the earlier G4 GeoMIP experiment, which requires an emission of sulfur dioxide (SO2) into the model, a prescribed aerosol forcing file is provided to the community, to be consistently applied to future model experiments between 2020 and 2100. This stratospheric aerosol distribution, with a total burden of about 2 Tg S has been derived using the ECHAM5-HAM microphysical model, based on a continuous annual tropical emission of 8 Tg SO2 yr−1. A ramp-up of geoengineering in 2020 and a ramp-down in 2070 over a period of 2 years are included in the distribution, while a background aerosol burden should be used for the last 3 decades of the experiment. The performance of this experiment using climate and chemistry models in a multi-model comparison framework will allow us to better understand the impact of geoengineering and its abrupt termination after 50 years in a changing environment. The zonal and monthly mean stratospheric aerosol input data set is available at https://www2.acd.ucar.edu/gcm/geomip-g4-specified-stratospheric-aerosol-data-set.