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Cooling by strong volcanic eruptions and their side effects: Large volcanic eruptions can provide insight into the potential consequences of sulfur compounds artificially introduced into the stratosphere, as they significantly increase the stratospheric background aerosol concentration for one to two years. This chapter presents their major impacts on climate and the hydrological and carbon cycle.

The 21st June 2019 Raikoke eruption (48° N,153° E) generated one of the largest amounts of sulfur emission to the stratosphere since the 1991 Mt Pinatubo eruption. Satellite measurements indicate a consensus best estimate of 1.5 Tg for the sulfur dioxide (SO2) injected at an altitude of around 14–15 km. The peak northern hemisphere mean 525 nm Stratospheric Aerosol Optical Depth (SAOD) increased to 0.025, a factor of three higher than background levels. The Volcano Response (VolRes) initiative provided a platform for the community to share information about this eruption, which significantly enhanced coordination efforts in the days after the eruption. A multi-platform satellite observation sub-group formed to prepare an initial report to present eruption parameters including SO2 emissions and their vertical distribution for the modelling community. It allowed to make the first estimate of what would be the peak in SAOD one week after the eruption using a simple volcanic aerosol model. In this retrospective analysis, we show that revised volcanic SO2 injection profiles yield a higher peak injection of the SO2 mass. This highlights difficulties in accurately representing the vertical distribution for moderate SO2 explosive eruptions in the lowermost stratosphere due to limited vertical sensitivity of current satellite sensors (+/- 2 km accuracy) and low horizontal resolution of lidar observations. We also show that the SO2 lifetime initially assumed in the simple aerosol model was overestimated by 66 %, pointing to challenges for simple models to capture how the life cycle of volcanic gases and aerosols depends on the SO2 injection magnitude, latitude and height. Using revised injection profile, modelling results indicate a peak northern hemisphere monthly mean SAOD at 525 nm of 0.024, in excellent agreement with observations, associated with a global monthly mean radiative forcing of -0.17 W/m2 resulting in an annual global mean surface temperature anomalies of -0.028 K. Given the relatively small magnitude of the forcing, it is unlikely that the surface response can be dissociated from surface temperature variability.

Sufficient large-ensemble simulations with the same model and radiative forcing scenario but varying initial conditions have become a great tool in recent years to disentangle forced and internal variability. Here we use 100-member ensembles of the MPI-ESM-LR for idealized volcanic eruptions of different eruption strength and latitude to investigate whether there is a linear volcanic signal on tropical precipitation dependent on the eruption strength, and when does it emerge from tropical internal variability. Our results show that for idealized tropical eruptions global and large hemispheric mean precipitation anomalies seemed to be scalable with the sulfur emission strength in a certain size range. An emission of 10 Tg sulfur seems to be a threshold where the volcanic signal is discernible from internal variability. We find that seasonal and ensemble mean pattern correlation of precipitation anomalies are highly correlated, in particular for larger emission strengths in the tropics and strongly modulated by ENSO with an increasing tendency for a warm ENSO increase with eruption strength. While the emergence of cooling appears on a hemispheric scale, the precipitation response is more localized and mainly confined to the tropics and subtropics. In line with previous findings our modelling results suggesting that large NH extratropical volcanic eruptions migrate the ITCZ to the south, whereas SH extratropical eruptions move the ITCZ to the north. Large regional differences depend on how the local Hadley circulation responds to the volcanic forcing., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

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.