Stratospheric aerosol forcing for CMIP7 (part 2): Volcanic sulfur dioxide emissions

Abstract. Explosive volcanic sulfur emissions into the stratosphere form sulfate aerosol particles which are an important driver of climate variability. Volcanic sulfur emissions are required both by climate models using interactive stratospheric aerosol schemes, as well as simpler volcanic aerosol models typically used to provide stratospheric aerosol optical properties for climate models which cannot simulate stratospheric aerosols interactively from precursor emissions. Here we present an upper-tropospheric and stratospheric volcanic sulfur emission inventory covering the period 1750 to 2023, made for phase 7 of the Coupled Model Intercomparison Project (CMIP7). CMIP7 stratospheric aerosol optical properties, presented in a companion paper, are directly derived from this emission inventory for the 1750–1978 pre-satellite period. For the satellite era, the emission inventory is primarily derived from satellite observations. For the pre-satellite period, the inventory is derived from a bipolar ice-core dataset, which captures well eruptions injecting on the order of 10 Tg SO₂ or more, complemented by a high-resolution Greenland ice-core and the geological record, which support the inclusion of eruptions injecting on the order of 0.1–1 Tg SO₂. We use satellite-era eruptions and older eruptions confidently matched to sulfate deposition records in ice-cores to derive empirical relationships to attribute injection height from SO₂ mass and injection latitude from polar deposition asymmetry when these parameters are missing. The final CMIP7 emission inventory includes 463 eruptive eruptions injecting a total of 428.2 Tg SO₂ into the upper troposphere-stratosphere over 1750–2023, corresponding to a mean flux of 1.56 Tg SO₂ yr^-1. For the pre-satellite era, around 25 % of CMIP7 volcanic sulfur emissions originate from small-to-moderate magnitude eruptions, defined here as injecting ≤ 3 Tg SO₂, which not captured in many available ice core arrays.  Comparing the CMIP7 inventory with other inventories available for specific time periods, we highlight the large uncertainties characterizing volcanic emissions over the historical period. However, we show that CMIP7 is less biased in terms of the frequency-magnitude distribution of small-to-moderate magnitude eruptions which are largely underestimated in other pre-satellite era inventories. Although the long-term mean emissions associated with these eruptions is consistent with the satellite era in CMIP7, we show that their frequency-magnitude distribution likely remains biased with a lack of small-magnitude eruptions injecting on the order of 0.1 Tg SO₂, and potentially too many moderate-magnitude eruptions injecting on the order of 1 Tg SO₂. We discuss other key sources of uncertainties and directions for improvements for the dataset, including the addition of non-sulfur and non-volcanic stratospheric aerosol precursors, as well as needs for operationalizing the dataset production. To support consistent implementation of the dataset in stratospheric aerosol models, we also provide recommendations for the temporal, horizontal, and vertical distribution of emissions associated with each eruption.