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Abstract The Southern Ocean has emerged as a key region for constraining aerosol‐climate interactions due to its relatively low anthropogenic influence. Sulfate is an important aerosol over the Southern Ocean, and models suggest dimethyl sulfide (DMS) is the largest source of sulfate during summer. However, sulfur isotopes of sulfate (δ³⁴S(SO₄²⁻)) in Antarctic ice cores suggest a significant contribution from a previously unexplained non‐DMS source. Here we show that the fractional contribution from passive volcanic degassing (f_volc) explains observed δ³⁴S(SO₄²⁻), and that a global chemical transport model underestimatesf_volcacross Antarctica. Underestimatedf_volcimplies that the model mischaracterizes sulfate sources in this important region. The discrepancy between observed and modeled sulfate sources can be reconciled by increasing passive volcanic sulfur degassing emissions and decreasing DMS emissions. Our results imply that current biases in emissions inventories could bias assessments of aerosol‐cloud interactions in the Southern Ocean region and globally. 

Abstract The Arctic is warming at almost four times the global rate. An estimated sixty percent of greenhouse‐gas‐induced Arctic warming has been offset by anthropogenic aerosols, but the contribution of aerosols to radiative forcing (RF) represents the largest uncertainty in estimating total RF, largely due to unknown preindustrial aerosol abundance. Here, sulfur isotope measurements in a Greenland ice core show that passive volcanic degassing contributes up to 66 ± 10% of preindustrial ice core sulfate in years without major eruptions. A state‐of‐the‐art model indicates passive volcanic sulfur emissions influencing the Arctic are underestimated by up to a factor of three, possibly because many volcanic inventories do not include hydrogen sulfide emissions. Higher preindustrial volcanic sulfur emissions reduce modeled anthropogenic Arctic aerosol cooling by up to a factor of two (+0.11 to +0.29 W m⁻²), suggesting that underestimating passive volcanic sulfur emissions has significant implications for anthropogenic‐induced Arctic climate change.