Underestimated Passive Volcanic Sulfur Degassing Implies Overestimated Anthropogenic Aerosol Forcing

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.

Sulfur isotopes quantify the impact of anthropogenic activities on industrial-era Arctic sulfate in a Greenland ice core

“…Years with major volcanic eruptions prior to 1980 were not measured. Detailed methods for sample analysis are described in Jongebloed et al (2023). We analyze a total of 135 samples representing 184 sampled years, including one sample per decade from 1200 to 1750 CE at 1-to 2-year resolution, one sample every 4 years from 1750 to 1980 CE at 1-year resolution, and one sample each year at 1-year resolution from 1980 to 2006 CE.…”

“…The isotopic composition and concentration of ice core sulfate can be used to estimate the relative contribution from each of its main sources (Norman et al 1999, Patris et al 2002, Wasiuta et al 2006, Jongebloed et al 2023, as effects of oxidation and transport on sulfur isotopic composition are minimal (Uemura et al 2016, Ishino et al 2019. The fraction of sea-salt sulfate in each sample is calculated using the mass fraction of bulk sea water SO 4 2− /Na + = 0.25 (Holland 1978), and sea salt contributes 2.1% of total ice core sulfate on average.…”

“…In the preindustrial samples (prior to 1850), the anthropogenic contribution to sulfate concentration is assumed to be negligible, and nssSO 4 2− originates from only DMS and volcanoes. Other natural sources of sulfate are thought to be negligible in the Arctic (Abbatt et al 2019, Patris et al 2002, Jongebloed et al 2023. The isotopic signature of biogenic sulfate (δ 34 S bio ) is well constrained at +18.8 ± 0.3 ‰ and the volcanic isotopic signature (δ 34 S volc ) was previously estimated to be +4.1 ± 0.5 ‰ (Jongebloed et al 2023).…”

“…Biomass burning is not a significant source of Arctic sulfate (Breider et al 2014, Abbatt et al 2019. Sulfur isotopes can be used to determine relative contribution of anthropogenic and natural sources to total sulfate when the isotopic signatures are known and sufficiently distinct from each other (Norman et al 1999, 2000, Patris et al 2002, Wadleigh 2004, Wasiuta et al 2006, Jongebloed et al 2023. In this study we present observations of sulfur isotopes of sulfate in Greenland ice core samples and use them to quantify anthropogenic and natural sulfate over the industrial era.…”

“…However, the estimated temperature increase due to a reduction in aerosol abundance is highly dependent on unknown natural Arctic and sub-Arctic sulfate abundance, which is sourced from volcanic sulfur emissions, marine phytoplankton dimethyl sulfide (DMS) emissions, and sea-salt aerosols (Norman et al 1999, Patris et al 2002, Abbatt et al 2019, Jongebloed et al 2023. Biomass burning is not a significant source of Arctic sulfate (Breider et al 2014, Abbatt et al 2019.…”