New marine sulfur chemistry amplifies natural aerosol cooling and increases estimated anthropogenic climate impact
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The marine sulfur cycle shapes Earth's climate through its influence on atmospheric composition, cloud properties and radiative forcing. Natural sulfate aerosols derive largely from the atmospheric oxidation of dimethyl sulfide (DMS) emissions from plankton. Recent work has revised this established mechanism in two ways: hydroperoxymethyl thioformate (HPMTF) forms rapidly during DMS oxidation and changes yields of different oxidation products, and methanethiol (MeSH) is now recognised as a substantial fraction of volatile marine sulfur. Enabled by synthesis of experimental and observational studies, here we implement both in a state-of-art Earth system model (UKESM). In pre-industrial (PI) and present-day (PD) simulations, the combined update produces strong top-of-atmosphere cooling in both climates (−1.31 ± 0.04 W m⁻² in PI; −1.16 ± 0.04 W m⁻² in PD). This cooling is concentrated over the Southern Ocean, dominated by aerosol–cloud interactions, and driven by contrasting chemical pathways. HPMTF scavenging by existing particles lowers the SO2 yield from DMS oxidation, yet simultaneously accelerates aerosol growth and enlarges the cloud condensation nuclei (CCN)-relevant population. MeSH emissions supply further SO2 and strengthen the cooling, by an amount that depends on the background DMS chemistry. Due to low background aerosol concentrations, PI clouds are highly susceptible to marine sulfur perturbations. Consequently, the increased natural sulfate loading from this new chemistry triggers a sharper rise in cloud droplet number concentrations (CDNC) in PI than in PD, resulting in stronger PI cooling. This elevated PI baseline CDNC reduces the PI-to-PD increase in cloud albedo. As a result, the attendant anthropogenic aerosol–cloud interaction offsets less greenhouse gas warming, leading to a 7% larger net anthropogenic radiative forcing. In turn, this suggests that climate sensitivity is smaller than previously estimated (since the PI-to-PD temperature response has been driven by a larger forcing than previously thought) and highlights the critical importance of including accurate multiphase sulfur chemistry in the Earth System models which inform the Intergovernmental Panel on Climate Change (IPCC).
Dr. Yao GE is a Research Fellow at the Yusuf Hamied Department of Chemistry, University of Cambridge, working with Professor Alex Archibald. In October 2024, she was elected to a Postdoctoral Research Associate position at Pembroke College. Prior to joining Cambridge, she completed her PhD in Chemistry at the University of Edinburgh and her MPhil in Chemistry at HKUST. Following her doctoral studies, she worked as a Research Scientist at the Norwegian Meteorological Institute (MET Norway) in Oslo. Her current research focuses on advancing our understanding of ocean–atmosphere sulphur cycles and their impacts on Earth's climate. She believes the most effective route to understanding Earth's atmosphere lies in combining state-of-the-art models with state-of-the-art measurements. Her long-term goal is to bring both to bear on questions that matter beyond the literature — delivering evidence and tools that support practical solutions to air pollution and climate change.