Constraining the role of the marine sulfur cycle in the Earth System (CARES)

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The marine sulfur cycle is an exemplar of a climatically important biogeochemical cycle within the Earth system (initially described by the CLAW hypothesis). Marine sulfate aerosol produced from biogenic dimethyl sulfide (DMS) is the main component of natural aerosol over many oceanic regions and sets a baseline aerosol concentration against which the magnitude of anthropogenic aerosol radiative forcing is determined. However, recent discoveries of new sulfur molecules formed from DMS (e.g., HPMTF) force us to radically re-examine the role of marine sulfur in the climate system. The understanding embedded in current climate models is now significantly challenged by new aerosol formation pathways and by observations of marine emissions of other biogenic sulfur species that were previously dismissed as unimportant: What do these molecules do? How do the processes they participate in affect the natural sulfur cycle? How do these discoveries alter the impact of DMS emissions on climate and anthropogenically driven climate change? The recently discovered species and chemical pathways are not included in any Earth system models that inform global climate change policies through the IPCC, even though aerosols from natural sources are a key driver of uncertainty in radiative forcing. The significant gap in understanding of the natural sulfur cycle is a major limitation when trying to constrain the pre-industrial climate system, which is itself crucial for determining the allowed emissions of greenhouse gases needed to meet climate stabilisation targets.The CARES project will fill these holes in our knowledge of the marine atmospheric sulfur cycle through a combination of intensive aircraft and ship observations as well as multi-scale model experiments. Advancements in models, further informed by new laboratory data, will allow us to better understand contemporary and historical sulfur and climate observations. This will deliver a substantial revision to our understanding of the fate and impact of natural sulfur emissions. The results will be used to rectify errors in the representation of sulfur processes in Earth system models, constrain the role of marine sulfur in the Earth System, and improve confidence in simulations that project future change.