ConstrAining the Role of sulfur in the Earth System (CARES)

海洋硫循环是地球系统内具有重要气候意义的生物地球化学循环的范例(最初由爪子假说描述)。由生物生成的二甲基硫化物(DMS)产生的海洋硫酸盐气溶胶是许多海洋地区天然气溶胶的主要成分，它设定了一个基线气溶胶浓度，并根据该浓度确定了人为气溶胶辐射强迫的大小。然而，最近发现的由二甲基硫(如HPMTF)形成的新硫分子迫使我们从根本上重新审视海洋硫在气候系统中的作用。新的气溶胶形成途径和对以前被认为不重要的其他生物硫物种的海洋排放的观察，严重挑战了当前气候模型中所包含的理解：这些分子有什么作用？它们参与的过程是如何影响自然硫循环的？这些发现如何改变DMS排放对气候和人类驱动的气候变化的影响？最近发现的物种和化学路径没有包括在任何通过政府间气候变化专门委员会为全球气候变化政策提供信息的地球系统模型中，尽管来自自然来源的气溶胶是辐射强迫不确定性的关键驱动因素。在试图限制工业化前的气候系统时，对自然硫循环的巨大理解差距是一个主要限制，而前工业化气候系统本身对于确定实现气候稳定目标所需的温室气体的允许排放量至关重要。CARE项目将通过密集的飞机和船只观测以及多尺度模型实验相结合，填补我们对海洋大气硫循环知识的这些空白。模型的进步，再加上新的实验室数据，将使我们能够更好地理解当代和历史上的硫磺和气候观测。这将对我们对自然硫排放的命运和影响的理解产生重大修订。这些结果将用于纠正地球系统模型中硫过程表示的错误，限制海洋硫在地球系统中的作用，并提高对预测未来变化的模拟的信心。

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.