UKRI-Norway: Figuring Out how to Reconstruct Common Era forcing of climate by VOLcanoes with novel data and modelling approaches (FORCE-VOL)

大规模的火山喷发可能会对气候产生重大影响，因为硫磺气体的排放会形成小水滴(气溶胶)，反射入射的阳光，使地球表面降温。当这些气溶胶在大气层的上层(平流层，海拔15-50公里)形成时，它们会在那里停留数年，导致显著的全球变冷。事实上，这种现象激发了有争议的提议，即通过人工注入平流层硫来冷却地球，以对抗全球变暖。然而，尽管火山喷发具有科学和社会意义，但由于大型火山喷发的观测记录有限，对其对气候影响的了解高度不确定：只有两次喷发--1991年的皮纳图博火山喷发和1982年的厄尔奇翁火山喷发--在卫星时代影响了全球气候。这些喷发至少比历史记录中最大的喷发小一个数量级，因此不能代表火山如何影响我们的气候的范围。这使得理解和准备应对未来大喷发的气候和社会影响具有挑战性。火山硫排放的有限观测记录也给气候模型带来了一个重大问题，气候模型需要知道在计算机化的平流层中添加多少硫才能模拟历史上的气候变化事件。为了应对这些挑战，我们提出了一种新的方法，根据极地冰芯中发现的火山硫酸盐记录，重建过去2000年来大规模喷发的平流层硫酸盐的数量。虽然这种方法被广泛使用，但目前如何将冰芯中发现的硫酸盐转换为平流层中原始的硫酸盐数量仍存在重大不确定性。该项目将通过使用新的冰芯、新的测量技术和新的建模方法，大大改进这种转换--即所谓的“传递函数”。首先，我们将详细比较冰层中硫酸盐的含量与过去150年平流层喷发时进入平流层的硫磺含量的测量结果，这段时间是对大气进行直接观测(通过卫星或测量阳光的仪器)的时期。与上次进行校准相比，可用冰芯的数量已从11个增加到90个，从而实现了更好的空间覆盖和更具代表性的数据。我们还有一种测量硫同位素的新技术，使我们能够区分对气候重要的平流层硫酸盐与其他来源的硫酸盐到冰盖，进一步提高校准的准确性。一种新的计算机模拟方法也将被用来确保传递函数适用于广泛的不同喷发特征(如喷发的大小、季节和纬度)，并帮助我们描述传递函数的不确定性。来自冰芯校准和模拟的见解将被结合起来，以产生过去2000年火山喷发的平流层硫酸盐的新记录。这一记录将被广泛用于气候模型模拟，包括用于向国际气候变化专门委员会(IPCC)提供信息的模拟。事实上，这项工作可能会改进气候模型，因为如果历史喷发模型中添加的硫酸盐的量更为人所知，我们应该能够更好地评估哪些模型最准确地匹配相关的气候变化。展望未来，我们的工作对对自然灾害感兴趣的政策制定者和保险公司也将是有价值的，因为它将使他们更好地了解我们未来将发生的重大喷发的频率和潜在影响。

Large volcanic eruptions can have a major impact on climate, due to the emission of sulfur gases, which form small droplets (aerosols) that reflect incoming sunlight and cool the Earth's surface. When these aerosols form in the upper levels of the atmosphere (the stratosphere, 15-50 km altitude) they remain there for several years, resulting in pronounced global cooling. Indeed, this phenomenon has inspired controversial proposals to cool the planet to combat global warming through artificial stratospheric sulfur injections. However, despite its scientific and societal significance, understanding of volcanic impacts on climate is highly uncertain, due to the limited observational record of large explosive volcanism: only two eruptions, Pinatubo in 1991 and El Chichón in 1982, have impacted global climate within the satellite era. These eruptions are at least an order of magnitude smaller than the largest eruptions in the historical record, and so are not representative of the scope of how volcanoes can impact our climate. This makes it challenging to understand, and prepare for, the climatic and societal impact of large eruptions in the future. The limited observational record of volcanic sulfur emissions also creates a major issue for climate models, which need to know how much sulfur to add to their computerised stratospheres in order to mimic historical climate change events. To address these challenges, we are proposing a new way to reconstruct the amount of stratospheric sulfate from large eruptions over the last 2000 years, based on the record of volcanic sulfate found in polar ice cores. Although this approach is widely used, at present there are major uncertainties in how to convert the amount of sulfate found in ice cores into the original amount of sulfate that was in the stratosphere.This project will substantially improve this conversion - known as the "transfer function" - by using new ice cores, new measurement techniques, and new modelling approaches. First, we will make detailed comparisons of the amount of sulfate in the ice to measurements of the amount of sulfur that went into the stratosphere for eruptions during the last 150 years, a time period in which direct observations of the atmosphere (either by satellites or instruments that measure sunlight) exist. Compared to the last time this calibration was done, the number of available ice cores has grown from 11 to 90, allowing for much better spatial coverage and more representative data. We also have a new technique that measures sulfur isotopes to allow us to distinguish the climatically-important stratospheric sulfate from other sources of sulfate to the ice sheets, further improving the accuracy of the calibration. A new computer modelling approach will also be used to make sure that the transfer function is applicable to a broad range of different eruption characteristics (such as the size, season, and latitude of the eruption), and to help us characterise the transfer function's uncertainty.The insights from the ice core calibration and the modelling will be combined to generate a new record of stratospheric sulfate from volcanic eruptions over the last 2000 years. This record will be used widely in climate model simulations, including those used to inform the International Panel on Climate Change (IPCC). Indeed this work may lead to improvements in climate modelling, as if the amount of sulfate to be added to the models for historical eruptions is better known, we should be able to make better assessments of which models most accurately match the associated changes in climate. Looking forward, our work will also be valuable for policy makers and insurance companies interested in natural hazards, as it will allow them to better understand the frequency and potential impacts of the major eruptions that will occur in our future.