Attributing impacts of external climate drivers on extreme weather in Africa

鉴于在减少温室气体排放方面进展有限，而且对许多影响的适应潜力不确定，脆弱地区和部门的注意力正转向“损失和损害”问题。谁应该承担人类对气候影响的代价，而这种影响不能通过适应来抵消？这场辩论因缺乏对这些成本的可靠估计而受阻。尽管各方共同努力编制排放清单，但我们仍然没有商定的方法来确定国家、公司或个人如何在区域环境变化的其他驱动因素的背景下受到人为气候变化的不利影响。气候变化的许多最重要影响在某种程度上与洪水、风暴和干旱等高影响天气事件有关。编制影响清单需要记录单个事件的影响以及这些事件如何受到多种气候驱动因素和内部气候变异性的影响。我们将继续开展对高纬度水环境及其影响的研究，评估气候变化与极端天气之间的联系，迄今为止主要侧重于中纬度现象和温室气体上升的影响。然而，在许多热带地区，短期气候强迫（SLCF），如硫酸盐，矿物和黑碳气溶胶和对流层臭氧可能在改变天气风险模式方面发挥了更大的作用。仅需20年即可实现人为SLCFs的大幅减少。包括已经计划的减少硫酸盐气溶胶前体排放的措施在内，SLCF可能主导天气风险的近期变化。用于适应规划的气候影响评估通常侧重于以温室效应引起的变暖为主的几十年人为净变化。很少有人讨论SLCF强迫和响应的不确定性。因此，依赖这些数据以及对近期趋势的推断，有可能“适应昨天的问题”，因为区域天气的主要驱动因素发生了逆转。评估外部驱动因素对极端天气的影响具有挑战性，因为最重要的事件通常很少发生。唯一的解决办法是依靠模拟模型，其可靠性可以进行测试，必要时可以使用为季节性预测而制定的成熟程序进行重新校准。我们还将使用陆面模式JULES在气象数据稀疏的地区进行间接验证。考虑到外部驱动因素和模拟模型的不确定性，需要大量相对高分辨率的气候模型模拟，以便对极端天气事件进行可靠的统计，该项目利用了climateprediction.net weatherathome全球志愿者计算项目。我们将量化各种外部气候驱动因素对非洲极端天气风险变化的作用，方法是在CORDEX-Africa域上实施区域气候模型，并使用数千个成员的集合模拟近几十年来观测到的天气统计数据，系统地排除不同气候驱动因素的影响，以量化其影响。迄今为止，HIWE的归因研究通常集中在水文气象事件本身，而不是一路模拟它们的影响。这可能会导致“过度归因”：如果一个破纪录的天气事件发生，而这个事件更有可能是由一些外部驱动因素造成的，那么人们往往会将该事件的大部分影响归咎于该驱动因素。但这种影响在很大程度上也可能是由一个较小的、非破纪录的事件造成的。因此，准确的评估需要对不断变化的影响风险进行明确的建模，而不仅仅是天气风险，因此该项目的一个主要重点是使用JULES来调查各种影响，并与非洲各地的影响建模人员合作，评估我们的天气模拟对其他部门不断变化的影响风险的影响。

Given limited progress in reducing greenhouse gas emissions and uncertain potential for adaptation to many impacts, attention in vulnerable regions and sectors is turning to the question of "loss and damage". Who should bear the costs of human influence on climate that cannot be neutralized by adaptation? This debate is impeded by lack of robust estimates of what these costs are. Despite concerted efforts to compile inventories of emissions, we still have no agreed method of establishing how countries, companies or individuals are being adversely affected by anthropogenic climate change in the context of other drivers of regional environmental change.Many of the most important impacts of climate change are related in some way to high-impact weather events (HIWEs), such as floods, storms, and droughts. Compiling an impact inventory requires documenting the impacts of individual events and how these events are affected by multiple climate drivers and internal climate variability. We will build on research into HIWEs and their impacts under THORPEX-Africa.Studies assessing the link between climate change and extreme weather have so far focused primarily on mid-latitude phenomena and the impact of rising greenhouse gases. Yet in many tropical regions, short-lived climate forcings (SLCFs) such as sulphate, mineral and black carbon aerosols and tropospheric ozone may have played a larger role in changing patterns of weather risk to date. Substantial reductions in anthropogenic SLCFs could be achieved in only 20 years. Including measures already planned to reduce emissions of sulphate aerosol precursors, SLCFs may dominate near-term changes in weather risk. Climate impact assessments used for adaptation planning typically focus on net multi-decadal anthropogenic change, dominated by greenhouse-induced warming. Few address uncertainty in SLCF forcing and response. Hence relying on these and extrapolation of recent trends risks "adapting to yesterday's problem" as key drivers of regional weather are reversed.Assessing the influence of external drivers on extreme weather is challenging because the most important events are typically rare. The only solution is to rely on simulation models, whose reliability can be tested and if necessary re-calibrated using well-established procedures developed for seasonal forecasting. We will also use the land-surface model JULES for indirect validation in regions with sparse meteorological data. Large ensembles of climate model simulations at relatively high resolution are required for robust statistics of extreme weather events, allowing for uncertainty in both external drivers and simulation models.This project makes use of the climateprediction.net weatherathome worldwide volunteer computing project. We will quantify the role of various external climate drivers on changing risks of extreme weather in Africa by implementing a regional climate model over the CORDEX-Africa domain and simulate observed weather statistics over recent decades using multi-thousand-member ensembles, systematically excluding the influence of different climate drivers to quantify their effects.Attribution studies of HIWEs to date have typically focussed on hydrometeorological events themselves, rather than modelling all the way through to their impacts. This can lead to "over-attribution": if a record-breaking weather event occurs that has been made more likely by some external driver, people tend to blame most of the impact of that event on that driver. But much of this impact might also have been caused by a lesser, non-record-breaking, event. Hence accurate assessment requires explicit modelling of changing impact risk, not simply weather risk, so a major focus of this project will using JULES to investigate various impacts and working with impact modellers across Africa to assess the implications of our weather simulations for changing impact risk in other sectors.