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Extreme weather impacts of climate change: an attribution perspective [1]

['Ben Clarke', 'Environmental Change Institute', 'University Of Oxford', 'Oxford', 'United Kingdom', 'Http', 'Friederike Otto', 'Grantham Institute', 'Imperial College London', 'London']

Date: 2023-07

Every year, climate change manifests through more intense extreme weather events, including heatwaves, droughts and heavy rainfall. This leads to impacts upon people, property and nature that would not have occurred in the absence of these increases in events' likelihood and intensity. Unlike some other impacts of climate change, extreme weather events are manifesting on immediate timescales and changes in extremes are poorly described by the climatological means studied in many projections. Unfortunately, there is currently no standardised way of, or effort towards, documenting climate change related harms systematically. As a result, there is no systematic basis to quantify the major contribution of human influence on extreme weather to the costs of climate change. This contributes to the challenge that measures taken to mitigate and adapt to current (of ∼1.20 °C at the time of writing (Masson-Delmotte et al 2021)) and future levels of global warming are not based on what could be the best available evidence.

Extreme event attribution is the method through which the role of climate change in an individual event can be assessed and quantified (Allen 2003, Philip et al 2020, van Oldenborgh et al 2021). Over the past two decades, more than 350 studies have quantified the role of climate change in over 400 extreme events (Carbon Brief 2021). This growing body of evidence is complementary to other analysis, such as work documenting observed and modelled trends in extremes due to climate change, and projections of future risk. The evidence from attribution studies adds value by highlighting the role of climate as a risk driver in experienced events, which in turn is useful for building future resilience (Raju et al 2022), and it enables the attribution of impacts, which is useful for cost-benefit analysis of mitigation and is a potential avenue for the exploration of drivers of loss and damage from climate-related extremes (James et al 2019). There is currently very little discussion of the role of science in determining loss and damage from anthropogenic climate change.

However, the number of events that have been studied using attribution methods is just a small fraction of all impactful extreme weather events that occurred over the same period. It is almost impossible to document this comprehensively due to data, time and resource constraints (Harrington and Otto 2020, van Oldenborgh et al 2021). Furthermore, these studies overwhelmingly focus on events that occurred in the global north (Otto et al 2020a). This pattern is mirrored in the unequal recording of impacts from extreme events, although nations in the global south are experiencing the most rapid changes in risk (Byers et al 2018) and often have high levels of underlying exposure and vulnerability to climate-related events.

The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) provides a synthesis of attributable changes in extremes in regions across the world (Masson-Delmotte et al 2021). This forms a core part of the evidence base in this literature review. However, in order to link regional assessments of attributable risk to impacts that have occurred, it is necessary to compile attribution statements for individual events. In doing so this review builds on the IPCC assessment, linking broad changes in hazards to the ways in which these manifest and interact with exposure and vulnerability, and result in tangible impacts, with a bottom-up methodology, in which we develop insight into human influence on assessed extreme weather event categories based on literature on individual events. Further, the distinction between regional assessment and individual events is particularly important in adding evidence for more complex events, such as droughts, and at smaller spatial scales than the IPCC's regional aggregations, at which other phenomena may affect the influence of climate change.

Overall, the review brings together the best evidence we have on changes in extreme weather hazards and the impacts of past events, covering five key hazards: heatwaves, rainfall-based flooding, droughts, wildfires, and tropical cyclones. It is important to note that several of these hazards fall along different parts of the causative chain that stretches from anthropogenic climate change to impacts; each was selected as an appropriate intersection between attribution science and impact-relevance. As such, for each hazard, we assess the degree to which past impacts can be attributed to anthropogenic climate change, and the limiting factors associated with this. In doing so, we also build a picture of where evidence is most lacking, and therefore most urgent, for both attribution science and the documenting of impacts.

Each hazard section is laid out as follows. First, we describe attributed changes in extremes on global and regional scales, then how individual event attribution fits within this. Second, we discuss why it matters, describing the causative chain that results in key impacts arising due to such hazards. Finally, we discuss the 'attributable impacts' for each hazard. The intent of this subsection is not to suggest that the impacts of an event with any degree of anthropogenic influence, no matter the scale, are all attributable to climate change. Instead, these sections contain individual attribution statements alongside the impacts of those events, in the context of their connections as described previously. This should be interpreted as a snapshot, constrained by the body of existing attribution statements and to date, of the types and magnitude of impacts that have manifested to some degree by anthropogenic climate change. We posit that this is currently the most we can know based on existing evidence. And, in the absence of science to accurately attribute impacts using an end-to-end system for every event (and understanding that a fractional attributable risk (FAR)-based attribution of impacts is most applicable to classes of event (Perkins-Kirkpatrick et al 2022)) this is nonetheless more useful than neglecting to combine such information at all. Tables 1 and 2 summarise this information on a global scale.

Table 1. Direct physical health impacts of different types of disaster between 2000 and 2020, as recorded by EMDAT, and the attributable influence of climate change on each hazard (EMDAT 2019). Observed direct impacts Attributable influence of climate change on hazard severity/likelihood (confidence level) Hazard Deaths Injured Total affected Heatwaves 157 000 193 000 320 000 Increase (high) Cold waves and severe winter conditions 14 900 1.86 million 96.1 million Decrease (high) Floods 111 000 304 000 1.66 billion Increase (medium) Droughts 21 300 N/A 1.44 billion Increase (medium) Wildfires 1570 7260 3.38 million Increase (medium) Storms 201 000 337 000 773 million Rainfall increase (high) Other impacts no change (low)

Table 2. Direct damages of different types of disaster between 2000 and 2020, as recorded by EMDAT, and the attributable influence of climate change on each hazard (EMDAT 2019). Note that these values are likely to be substantial underestimates of the true magnitude of damages. Observed direct impacts Attributable influence of climate change on hazard severity/likelihood (confidence level) Hazard Insured damages (USD) Total damages (USD) Heatwaves 10 000 13.4 bn Increase (high) Cold waves and severe winter conditions 4.63 bn 31.3 bn Decrease (high) Floods 74.1 bn 610 bn Increase (medium) Droughts 21 bn 119 bn Increase (medium) Wildfires 51.3 bn 94.3 bn Increase (medium) Storms 499 bn 1.30 trillion Rainfall Increase (high) Other impacts no change (low)

This is not a review of risk, because that is impossible without also discussing vulnerability and exposure to changing hazards. However, it provides a platform on which to begin such a discussion. Further, it is a basis for placing a price tag on the diverse impacts of climate change, with implications for mitigation and adaptation considerations at all levels of decision-making. It concludes with several key areas in which further work will improve upon quantification of climate-related impacts, and the applications of this to address the inequity at the heart of the climate crisis (Pelling and Garschagen 2019, Stone et al 2021, Raju et al 2022).

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[1] Url: https://iopscience.iop.org/article/10.1088/2752-5295/ac6e7d

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