Extreme Events and Climate Change. Группа авторов

weather across multiple types of impacts. It considers existing global synthesis efforts rather than developing a new analysis based on other chapters in this book. It includes discussion of the motivation for such assessments, challenges in performing syntheses related to extremes, and possible methods for assembling a synthesis. The focus is on the detection and attribution of impacts during the past half‐century, but implications for predicting and, ultimately, documenting future changes in risk are also discussed. The only synthesis assessment of past impacts related to extreme weather is reviewed, noting that its shortcomings can be overcome only through further developments in a number of areas, including monitoring and process understanding.

1.1. A REASON FOR CONCERN

      In 1992, the nations of earth agreed to “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” according to the prescriptions of the United Nations Framework Convention on Climate Change (UNFCCC) (United Nations, 1992). The meaning of “dangerous” was not specifically defined, but it was made clear that action should be taken so as “to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.” Since 1992, the world’s nations have continued developing the UNFCCC, and more recently they noted “the importance of averting, minimizing and addressing loss and damage associated with the adverse effects of climate change, including extreme weather events …” (United Nations, 2015, p. 26). In doing so, the countries recognized that “adverse effects of climate change” will impose “loss and damage,” but they remained silent on the conditions under which such adverse effects, loss, and damage might be considered “dangerous.” Such conditions might be reached, for instance, once a certain threshold of damage is achieved or if the rate of increase of loss becomes too high. The nature of those conditions might be different for the viability of the insurance industry, the stability of an economy, the reliability of a food supply, or the steadiness of a political system. Hence, whatever might ultimately be designated as dangerous, it will need to be informed by assessment of impacts around the world and across natural, managed, and human systems. This assessment not only needs to note the global and cross‐system averages but also the existence of any localized but transformative impacts, such as might occur around an ice‐free Arctic Ocean, as well as disparities in impacts, for instance between wealthy and poor populations. In this chapter we will refer to such an assessment as a synthesis.

This chapter is concerned with possibilities and challenges of syntheses that might inform the UNFCCC process (and we hope other national and international activities) with specific respect to adverse effects inflicted by extreme weather events. It is not intended to provide a synthesis assessment itself, a major multidisciplinary endeavor. Why the focus on extreme weather? Does it matter whether impacts are a consequence of extreme weather rather than of other manifestations of anthropogenic climate change?

      Much contemporary risk management focuses on reducing exposure and vulnerability to, and increasing resilience against, natural disasters. Infrastructure is designed to withstand certain thresholds of extreme weather, and insurance is purchased as a hedge against damage from uncertain but plausible extreme weather. Thus one possible lens for defining “dangerous” is through the definition implicit in current design specifications and in what is considered affordable levels of insurance: in other words, through risks associated with extreme weather. So, to answer the question from the previous paragraph, for some purposes it may indeed be relevant to focus on impacts that are a consequence of extreme weather. This point features in reports from the Intergovernmental Panel on Climate Change (IPCC), the international body tasked with assessing current understanding of anthropogenic climate change in order to inform the UNFCCC process. In its 2001 report, the IPCC identified five “reasons for concern” (RFCs), each “consistent with a paradigm that can be used … to help determine what level of climate change is dangerous” (Smith et al., 2001, p. 915). These RFCs have continued to provide synthesizing structure through to the most recent reports (Cramer et al., 2014; Hoegh‐Guldberg et al., 2018; Oppenheimer et al., 2014; Smith et al., 2009). One of these RFCs is the relationship between anthropogenic climate change and risks associated with extreme weather events.

      In keeping with the use of the RFCs as summary measures for informing the UNFCCC process, this chapter focuses on understanding how synthesis assessments might provide status updates on risks associated with extreme weather events. In particular, the chapter will concentrate on understanding the detection and attribution of recent impacts, that is, evaluating the combined evidence from monitoring and system understanding, including their comparison, in order to document how anthropogenic emissions have already affected various aspects of human, managed, and natural systems around the world via extreme weather. A benefit of the focus on detection and attribution is that it highlights the role of monitoring. Implications for predicting future changes in risk will be discussed at the end, including the role of continued documentation of impacts for monitoring progress toward the UNFCCC objective. One thing to note at this point, though, is that analysis of the past considers impacts, that is, the outcomes of certain risks, whereas in the future we can consider only the risks themselves. For simplicity, in this chapter we will tend to consider impacts, outcomes, and risks to be different facets of the same thing.

      The chapter consists of three further sections. The next (second) section will examine various steps involved in generating a synthesis assessment, particularly focusing on challenges. The third section will then review the single existing synthesis assessment of past changes in risk associated with extreme weather. That assessment was conducted as part of the chapter on “Detection and Attribution of Observed Impacts” in the IPCC Fifth Assessment Report (Cramer et al., 2014) in order to document current understanding of the “risks associated with extreme weather events” (their section 18.6.4). Other synthesis approaches will also be mentioned, but as yet they have not been applied to the specific topic of the impacts of extreme weather. The final section will describe implications for predicting future global, cross‐sectoral, extreme‐weather‐related risk.

1.2. OF TRUTHS AND TRIVIALITIES

      Niels Bohr, one of the pioneers of quantum mechanics, used to say that it was the task of science to reduce deep truths to trivialities (Pais, 1991). When it comes to informing climate policy, however, the opposite might be a more useful dictum. A substantial component of current disagreement over the impacts associated with extreme weather events comes from a lack of clarity over what is meant by impacts of extreme weather events. This means that trivialities about natural hazards, such as that more intense hurricanes have the potential to induce more damage than do weaker hurricanes, are often taken as truths about impacts of climate change. But the truth is a much more complicated amalgam of weather hazard, policy, economics, community organization, and just plain luck. Understanding this truth will be easier if we clarify exactly what question interests us, what possible tools we have for exploring that question, and what challenges we face in applying those tools. This section discusses some of these issues.

      1.2.1. Weather Extremes or Impact Extremes?

We will start first with the distinction between weather and impacts (of weather). Although the distinction is generally commonly understood for long‐term impacts of long‐term climate changes, this is not the case with extremes. Extreme weather is often confused with natural hazards. For instance, in its review titled Attribution of Extreme Weather Events in the Context of Climate Change, the US National Academy of Sciences in fact considered natural hazards including floods and wildfires (National Academies of Sciences, Engineering, and Medicine, 2016). However, in the most recent IPCC assessment report, floods and wildfires are considered to occur outside of the climate system in the hydrological and ecological systems, respectively (Cramer et al., 2014; Settele et al., 2014).

      In this chapter we will distinguish between “extreme weather events” and, for lack of a better term (Cramer et al., 2014), “extreme impact events.” We will consider an “extreme weather event” to be any event in the climate system that is episodic in nature and is far from average in some standard