Extreme events: trends and risk assessment methodologies

In this chapter we discuss extreme weather events as they relate to climate adaptation. First, we summarize the state of the science regarding how different kinds of extreme events have been or will be influenced by global warming. Second, we describe the different kinds of tools that exist for extreme event hazard assessment. These include extreme value theory, which allows inference of rare event statistics from observational records too short to resolve them; “catastrophe models” developed for the insurance industry; and dynamical models developed for climate science. Third, we describe how these different tools may be relevant to different activities that might fall under the broad rubric of “climate adaptation science”. We advocate a pragmatic approach, recognizing that anthropogenic climate change is one of multiple factors influencing extreme event hazard, and that, when the most extreme events are considered, much human settlement and infrastructure is inadequately adapted even to the historical climate. Impact of climate change on extremes There is no simple statement which accurately describes the state of the science on how extreme weather events respond to climate change. Statements such as “climate change is making weather more extreme” are oversimplifications with the potential to be misleading. The statement in quotations above does carry two truths, however. First, much of the damage from climate change will be felt through changes in extreme events. Second, for many kinds of events, despite the uncertainties, current science justifies a legitimate concern that the risk – a probabilistic concept that can include scientific uncertainty as well as other forms – is increasing. But the dependence of extreme events on climate is substantially different for different types of events, as is our degree of scientific knowledge and understanding. In this section we give a brief summary of the current understandings regarding a subset of extreme event types. The recent review by IPCC (2012) remains relevant and can be consulted for more details. The more recent National Academy (2016) report addresses individual extreme event attribution the science of making quantitative statements about how different causes, including anthropogenic global warming, contribute to specific individual events but contains a wider-ranging discussion illustrating the broader point that our confidence in our understanding of the human influence on any event type is affected by many factors, including the quality and length of observational records, the quality of numerical models in simulating and predicting that event type, etc. Fig. 1, adapted from that report, provides a graphical assessment of the state of attribution science for different types of extreme events, as explained briefly in the caption and in more detail in the report itself. More broadly, the National Academy (2016) report emphasizes that our degree of scientific understanding of different types of events’ relation to warming is greatest for events most closely related to atmospheric temperature, since temperature is the variable in which greenhouse gas influence is first and most directly felt. Thus the human influence is most clear on heat waves and cold snaps, as indicated by their position above and to the right of other event types in Fig. 1.