Attribution of Extreme Weather Events in the Context of Climate Change (2016) / Chapter Skim
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This emerging area of science also has drawn the interest of the public because of the frequently devastating impacts of the events that are studied. This is reflected in the strong media interest in the connection between climate change and extreme events, and it occurs in part because of the potential value of attribution for informing choices about assessing and managing risk and in guiding climate adaptation strategies.
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Some of the study methods are still relatively novel, however, and there are a range of views about how to conduct and interpret the analyses. This report examines the science of attribution of specific extreme weather events to human-caused climate change and natural variability3 by reviewing current understanding and capabilities.
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Weather and climate model-based approaches to extreme event attribution compare model-simulated weather and climate phenomena under different input conditions: for instance, with and without human-caused changes in GHGs. Many studies rely on coupled atmosphere-ocean climate models, while others may use global atmospheric models, regional models, or models that are constructed specifically to represent a particular class of weather events, such as hurricanes.
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Some recent studies also have used models to attempt to follow the evolution of a particular extreme weather event -- for example, through the use of a set of shortterm forecasts using a weather model. This allows detailed study of particular extreme events with a model capable of representing those specific events with fidelity and quantification of the effect of certain aspects of climate change (e.g., increased m oisture-holding capacity of a warmer atmosphere)
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In general, confidence in attribution results is strongest for extreme event types that • have a long-term historical record of observations to place the event in an appropriate historical context; 5
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against frequency of occurrence, while colored dots show the return times in a world that might have been, constructed by removing the pattern of human influence on sea surface temperatures (SSTs) from four different climate models: HadCM3 (brown, a)
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Furthermore, confidence in attribution results that indicate an influence from anthropogenic climate change is strongest when there is an understood and robustly simulated physical mechanism that relates a given class of extreme events to longterm anthropogenic climate changes such as global-scale temperature increase or increases in water content of a warmer atmosphere. More frequent occurrences of extreme heat and less frequent occurrences of extreme cold are examples of changes that are consistent with increasing global mean temperatures.
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The vertical position of each event type indicates an assessment of scientific confidence in current capabilities for attribution of specific events to anthropogenic climate change for that event type, which draws on all three columns of Table S.1. A position below the 1:1 line indicates an assessment that there is potential for improvement in attribution capability through technical progress alone (such as improved modeling, or the recovery of additional historical data)
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Understanding of Physical Mechanisms = high Capabilities of Climate That Lead to Changes in = medium Models to Simulate Quality/Length of the Extremes as a Result of = low Event Type Observational Record Climate Change Extreme cold events Extreme heat events Droughts Extreme rainfall Extreme snow and ice storms Tropical cyclones Extratropical cyclones Wildfires Severe convective storms 9
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Gaps in understanding and limitations in the historical data lead to differences in confidence in attribution of specific events among different event types. Attribution of events to anthropogenic climate change may be complicated by low-frequency natural variability, which influences the frequencies of extreme events on decadal to multidecadal timescales.
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For example, it may be possible to make confident statements about how some class of extreme events is expected to change because of humaninduced climate change, while at the same time an attribution study of an individual event of that type may be unable to make a confident statement about the human influence on that one specific event. Thus, for all of these reasons, counts of available attribution studies with any positive, negative, or neutral results are not expected to give a reliable indication of the overall importance of human influence on extreme events.
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• Statements of confidence accompany results so users understand the strength of the evidence. Bringing multiple scientifically appropriate approaches together, including multiple models and multiple studies helps distinguish results that are robust from those that are much more sensitive to how the question is posed and the approach taken.
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• Clear communication of remaining uncertainties and assumptions made or conditions imposed on the analysis. THE PATH FORWARD Improving Extreme Event Attribution Capabilities Continued research efforts are necessary to increase the reliability of event attribution results, particularly for event types for which attribution is presently poorly understood.
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; and • the representation of a counterfactual world that reliably characterizes the probability, magnitude, and circumstances of events in the absence of human influence on climate. Research efforts targeted specifically at extreme events, including event attribution, could rapidly improve capabilities and lead to more reliable results.
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Systematic criteria for selecting events to be analyzed would minimize selection bias and permit systematic evaluation of event attribution performance, which is important for enhancing confidence in attribution results. Studies of a representative sample of extreme events would allow stakeholders to use such studies as a tool for understanding how individual events fit into the broader picture of climate change.
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The activity would involve rigorous approaches to managing and implementing system enhancements to continually improve models, physical understanding, and observations focused on extreme events. Although situating some future event attribution activities in an integrated weather-to-climate forecasting effort would lead to more coordination, the committee encourages continued research in event attribution outside of an operational context to ensure further innovation in the field.
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Summary meet the information needs of stakeholders. Further improvement will depend not only on addressing scientific problems specific to attribution but also on advances in the basic underlying science, including observations, modeling, and theoretical understanding of extreme events and their relation to climate change.
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