Attribution methods related to past and projected changes, and to extreme events

Over recent decades, there has been impressive progress in the ability to detect and attribute the influence of external forcing, and particularly anthropogenic forcing, on the climate system. This includes temporal changes in a wide variety of weather, climate and ocean extremes, as well as in the occurrence of individual extreme events. This is opening up opportunities to operationalise extreme event attribution. The increasing maturity of event attribution methodologies and the growing recognition of the role of compound extremes in high-impact events have together resulted in classical event attribution now being complemented by methods which operationalise a new compound event attribution framework. Another significant methodological advance now makes it possible to identify major changes in the probability, predictability and persistence of atmospheric circulation patterns associated with selected extreme events. In order to assist the public, planners, decision makers and other end users to better appreciate the role of anthropogenic drivers of changes in extremes, and act on this understanding, considerable effort has also gone into developing clear and inclusive language with which to communicate the specific findings of an attribution study, including the associated scientific uncertainties. Multiple challenges still limit the ability to provide rigorous attribution statements. These include the availability of observations of sufficient quality and duration, the limited ability to understand and hence model the dynamic and thermodynamic processes involved in the generation of extreme events and in the ability to characterise the natural variability of extremes as well as the statistical properties of unprecedented extreme events.