It has become increasingly common for the general media to say that weather-related disasters have been caused by climate change. Such headlines have largely been frowned upon by the scientific community as hyperbole because scientists could not say with confidence that any given weather-related event was the result of climate change. That’s beginning to change.

Executive Summary

AIR Worldwide's Peter Sousounis discusses approaches used to determine if classes of extreme weather events and individual historical natural catastrophe events occurred as the result of climate change or current atmospheric conditions at the time. Approaches include historical observation, the use of climate models that compare how extreme events unfold in the presence or absence of rising temperatures, and assessing explanability (the level of scientific knowledge about how global warming will affect the atmospheric processes that produce certain types of events) for classes of events. Fractions of attributable risk measure the proportion of an individual adverse event attributable to the presence of greenhouse gas emissions.

Advances in climate models and data science are being fed by a deeper physical understanding of the mechanisms that cause extreme weather events and have opened up a relatively new branch of climate change science: the science of event attribution.

Two approaches to event attribution—using observation data and climate models—are discussed below, although most studies typically use a combination of the two. A physical understanding of how weather events evolve and how a changing climate might impact that evolution is what gives us confidence in our findings.

Extreme weather events are a natural part of Earth’s climate system. We have seen them throughout recorded history, and there is no reason to believe that we’ve seen the worst even in the absence of climate change. Extreme events are, by definition, rare, so observational data for them is naturally scarce in the short historical record and we should expect—and prepare—for loss records to be broken. This is why catastrophe models include correspondingly rare events that cause losses well above those in the historical record.

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