A report by the National Academies of Science, Engineering and Medicine has concluded that it is possible to link individual extreme weather events to global warming, and provided guidelines in doing so. Attribution isn't always possible, at least with confidence, but it is no longer true to say it can never occur.
It has become almost a ritual after big weather events. Climate activists make the connection to fossil fuel emissions, noting that we can expect more of this unless we stop emitting carbon. Those hostile to action taken respond that such events have always happened, and argue no one event can be attributed to a changing climate.
It may not bother people who have been denying the science for decades, but one of the world's most prestigious scientific bodies has now stated that we can, in fact, see the hand of human action behind individual natural disasters, at least some of the time.
"An increasingly common question after an extreme weather event is whether climate change 'caused' that event to occur," said Pennsylvania State University Professor David W. Titley in a statement. "While that question remains difficult to answer given all the factors that affect an individual weather event, we can now say more about how climate change has affected the intensity or likelihood of some events."
Titley is chair of a committee that published a report on when and how such connections can be made with confidence. The 144-page document does not skimp on details.
“The observed frequency, intensity and duration of some extreme weather events have been changing as the climate system has warmed,” the report begins. “Such changes in extreme weather events have also been simulated in climate models, and some of the reasons for them are well understood.”
However, it continues, “The extent to which climate change influences an individual weather event is more difficult to determine.” Multiple approaches to attribution are discussed, including the use of climate models and comparisons with past events. The report points out that “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;
are simulated adequately in climate models; and
are either purely meteorological in nature (i.e. the event is not strongly influenced by built infrastructure, resource management actions, etc) or occur in circumstances where these confounding factors can be careful and reliably considered.”
The report concludes that the events we are most able to link to global warming are changes in the frequency of extreme cold snaps, closely followed by intense heatwaves. The lowest confidence comes with severe convective storms and wildfires.
Wildfires are among the hardest natural events to individually attribute to climate change, even though overall they are becoming more common. Tom Reichner/Shutterstock
The report stresses the importance of defining events rigorously, with duration, geographic extent, and physical variables carefully spelled out.
And the authors suggest that more consistency in definitions is needed to allow comparisons between studies, which would increase confidence, both in attribution after events, and predictions of future frequency.