Weather extremes now surpassing the realm of natural possibilities

A new report from the American Meteorological Society makes a rather stunning statement about climate change. For the first time, researchers have concluded that specific weather-related events could not have happened without the influence of climate change caused by human activity.

Three events studied in 2016 were so extreme that they did not fit into the context of natural climate conditions, according to researchers working on separate projects. One involved the global heat record for 2016; another was focused on warmth across Asia; and the third was the “blob” of warm ocean water familiar to folks who follow weather in the Pacific Northwest.

A “blob” of warm water off the Northwest coast from 2013 to the end of 2016 could not have occurred without human-induced climate change, experts say.
Map: NOAA’s Earth System Research Laboratory

“This report marks a fundamental change,” said Jeff Rosenfeld, editor-in-chief of the Bulletin of the American Meteorological Society, in a news release. “For years scientists have known humans are changing the risk of some extremes. But finding multiple extreme events that weren’t even possible without human influence makes clear that we’re experiencing new weather, because we’ve made a new climate.”

Personally, I did not expect to see this sort of demonstrable statement about man-made climate change anytime soon. In classes and seminars on the subject of climate change, I’ve often seen lecturers present frequency curves that show the number of times that certain weather-related phenomena — such as temperatures or rainfall — are observed over a given time.

We’re told by climatologists that many of these curves are steadily shifting, so that fairly extreme conditions occur more often and truly extreme conditions emerge for the very first time in certain locations.

Researchers are loathe to say that a given storm, drought or hurricane is the result of climate change. They would rather say climate change affects the likelihood of extreme weather events, plotted at the end of the frequency curve. In the realm of statistics, there is a tendency to hold onto the idea that almost any kind of weather could occur almost anytime, provided that a perfect storm of conditions line up together.

Against that backdrop, comes the new report titled “Explaining extreme events of 2016 from a climate perspective,” which examines extreme weather events throughout the world.

“First, it is important to note that climate scientists have been predicting that … the influence of human-caused climate change would at some point become sufficiently strong and emergent to push an extreme event beyond the bounds of natural variability alone,” state the six editors in an introduction to the report.

“It was also anticipated that we would likely first see this result for heat events where the human-caused influences are most strongly observed,” they continue. “It is striking how quickly we are now starting to see such results, though their dependence on model-based estimates of natural variability … will require ongoing validation …”

In other words, the conclusion comes from computer models that can analyze the probability of an extreme event taking place when greenhouse gases are found at different concentrations. Results using today’s observed conditions are compared with results using conditions before the industrial release of greenhouse gases.

In the three highlighted papers, the researchers calculated the “fraction of attributable risk,” or FAR, for the extreme event they were studying. FAR is a statistical approach used in epidemiology to measure the likelihood of an event under various conditions. For explanations, see Boston University School of Public Health and the 2007 IPCC report.

“All three papers concluded that the FAR was 1, meaning that the event was not possible in the ‘control’ planet and only possible in a world with human-emitted greenhouse gases,” the editors say.

Although this is the first time that researchers have concluded that extreme events could not have happened without human-induced climate change, the editors are quick to point out that the same phenomenon may have occurred unnoticed in the past on a smaller geographic scale.

These findings do not mean that the climate has reached any kind of tipping point. It simply adds to the evidence that mounting weather extremes are not the result of natural processes.

Reporters Brad Plumer and Nadja Popovich of the New York Times do a nice job of delving into the concept of attribution science while mentioning five of the extreme events covered in the new report. They quoted Heidi Cullen, chief scientist at Climate Central, which produces news stories about climate issues.

“In 2011, people were still of the mind-set that you couldn’t attribute any individual event to climate change,” Cullen said. “But with each subsequent issue (of the BAMS report), people are able to say that climate change really is increasing the risk” that extremes will occur.

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