Science and journalism revisit the attribution of extreme weather to climate change
DOI: 10.1063/PT.5.8170
Climatologist Heidi Cullen serves as chief scientist for Climate Central , an independent research and public reporting organization. Two weeks ago in a New York Times op-ed , she introduced a new National Academies report with a hot-topic title: Attribution of Extreme Weather Events in the Context of Climate Change.
Climatologist Patrick Michaels directs the Cato Institute’s Center for the Study of Science , which asks, “What happens if science is being systematically distorted by the incentive structure for advancement in one’s profession?” Two months ago Michaels’s Wall Street Journal op-ed “The climate snow job” mocked the attribution of extreme weather to human causes with this subhead: “A blizzard! The hottest year ever! More signs that global warming and its extreme effects are beyond debate, right? Not even close.”
Physicist John Holdren directs the federal Office of Science and Technology Policy as President Obama’s science adviser. A dozen years ago in a formal statement , he dismissed Michaels “for his shrill op-ed pieces and indiscriminate denunciations of virtually every finding of mainstream climate science.”
Mainstream climate science has long struggled with the complex and technopolitically volatile issue of extreme-weather attribution. So have journalists. Science magazine’s article on the new National Academies report recalls that the “first major study that tried to link a specific weather event to climate change was published in Nature in 2004.” Science‘s Warren Cornwall continued:
That analysis found that global warming had at least doubled the chances of a 2003 heat wave associated with more than 70,000 deaths in Europe. To make the link, researchers relied on historic temperature records and computerized climate simulations. The simulations allowed the scientists to compare how different levels of warming gases affected the risk of heat waves. Since then, a string of similar studies have linked climate change to heat waves in Russia and Australia, several rounds of flooding in the United Kingdom, and a Texas drought, among other weather events.
The National Academies report describes itself:
As climate has warmed over recent years, a new pattern of more frequent and more intense weather events has unfolded across the globe. Climate models simulate such changes in extreme events, and some of the reasons for the changes are well understood. Warming increases the likelihood of extremely hot days and nights, favors increased atmospheric moisture that may result in more frequent heavy rainfall and snowfall, and leads to evaporation that can exacerbate droughts.
Even with evidence of these broad trends, scientists cautioned in the past that individual weather events couldn’t be attributed to climate change. Now, with advances in understanding the climate science behind extreme events and the science of extreme event attribution, such blanket statements may not be accurate. The relatively young science of extreme event attribution seeks to tease out the influence of human-cause climate change from other factors, such as natural sources of variability like El Niño, as contributors to individual extreme events.
Event attribution can answer questions about how much climate change influenced the probability or intensity of a specific type of weather event. As event attribution capabilities improve, they could help inform choices about assessing and managing risk, and in guiding climate adaptation strategies. This report examines the current state of science of extreme weather attribution, and identifies ways to move the science forward to improve attribution capabilities.
The report’s conclusions chapter makes clear that although the field has seen “great strides,” it’s still emerging and requires continued research. The chapter discusses factors causing variation in confidence concerning attribution results. It emphasizes that “a definitive answer” to the perennial question of whether a specific weather event was caused or affected by climate change “cannot usually be provided in a deterministic sense, because natural variability almost always plays a role.” The chapter cautions that “unambiguous interpretation of an event attribution study” is a tricky business.
Media discussions of the report have generally acknowledged the tentativeness and the caveats, but reporters and commentators have also joined National Academies study chairman David Titley in asserting that great strides have indeed been made and that something new has begun to appear. Under the headline “Efforts to link climate change to severe weather gain ground,” Cornwall in Science reports that over “the last dozen years, scientists have gone from saying it is impossible to link climate change to any single bout of bad weather to confidently declaring that, in certain cases, they can.” He quotes Titley: “It is now possible to estimate [the] influence of climate change on some types of specific extreme weather events and in particular, heat and cold events, drought, and precipitation.”
At least one passage in the report, from the end of the opening summary chapter , unambiguously supports Titley’s declaration:
In the past, a typical climate scientist’s response to questions about climate change’s role in any given extreme weather event was “we cannot attribute any single event to climate change.” The science has advanced to the point that this is no longer true as an unqualified blanket statement. In many cases, it is now often possible to make and defend quantitative statements about the extent to which human-induced climate change (or another causal factor, such as a specific mode of natural variability) has influenced either the magnitude or the probability of occurrence of specific types of events or event classes. The science behind such statements has advanced a great deal in recent years and is still evolving rapidly.
USA Today quoted report contributor Marshall Shepherd, a University of Georgia meteorologist, who sees the report as establishing a “gold standard” for attribution science.
Report contributor Adam Sobel published an online Washington Post commentary . “A new area of scientific research, known as ‘extreme event attribution,’ has emerged to provide more substantive and quantitative answers,” he wrote. “Our science has reached the point where we can look for the human influence on climate in single weather events, and sometimes find it.” Also at the Post, science writer Chris Mooney published an article under the headline “It’s official: We can now say global warming has made some weather events worse.”
In an AP piece , science writer Seth Borenstein began, “Climate science has progressed so much that experts can accurately detect global warming’s fingerprints on certain extreme weather events, such as a heat wave, according to a high-level scientific advisory panel.” Borenstein quoted two professors well known in public discussions of climate:
* Michael Oppenheimer from Princeton University: “The fog of uncertainty that obscured the human role in individual events is finally lifting.”
* Katharine Hayhoe from Texas Tech: It is “very important to clarify to people how no event is black or white: No individual weather event can be said to be ‘solely’ human nor ‘solely’ natural any more. They are somewhere on the sliding scale between.”
The Chinese news agency Xinhua quoted a report contributor:
“Scientists used to say that we can’t attribute any one event to climate change,” said Philip Mote, an Oregon State University (OSU) climatologist and co-author on the report.
“But that is a copout. Every extreme weather event has the fingerprint of climate change. The question is not whether global warming caused Hurricane Sandy; but rather how much stronger it was because of global warming.”
“There is little doubt that Hurricane Sandy would have had less impact without climate change,” Mote said.
Scientific American‘s article began, “Scientists can now say with confidence whether heat waves, such as the one that struck Russia in 2010 and caused 55,000 deaths, are linked to climate change.”
Cullen in her Times op-ed declared that the National Academies report “has outlined a rigorous, defensible, science-based system of extreme weather attribution to determine which events are tied to climate change.” Then she elevated it to historical status: “Like the surgeon general’s 1964 report connecting smoking to lung cancer, the report ... connects global warming to the increased risk and severity of certain classes of extreme weather, including some heat waves, floods and drought.”
She continued: “Climate change can no longer be viewed as a distant threat that may disrupt the lives of our grandchildren, but one that may be singled out as a factor, possibly a critical factor, in the storm that flooded your house last week. The science of extreme weather attribution brings climate change to our doorsteps.”
Cullen reported that “one recent study found that an extreme heat wave last May in Australia was made 23 times more likely because of climate change. When the numbers get that big, it’s fair to say that some episodes of extreme heat would have been virtually impossible (but never absolutely impossible) without climate change.”
She also offered what she called “an example that underscores the predictive power of extreme event attribution":
A recently published study in the journal Nature Climate Change analyzed record-breaking rains in Britain that flooded thousands of homes and businesses and caused more than $700 million in damage in the winter of 2013-14. Scientists found that such an event had become about 40 percent more likely. As a result, roughly 1,000 more properties are now at risk of flooding, with potential damage of about $40 million.
Climate-change-attributable dislocations and property losses? Cornwall at Science conjectured about “the possibility that attribution science could end up in the courtroom, as those harmed by climate-driven weather try to extract damage payments from those who produce greenhouse gases.” He reported that some countries, “such as small island nations threatened by sea-level rise, are already pressing for financial support from industrialized nations responsible for the bulk of greenhouse gas emissions. Those demands include compensation for extreme weather, too.” He proposed that architects, planners, and engineers “could be hit with lawsuits over whether they properly considered climate change when designing infrastructure such as flood-control levees.” He also cited “the possibility of pursuing weather-related damage claims against major carbon polluters, such as fossil fuel companies.”
Cornwall summed up his litigation subtopic with this anecdote: “In 2015, two attorneys for the London-based environmental firm ClientEarth took to the pages of Nature Geoscience to offer this advice to climate attribution scientists: ‘Be prepared to testify as an expert witness.’”
But even given the tentativeness and the caveats accompanying this new stage of attribution science, not everybody is persuaded. Georgia Tech climatologist Judith Curry wrote in a recent blog post, “I remain highly critical of efforts to attribute extreme weather events to human caused climate change.” She added, “I find nothing in this report that convinces me that extreme event attribution is feasible.”
Curry closed by repeating something she wrote five years ago:
Not sure what the motive is for the attribution of extreme events, other than to build political will for climate change policies. More comprehensive analysis of regional extreme events (including those in the paleo records, of which we need more) in the context of known modes of natural climate variability is probably the single most useful thing that could be done in this regard.
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Steven T. Corneliussen, a media analyst for the American Institute of Physics, monitors three national newspapers, the weeklies Nature and Science, and occasionally other publications. He has published op-eds in the Washington Post and other newspapers, has written for NASA’s history program, and was a science writer at a particle-accelerator laboratory.