The answer scientists gave for years was that greenhouse gases created by humans likely contributed to extreme weather, but it was hard to definitively tie the warming atmosphere to any single episode.
But that cautious approach, repeated in thousands of news reports for more than a decade, has been changing in recent months. Now, scientists say that they will increasingly be able to link extreme weather events to human-caused global warming and to make such determinations quickly, sometimes within days.
So when a heat wave beset Northern Europe early this summer, bringing temperatures in Scandinavia into the 90s, a consortium of researchers operating under the name World Weather Attribution whipped together a series of computer simulations. Within three days, the scientists issued a finding that the hot spell had been made at least twice as likely because of human-driven climate change.
In less frequent instances, scientists taking more time have reached even bolder conclusions — finding that some extreme events would not have happened at all in a pre-industrial era, when Earth's atmosphere had not been pumped full of carbon dioxide.
The trend promises to become even more pronounced in the coming years, because national weather agencies in countries like Germany and Australia, and the weather service for the European Union, expect to begin issuing regular findings on whether unusual weather events grew out of climate change.
“Usually scientists have been quiet or said only that ‘This is the kind of event that we would expect to happen more often,'" said Friederike Otto, deputy director of the Environmental Change Institute at Oxford University in England. "But now we can, and will, be able to say more."
The science of attributing extreme weather to climate change
Otto is one of the leading scientists in the rapidly evolving field of extreme events attribution. The discipline is being driven by an increasing focus among academics, by better data collection worldwide and by open-source computer models that allow researchers ready access to complex climate simulations, particularly of what Earth’s temperatures likely would have looked like without the profusion of carbon dioxide and other greenhouse gases over the last century.
Many of the researchers in the field are determined to ensure that experts, not amateurs, drive the discussion of unusual weather. “If the answer is not given by scientists, it will be given by politicians or someone with an agenda,” Otto said. “We want to make sure there is scientific evidence in this debate.”
Martin Hoerling, a scientist at a National Oceanic and Atmospheric Administration lab in Boulder, Colorado, said another factor is influencing the more definitive statements about the impact of global warming. “That signal from climate change is becoming larger, large enough to be detected in the data itself," Hoerling said, "and also in the computer models” that extrapolate on that data.
But scientists say they remain uncomfortable with more definitive statements, such as the question of whether global warming caused a string of wildfires, or a deluge of rain or a particular heat wave.
“The thing we are trying to do is not to give you a binary ‘yes’ or ‘no’ answer,” said Stephanie Herring, a NOAA scientist who edits a report on attribution studies that has been published annually since 2012. “We are trying to show how much of an impact climate change is having now and to suggest how much of an impact it might have in the future.”
A couple of analogies help provide a better way for people to understand the role of climate in extreme events, said J. Marshall Shepherd, a professor of geography and atmospheric science at the University of Georgia: Climate is like fertilizer to grass. The grass would grow anyway, just faster with the added nutrients. Or climate is like the steroids taken by a baseball player, who would have been a big hitter anyway, but who slugs more home runs because of the artificial enhancement.
How climate and weather modeling works
Scientists who conduct attribution studies are reliant on computer modeling. They take a finite event, such as a temperature spike of 100 degrees that lasts more than a week in a single location. They then design a computer simulation, altering key factors to create a “counter-factual.” One key alteration: setting carbon dioxide levels at the roughly 280 parts per million that existed in the atmosphere on pre-industrial Earth, compared to the current 400 parts per million.
They then run thousands of computer simulations, each offering slight alterations in conditions, to try to mimic Earth’s climate. That shows how often 100-degree-plus days would show up in these pre-industrial conditions. The result is compared with the recent, real-world measurements.
Researchers said they are most adept at measuring the climate change impact on events where the historical record is most complete and the dynamics of the episode are most clear — such as heat waves, droughts and heavy rains. They are less confident in quantifying the role of climate change in more complex incidents that also have sketchier historical records, such as wildfires, tornadoes and thunderstorms.
More results, faster
Over the last six years, the American Meteorological Society has published 131 attribution studies in its annual reports. About 65 percent of those identified climate change as playing a role in extreme events, while the rest found global warming’s role was not appreciable, or detectable.
Last year, for the first time, the annual report concluded that three events would not have happened without human-caused global warming: the overall increase in temperatures globally, a heat wave that beset Asia and a “blob” of warm ocean that settled in the Bering Sea near Alaska.
“While remarkable, these results are not surprising," Herring wrote in describing the research. "Scientists have long predicted we would eventually reach a point where human-caused climate change altered Earth's system to such a degree that we would begin to see weather and climate events that would not have been possible without human contributions."
Those American Meteorological Society papers all went through rigorous peer reviews that lasted up to a year. But researchers say more rapid rulings are now not only possible, but advisable. They argue that results need to be produced during the time when the public and decision-makers are paying attention. And they say that peer review is either unnecessary (because methodologies from earlier peer-reviewed studies are merely being repeated) or can be completed later.
“If it’s like a weather forecast and the methodology has been tested many times before, we can do this faster and provide the scientific evidence more quickly,” said Otto, who helped found the World Weather Attribution group to speed up the studies.
What's next for extreme weather science
The next major area for advancement in the field is expected to involve the impacts of extreme weather on people. In 2016, for instance, a paper in Environmental Research Letters attributed 506 of the 735 deaths in a punishing heat wave in Paris to extra heat brought on by climate change.
NOAA’s Herring is currently reviewing the hospitalizations that might be attributed to global warming during a heat wave in North Carolina. By improving understanding of global warming's impact, “we can improve preparedness and minimize the cost of these events,” Herring said.
The government-sponsored weather services in Germany and Australia aim to complete regular climate attribution reports within the next two or three years. The European Centre for Medium-Range Weather Forecasts in England is launching a pilot program to study extreme events for the European Union by 2020.
Would the United States launch such a program, via the National Weather Service, or its parent agency, NOAA? Not likely, in the current political climate.
Even when President Barack Obama was in the White House, a Republican-controlled House of Representatives in 2011 killed NOAA’s proposal to form a National Climate Service. And hostility to climate science has increased since Donald Trump became president. A Trump transition team member accused the “left” of “exploiting Hurricane Harvey to try to advance their political agenda.”
But others who support the mainstream view of climate science said it is wrong to leave voters and public officials without science's view of how climate change is impacting the weather.
“People don’t see how these extreme events in their lives are linked to climate change,” said Anthony Leiserowitz, who measures public opinions on climate issues as director of the Yale Program on Climate Change Communication. “And that shows up in our politics. If there is no connection made to climate, then there are no rewards and no punishments for politicians and the way they respond to climate change.”