Hesitance to Link Some Weather Events to Climate Change 'No Longer Appropriate'

Fast study showed climate change's influence on Louisiana's devastating floods. The method is being used to more quickly assess some types of extreme weather.

Downpours in Louisiana in August destroyed 60,000 homes.

The downpours in Louisiana damaged 60,000 homes and tested scientists' ability to quickly determine climate change's role in the disaster. Credit: Reuters

The death and destruction from three days of relentless rain in Louisiana were just coming into grim focus when a team of international climate scientists started discussing whether they could quickly assess the downpour's link to climate change.

By the time their study was published less than a month later, concluding that climate change made such an extreme rain event at least 40 percent more likely in the central Gulf Coast area in 2016 compared to 1900, the storm's staggering toll was clear. At least 13 people were killed and more than 60,000 homes damaged. More than 140,000 people have applied for federal aid, according to FEMA, and 20 Louisiana parishes have been declared federal disaster areas.

In the month's wait for the climate change verdict, plenty of people weighed in ahead of the science. Former Vice President Al Gore said the Louisiana floods were impacted by man-made climate change. Bill Nye the Science Guy agreed. Several scientists weighed in, too, explaining how the event had some clear climate fingerprints. (Others known to deny that climate change exists or downplay its severity dismissed any such association.)

That's why researchers with the group World Weather Attribution (WWA), along with scientists at the National Oceanic and Atmospheric Administration (NOAA), are among those trying to determine climate change's role in extreme weather as fast as possible.

"What we are trying to do is provide a first best quantitative answer to the question: 'Did climate change play a role?' Because otherwise there's a lot of speculation," Heidi Cullen, chief scientist at Climate Central and a member of the WWA team, told InsideClimate News.

For years, scientists hesitated to link any extreme weather event to climate change. But that's changed in the last decade due to the emerging field of attribution science, which uses observational data, climate models or both to calculate how specific weather events are impacted by all the extra heat trapped in the Earth's atmosphere. The world is about 1 degree Celsius hotter than it was in 1880, which may seem barely perceptible, but it creates tremendous energy spread across the entire surface of the planet. Its complex impacts include creating extra moisture that can fuel weather extremes like severe downpours.

"We are at a point now where it's no longer appropriate to say you can't attribute an individual event to climate change," said Cullen.

That was also the takeaway of a review of attribution studies published in March, conducted by the National Academies of Sciences, Engineering and Medicine. According to that study, scientists have the most confidence in determining possible climate impacts on temperature extremes, such as heat waves, and moderate confidence in assessing droughts and precipitation events. The WWA group has limited its studies so far to these three types of events.

Most attribution studies take months to years to complete, as they go through an extensive peer-review process before being submitted for journal publication. That long wait strikes Cullen and others as too long to influence the way a community builds back from a particular storm or prepares for the next heat wave.

Cullen helped form WWA in 2014 with the goal of doing attribution studies fast. The other climate experts in the coalition come from the University of Oxford in England, the University of Melbourne in Australia, the Netherlands-based Red Cross Red Crescent Climate Centre and the Royal Netherlands Meteorological Institute. For each study, WWA teams up with outside researchers who have expertise in local science, data or modeling.

"This is all still fairly cutting-edge stuff, but there is this interest in doing more [standardized] attribution," said Cullen, adding that WWA members "felt we could begin to lay out a process for that."

While WWA was one of the first research teams to try rapid attribution, and it remains one of the only teams to conduct these analyses consistently, interest in the field is growing. "I think it's going to become like weather forecasting––done routinely and maybe in operational centers," said Adam Sobel, a Columbia University professor who contributed to the National Academies' report.

WWA so has far completed four rapid studies. The team's first assessment, which took a little more than a week, was for a heat wave in Europe last July. It concluded such extreme multi-day events are now four to eight times more likely in certain cities such as Madrid and Zurich because of climate change.

Each time, the team updates its process. ("We haven't made any mistake twice," joked KNMI's Geert Jan van Oldenborgh.) One of the lessons from the first analysis was to be more transparent about the study's methodology.

WWA's fastest analysis clocked in at four days and four hours when the scientists raced to analyze the possible climate fingerprints of the United Kingdom storm Desmond before the Paris climate agreement was finalized last December. "I broke a record for how fast I could write a paper," said van Oldenborgh, who quickly added that the paper should have been done more slowly and carefully.

The team's third event analysis, completed in two weeks, was also in Europe. It focused on strong rains that caused the French River Seine to flood in May. The rain triggered the removal of precious artwork from the Louvre museum in Paris and forced about 20,000 residents to evacuate their homes across France.

"The main criteria to get events on our radar is human impact," said Maarten van Aalst, a WWA team member from the Red Cross Red Crescent Climate Centre.

"Unfortunately, for now, that also means that we tend to have focused on disasters in developed countries." He said the team is working to remedy this and is aiming to do rapid analyses in East Africa (Ethiopia and Kenya) and South Asia (India and Bangladesh).

The Louisiana floods were the team's biggest test so far. It reached out to the NOAA's Geophysical Fluid Dynamics Laboratory, which had the high-resolution climate models it needed. It also constrained its study parameters to look at rainfall levels from any precipitation events (from thunderstorms to hurricanes) for the larger central Gulf Coast area.

At NOAA, scientists used two climate models to review two scenarios: one based on current climate conditions and another using 1860 conditions to simulate a world without a rise in human-caused greenhouse gas emissions and other air pollution. The researchers also reviewed a third simulation using both models, one where greenhouse gas emissions, ozone and aerosols were gradually increased over time. Meanwhile, WWA researchers performed separate analyses using observational data.

NOAA scientists quickly cranked out their analyses because they didn't need to run new model simulations; they already had past results that they could apply. It's standard practice to rely on available observational and model data and statistical methods that have previously been used for similar analyses, RNMI's Oldenborgh called this "pre-cooking" the data.

"I was very impressed with this study," said David Titley, chair of the committee that wrote the National Academies' report. He praised the multiple lines of analysis and clearly stated methodology and assumptions.

The researchers have submitted a paper on their process and results to the journal Hydrology and Earth System Sciences, where it will go through an open-peer review process.

"This is not the final answer," said Karin van der Wiel, lead author on the journal article and a NOAA scientist . "This is a big event. I expect there to be a lot more work [in the future] and that's great."

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