As floodwaters from the swollen River Thames crept closer to the walls of Myles Allen's south Oxford home in the United Kingdom, he was thinking about climate change—and if scientists could figure out if it was affecting the climbing water outside.

It was January 2003, and as Allen—a climate expert at the University of Oxford—monitored the rising waters from the safety of his house, a voice on the radio was telling him that it couldn't be done. Sure, the flood was the type of event likely to be made more frequent by global warming, the representative of the United Kingdom's Met Office said on the show. But ascertaining anything more concrete was out of reach.

At the time, the Thames River Basin had seen some of its greatest rainfall in decades, and by early January, the flow in some parts of the river was the highest it had been since 1947.

But the radio voice added that it would be "impossible to attribute this particular event [floods in southern England] to past emissions of greenhouse gases," said Allen in a commentary published in Nature shortly thereafter.

In 2003, that was the predominant view in the scientific community: While climate change surely has a significant effect on the weather, there was no way to determine its exact influence on any individual event. There are just too many other factors affecting the weather, including all sorts of natural climate variations.

But Allen wasn't so sure.

"At the time, everybody was saying, 'Well, you can't attribute a single event to climate change,'" he said in an interview with E&E News. "And this prompted me to ask, 'Why not?'"

So he drafted his commentary as the floodwaters inched closer to his kitchen door. He wrote that it might not always be impossible to attribute extreme weather events to climate change—just "simply impossible at present, given our current state of understanding of the climate system." And if researchers were ever able to make that breakthrough, he mused, the science could potentially influence the public's ability to blame greenhouse gas emitters for the damages caused by climate-related events.

His hunch held true. Nearly 15 years later, extreme event attribution not only is possible, but is one of the most rapidly expanding subfields of climate science.

"The public stance of the scientific community about individual event attribution in the year 2000 is that it's not something that science does," said Noah Diffenbaugh, a Stanford University climate scientist and attribution expert. "And so to go from that to now, that you'll find a paper every week ... that's why we say there's been an explosion of research. It's gone from zero to 60, basically."

Over the last few years, dozens of studies have investigated the influence of climate change on events ranging from the Russian heat wave of 2010 to the California drought, evaluating the extent to which global warming has made them more severe or more likely to occur.

The Bulletin of the American Meteorological Society now issues a special report each year assessing the impact of climate change on the previous year's extreme events. Interest in the field has grown so much that the National Academy of Sciences released an in-depth report last year evaluating the current state of the science and providing recommendations for its improvement.

And as the science continues to mature, it may have ramifications for society. Legal experts suggest that attribution studies could play a major role in lawsuits brought by citizens against companies, industries or even governments. They could help reshape climate adaptation policies throughout a country or even the world. And perhaps more immediately, the young field of research could be capturing the public's attention in ways that long-term projections for the future cannot.

"I think the public and many policymakers don't really take those 100-year forecasts very seriously," said Allen, who is now one of the world's leading experts in attribution science. "They are much more seriously interested in the question of what is happening now and why—which boils down to attribution."

The birth of a new field

Less than two years after his flood-inspired commentary, Allen was publishing another paper in Nature — this time, one that would help turn extreme event attribution from a scientific impossibility to a burgeoning new field.

In 2004, he and Oxford colleague Daithi Stone and Peter Stott of the Met Office co-authored a report that is widely regarded as the world's first extreme event attribution study. The paper, which examined the contribution of climate change to a severe European heat wave in 2003—an event which may have caused tens of thousands of deaths across the continent—concluded that "it is very likely that human influence has at least doubled the risk of a heat wave exceeding this threshold magnitude."

Before this point, climate change attribution science had existed in other forms for several decades, according to Diffenbaugh. Until 2004, much of the work had focused on investigating the relationship between human activity and long-term changes in climate elements like temperature and precipitation. More recently, scientists had been attempting to understand how these changes in long-term averages might affect weather patterns in general.

The breakthrough paper took the existing science a step further. Using a climate model, the researchers compared simulations accounting for climate change with scenarios in which human-caused global warming did not exist. They found that the influence of climate change roughly doubled the risk of an individual heat wave. The key to the breakthrough was framing the question in the right way—not asking whether climate change "caused" the event, but how much it might have affected the risk of it occurring at all.

Despite a reluctance to attempt this type of research, the response from other scientists was "not particularly controversial," according to Allen. Instead, he said, "much of the reaction was more along the lines of that it was kind of obvious."

Since then, interest in extreme event attribution has continued to grow—slowly at first, and now with increasing momentum. According to last year's National Academy of Sciences report, "An indication of the developing interest in event attribution is highlighted by the fact that in 4 years (2012-2015), the number of papers increased from 6 to 32."

According to Friederike Otto, an attribution expert at the University of Oxford, the progression of technology—namely, the improvement of climate models—is driving the recent surge.

"Extremes are, by definition, rare," she told E&E News. So in order to actually get one to crop up in a simulation, models need to accurately represent the physical factors that help these extremes occur, and researchers need to be able to run them over and over again. The development and improvement of climate ensembles—large groups of slightly different climate models—have improved scientists' ability to simulate weather events under different conditions.

"I think it's not so much that the philosophy changed, but that the technology has changed," Otto said.

Asking the right questions

In 2010, a record-breaking heat wave swept through Russia, driving temperatures in some places above 100 degrees Fahrenheit. According to some estimates, the extreme temperatures contributed to the deaths of more than 50,000 people.

Two separate studies attempted to quantify the influence of climate change on that event and appeared to come to very different conclusions, inspiring a confusing series of headlines in the news. One research paper, published in Geophysical Research Letters, suggested that the heat wave was mainly the product of natural climate variations, while the other, in Proceedings of the National Academy of Sciences, claimed that human-caused climate change was a major factor.

"That, of course, sounded as if they were contradictory," said Otto, the Oxford attribution expert. For a brief time, scientists were bemused—the two sets of findings had to be at odds with one another.

But in a separate paper, published in 2012 in Geophysical Research Letters, Otto, Allen and several other colleagues demonstrated that the two studies were actually investigating two different questions—and their conclusions were compatible.

The first study, they found, explored the extent to which climate change had affected the heat wave's magnitude, or severity, and concluded that natural climate variations were mainly accountable. The second had investigated global warming's influence on the heat wave's overall probability of occurring. It's possible for climate change to have a significant effect on one factor, but not the other, for the same event, Otto and her colleagues pointed out.

Today, scientists still generally agree that it's impossible to attribute any individual weather phenomenon solely to climate change. Storms, fires, droughts and other events are influenced by a variety of complex factors. And they're all acting at once, including both natural components of the climate system and sometimes unrelated human activities. For instance, a wildfire may be made more likely by hot, dry weather conditions, and by human land-use practices.

But what scientists can do is investigate the extent to which climate change has influenced a given event. Generally, researchers do this with the help of climate models, which allow them to run simulations accounting for the influence of climate change alongside simulations that assume that climate change did not exist. Then they compare the outcomes. The focus is typically on highly unusual or even unprecedented events where the influence of human-caused climate change, as opposed to natural climate variability, is likely to be clearer.

Certain types of events lend themselves to analysis better than others. For instance, researchers have high confidence when investigating heat waves, droughts or heavy precipitation. But they have less confidence when it comes to hurricanes and other more complex phenomena.

Still, scientists are investigating all kinds of weather events. The special issue of the Bulletin of the American Meteorological Society issued last month included about two dozen papers on a variety of extreme events from 2016, ranging from snowstorm Jonas to the heat-induced bleaching of the Great Barrier Reef.

It also contained some surprises: Three papers, for the first time in the Bulletin's history, suggested that the studied events not only were influenced by climate change but could not have occurred without it (Climatewire, Dec. 14, 2017). The studies determined that the record-breaking global temperatures in 2016 (the hottest year ever recorded), extreme heat in Asia and an unusually warm "Blob" of water off the coast of Alaska would all have been impossible in a world where human-caused climate change did not exist.

Scientists have cautioned that the findings don't necessarily overturn the existing narrative that no single event can be attributed to climate change. Even events that would not have been possible without warming are still influenced by the Earth's natural climate and weather systems. But the research does make it clear that the planet has reached a new threshold in which climate change has become not only a component of extreme weather events but an essential factor for some.

As scientists continue to investigate the weather and climate events that reflect the changing planet, the two questions asked by the Russian heat wave studies—one focusing on probability, and the other on magnitude—have emerged as two main approaches used in attribution studies. The probability approach is perhaps most significant from a policy perspective, Otto suggested, because it helps identify the types of events that might become more common in the future and where they may occur.

The second method, sometimes called the "anatomy of an extreme event," advances scientists' understanding of the components that cause these events, and how changes to the climate system may affect them.

Both approaches are strengthening the body of evidence that climate change can influence the kinds of damaging weather events formerly thought of as "natural" disasters. As a result, some experts now believe that extreme event attribution could be the cutting edge not only of climate science but of climate litigation, as well.

New frontiers

In the aftermath of Hurricane Katrina, which devastated the Mississippi and Louisiana shorelines in 2005, residents of the U.S. Gulf Coast felt that Mother Nature wasn't the only one to blame for the damage. A month after the storm struck, a group of citizens filed a lawsuit, Comer v. Murphy Oil, against a group of oil and energy companies for releasing greenhouse gas emissions. The plaintiffs said the emissions had contributed to climate change, which intensified the hurricane's effects.

After an unusual series of legal maneuverings, including dismissals, appeals, reversals and recusals, the case was ultimately dismissed (Greenwire, June 1, 2010). It never went to trial. But the message was clear: The public is paying attention to the links between climate change and harmful extreme weather.

Allen, the Oxford scientist, had hinted at such litigation two years before Hurricane Katrina occurred. In his Nature commentary, he mused about the possibility of massive class-action lawsuits—carrying the potential for "up to six billion plaintiffs" around the world—attempting to hold greenhouse gas emitters liable for damages.

Attribution studies could help to apportion the blame, he noted. For instance, he wrote, "If, at a given confidence level, past greenhouse-gas emissions have increased the risk of a flood tenfold, and that flood occurs, then we can attribute, at that confidence level, 90% of any damage to those past emissions."

More recently, other legal experts have suggested that the fossil fuel industry isn't the only player at risk of being sued over climate-related damages. In the future, attribution studies could become evidence in cases against governments or private companies for failing to protect property, or public infrastructure, against extreme weather in a warming world.

In a recent paper published in August in the journal Nature Geoscience, legal experts from the United States and the United Kingdom argued, "Improvements in attribution science are affirming the foreseeability of certain climatic events and patterns in specific locations, and in identifying increasing risks of consequential impacts on property, physical assets and people." As a result, they wrote, attribution studies may inspire an increase in climate change litigation in the future.

Citizens have already sued other parties for failing to protect them from natural disasters, even if they don't specifically blame climate change. In another post-Katrina case, for instance, a class-action case claimed that the U.S. Army Corps of Engineers, and other parties involved in the planning and construction of Louisiana's levees, should be held accountable for the levees' failures. In that case, a settlement was reached—and as with the case brought against the energy companies, it's unclear how the lawsuit might have panned out had it gone to court.

Now, more than 12 years after Katrina, with a growing stack of studies pointing to the link between climate change and damaging weather events, experts are warning that these types of lawsuits may become more commonplace. The idea is that if decisionmakers are aware that climate change can make certain events more frequent or more severe, they may be held legally responsible for failing to prepare for the worst.

As a general rule, extreme event attribution studies don't predict the likelihood of a future event. They focus on how climate change has affected events that have already happened. Even the National Academy of Sciences report warned, "Attribution studies of individual events should not be used to draw general conclusions about the impact of climate change on extreme events as a whole."

But from a legal standpoint, the studies' implications could be broader. Lindene Patton, a member of the legal team at the Earth &Water Group and one of the authors of the Nature Geoscience paper, noted that as more and more attribution studies are released, they could accumulate a body of evidence suggesting that events occurring today are being influenced by climate change—meaning governments and businesses should be prepared for similar events in the future.

"When the science changes, when a body of knowledge to which a responsible professional is expected to keep up with and understand and pay attention to—when that changes, it changes what they have to do to protect people," she told E&E News. "It changes the standard of care."

Michael Burger, executive director of Columbia Law School's Sabin Center for Climate Change Law, cautioned that the field likely has some serious maturing to do before it becomes a major tool for climate litigation. There's no standardized method for conducting all attribution studies, he noted. Different research groups tend to use different models, ask different questions or use different criteria for selecting the events they investigate, making individual analyses difficult to compare.

"In court, expert testimony doesn't need to reflect a consensus view," Burger said. "It does need to be based, however, on established methodologies. And here, it's not clear that we're at the point yet where we have those established methodologies."

But that point may be coming. Some scientists hope to eventually launch a kind of standardized extreme event attribution service, similar to a weather forecasting service, that would release immediate analyses—with the same uniform methods used for each one—for every extreme event that occurs.

It's still unclear what such a service might look like, but one could imagine receiving an email or smartphone notification each time an extreme heat wave or flood rolls through, explaining its connection to climate change.

The Met Office is already working on such a project, although it's in early stages. The European Prototype demonstrator for the Harmonisation and Evaluation of Methodologies for attribution of extreme weather Events, or EUPHEME, is an ongoing project designed to "build the bridge between science and an operational service," according to Nikos Christidis, one of the scientists involved.

In the meantime, though, individual studies are expected to keep rolling out. This last summer alone, a wave of unusual events across the world—from Hurricane Harvey in the United States to devastating floods in Southeast Asia—sparked renewed interest in the link between extreme weather and climate change.

Scientists have already tackled some of them. Two separate studies published in December both found that climate change had influenced Harvey's record-breaking rainfall (Climatewire, Dec. 14, 2017).

As for the 2003 flood that sparked Allen's interest, its exact connection to global warming may remain in question. But scientists have investigated several similar events since then—Allen, himself, co-authored a paper published in Nature Climate Change concluding that the wet January that caused the south England floods of 2014 was made 43 percent more likely by climate change.

Future floods are less likely to go uninvestigated. According to Christidis, the Met Office scientist, extreme event attribution is not only a matter of scientific advancement but a public obligation.

"Every time we have a high-impact, catastrophic, perhaps extreme event happening, people are invariably asking the question, 'Is this climate change?'" Christidis said.

"The whole science of event attribution developed so that we can provide scientifically robust answers to these questions. If we the experts don't do this, then there will be people who are not qualified who will go and fill in the gaps. So this is the very important challenge that we are called to face."

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.