How scientists can attribute parts of 2020’s hurricanes and wildfires to climate change

Back in 2017, record-breaking hurricanes like Maria and wildfires like Tubbs hammered the United States. But the specific role of long-term global warming was a tentative part of the discussion, with scientists speaking of it cautiously, in broad strokes.

Compare that with 2020, where researchers now have far more data showing just how much climate change affects the frequency and likelihood of heat waves (and fires that follow them), ocean heat waves, droughts, and intense storms. That has risen alongside a growing public awareness of how climate change is playing out. A 2019 Pew Research poll found that 62 percent of Americans said climate change was impacting their local community. CBS News reported that a majority of Americans now believe climate change is contributing to extreme weather.

In turn, more public officials and media personalities are directly connecting climate change to some of the disasters underway.

During Tuesday’s first 2020 presidential debate, moderator Chris Wallace posed a question to President Donald Trump, after referencing the wildfires raging in the West: “Do you believe that human pollution, greenhouse gas emissions, contributes to the global warming?” Wallace asked.

“I think a lot of things do,” Trump responded, before launching into a meandering answer blaming California’s wildfires solely on forest management.

That such a question was asked during a presidential debate is a stark shift from the 2016 campaign, where climate change barely came up at all, let alone its role in disasters.

“I think the question has started to change from, ‘Was this event due to climate change?’ to, ‘How was this event changed because of climate change?’” said Kevin Reed, an associate professor at Stony Brook University who leads the climate extremes modeling group. “The science has definitely improved. But I think a big part of what’s improved is also the dialogue about trying to understand the nuances of what is the impact of climate change on extreme events.”

While the long-term heating of the planet resulting from humans’ greenhouse gas emissions is not the “cause” of massive wildfires and powerful hurricanes, it can be a component in their severity, frequency, or likelihood. We have more certainty about that, thanks to an emerging scientific field known as extreme-event attribution. Here, scientists construct models to evaluate the counterfactual of what would have happened in a certain event without climate change and compare it to observed results.

Scientists working in this field acknowledge that for phenomena as complicated as wildfires and hurricanes, there are many other factors at play. That includes natural variability from climate cycles like El Niño, as well as policy decisions like the suppression of naturally occurring wildfires and allowing forest fuel to accumulate.

Firefighter’s discuss the Glass Fire’s progress in St. Helena, California, on September 27.
Carlos Avila Gonzalez/The San Francisco Chronicle via Getty Images

The Glass Fire in Napa County and the Zogg Fire near Redding, California, have consumed tens of thousands of acres.
Kent Nishimura/Los Angeles Times via Getty Images

Homes leveled by the Glass Fire in Santa Rosa, California .
Noah Berger/AP

Yet as even attribution science has become more and more important to the public understanding of climate change and extreme events, some in conservative circles dismiss it, particularly around the disasters this year.

President Trump has long been dismissive of climate change. When told about the role of climate change in wildfires during a White House briefing, he said, “It’ll start getting cooler. You just watch.” Environmental Protection Agency administrator Andrew Wheeler also diminished the role of climate change in wildfires. “I do believe most of it is forest management issues,” he told Cheddar on September 22.

Some right-wing personalities have been much more blunt, claiming such events have “nothing to do with climate change.” Others promoted conspiracy theories like the one claiming wildfires are purely due to a wave of arson.

Water floods a road as Hurricane Sally come ashore in Alabama on September 15.
Joe Raedle/Getty Images

Understanding the interplay of all the variables in extreme weather isn’t just an academic discussion; the role of climate change in disasters affects how we plan for the future, how we reduce risk, and how we adapt. That’s why it’s worth highlighting how our understanding of these phenomena has improved in recent years, why it’s important to unpack how rising average temperatures are fueling destruction, and why it’s critical to address these concerns now.

Climate change is priming fuels to create massive wildfires

Just this past weekend, another round of wildfires ignited in Northern California, prompting middle-of-the-night emergency evacuation orders in places like Butte County and Sonoma County. The new blazes have killed at least three people. By Wednesday evening, the Glass Fire in Napa and Sonoma Counties and the Zogg Fire in Shasta County, California, had together consumed close to 100,000 acres.

Meanwhile, the National Weather Service has issued a red-flag warning for wide swaths of Southern California as another heat wave and high winds are raising fire risks.

These blazes are only adding to what has already been an unprecedented wildfire season across much of the Western United States. In California, it’s now the worst fire season on record in terms of area burned. And more blazes are likely in store.

Wildfires are a natural and essential part of many ecosystems, particularly in the Western United States. Periodic fires help clear decay in forest and grasslands, help plants germinate, and return vital nutrients to soils.

However, humans have been making wildfires worse in recent years, expanding their scale and their devastation. People are building closer to fire-prone regions, increasing the opportunities to ignite fires and raising the damage tolls of fires that do occur.

Paradoxically, decades of suppressing naturally occurring fires have led to vegetation in these ecosystems accumulating to high levels so that when they do dry out, there’s far more fuel to burn. In some forests, that fuel also changes the nature of fires, from low-intensity burns close to the forest floor to towering flames that torch tree canopies.

And humans are changing the climate.

There’s now a growing pool of research identifying the specific role of human-caused climate change in wildfires. A study from the World Weather Attribution research consortium examined Australia’s massive bushfires this year. It found that climate change increased the likelihood of the conditions that fueled the blazes by at least 30 percent.

Earlier this year, the bushfires in Australia killed at least 24 people, torched more than 15.6 million acres, and destroyed more than 1,400 homes.
Brook Mitchell/Getty Images

The Australian bushfires also killed 480 million animals, environmental officials told the Times in the United Kingdom, including nearly one-third of koalas.
Brook Mitchell/Getty Images

Another study looking at 2017’s record-breaking fire season in British Columbia reported that climate change made the conditions behind those fires two to four times more likely and increased the burned area between seven- and elevenfold.

In Arizona and New Mexico between 1984 and 2015, a study in the journal Forest Ecology and Management found that climate change is increasing wildfires, particularly high-severity fires.

One key risk factor for wildfires is the vapor pressure deficit. This is the difference between how much moisture the air could hold and how much moisture is actually there. Air can absorb about 7 percent more water for every degree Celsius the air warms. But just because the air can hold on to more moisture doesn’t mean that it does.

A high vapor deficit means that the air is very dry, which means it can draw more moisture out of vegetation. That leaves grasses, trees, and shrubs primed to burn.

This year, California experienced some of the highest vapor pressure deficits in decades, leaving the state’s forests and its semi-arid shrub land known as chaparral parched as record-breaking heat baked the state. Scientists have found that this metric has been increasing since the 1970s due to climate change.

“Among the many processes important to California’s diverse fire regimes, warming‐driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date,” researchers wrote in a 2019 study in the journal Earth’s Future.

John Abatzoglou, one of the co-authors of the study and an associate professor of climatology at the University of California Merced, however, noted that there are also unique elements at play in every wildfire.

“The climate change argument is going to be the strongest for measures of fuel dryness, measures of vapor pressure deficits, and less so for individual weather events,” he said. “Each fire has its own story.”

One variation between wildfires is in the ecosystems where they can burn. Ponderosa pine forests burn typically burn at different times of year, frequency, and intensity compared to coastal redwood forests or chaparral across Southern California. These ecosystems all receive a distinct combination of rainfall, heat, humidity, and vegetation, so the effects of climate change don’t emerge in all these areas in the same way at the same time.

Another variable is how the fires burn. Some smolder across forest floors while others produce towering flames that tear through tree canopies. Some wildfires inch across the landscape while others driven by high winds can consume a football field’s worth of vegetation in minutes.

But there are long-term changes underway that could have an impact on all of these blazes. In a 2016 study, Abatzoglou reported that half of the increasing fuel dryness in western forests since the 1970s was attributable to climate change caused by humans.

California is also still reeling from the effects of a massive drought from 2011 until 2017 that helped kill off more than 140 million trees across the state. That drought was exacerbated by climate change, and warming has increased the likelihood of extreme drought in the region.

And it’s not just the summer heat that’s rising due to climate change. Winters are warming up too. In fact, in some parts of the country, winters are warming faster than summers. That has critical effects on fire activity and lays the foundation for major fires months in advance.

Snow accumulated in the winter in places like the Sierra Nevada acts as a battery for water. As it melts throughout the spring, it discharges much-needed moisture for plants. But with shorter winters, less snow has time to build up, leading to drier vegetation.

As winters warm up, snowpack can melt earlier in the spring, which leads to a process that causes soils to become drier, an effect that can compound over years. That reduces the amount of water available to plants. Snow also reflects sunlight back into space, and with less snow on the ground, the soil can absorb more heat and dry out further.

A warmer winter also means more precipitation falls as rain rather than snow. Rain doesn’t store as easily, so it can contribute to flooding early in the season but quickly run off into the ocean, leaving less moisture for the rest of the year.

“From a climate change perspective, we’ve been predicting lower snowpack values,” said Sarah Kapnick, deputy division leader at the Geophysical Fluid Dynamics Laboratory at NOAA. “Observational records have been showing an earlier timing of snow melts, and those have been increasing. That’s one factor that affects fire risk, because it leads to drying.”

The Meadowood Napa Valley luxury resort burns in St. Helena, California.
Justin Sullivan/Getty Images

The Chateau Boswell Winery building was destroyed by the Glass Fire in St. Helena, California.
Justin Sullivan/Getty Images

Climate change is also tipping the scales toward larger fires. Monica Turner, a fire researcher and a professor of integrative biology at the University of Wisconsin Madison, said in an email that climate is a big driver of megafires, those burning through an area larger than 100,000 acres.

This year, both long-term trends and seasonal variability converged, causing extreme heat and dryness, as well as some unusual ignition events, like a dry lightning storm in Northern California that sparked more than 300 fires.

Turner also noted that when weather conditions reach extremes like those across the West this year, they tend to overwhelm other wildfire factors like the amount of fuel present. “With weather like 2020, fires will burn through forests of all ages, structures and densities,” she wrote in a Q&A for the University of Wisconsin.

So while the Western wildfires in 2020 have been unusual in their severity, many of their most important ingredients will continue being amplified by climate change.

The most destructive elements of hurricanes are getting worse as temperatures rise

The Atlantic hurricane season this year has been extremely active, so active that forecasters have completely run through their list of names for storms and are now using the Greek alphabet. There have already been 23 named storms as of September 25, some of which formed before the official start of the Atlantic storm season. Earlier in September, Hurricane Beta became the ninth named storm this year to make landfall in the United States, tying a record set in 1916.

There were several unique factors this year. Researchers were able to see some of this coming in the spring when they detected warmer than average sea surface temperatures in the Caribbean Sea and the Atlantic Ocean. The waters continued to heat up over the summer. That’s important because hurricanes need surface water to be at least 26°C (79°F). More warm water means more energy for hurricanes, lending them strength as they spool up.

“Our seasonal predictions back in May were that this was going to be a massive [tropical storm] season,” Kapnick said.

Another factor was that El Niño, a periodic warming and cooling pattern in the Pacific Ocean, was in its neutral phase this year, leading to more stable air over the Atlantic. Phenomena that result from unstable air like wind shear can rip apart hurricanes before they can gather strength, so calmer skies above the ocean served as ideal breeding grounds for tropical storms this year.

So those were the unique seasonal effects. Then how does climate change fit in?

Damage caused by Hurricane Laura as it passed through Lake Charles, Louisiana, on August 28.
Joe Raedle/Getty Images

Hurricane Laura left behind expansive flooding, with water reaching up to 9 feet high. In Lake Charles, Louisiana, it triggered a chemical fire.
Stringer/AFP via Getty Images

One problem with finding climate change signals in tropical storms is that they are relatively infrequent events, and having the right mix of ingredients doesn’t always mean that a storm will form. There is a lot of variability in hurricane patterns, both year to year and over the course of decades. That makes it hard to suss out trends and even harder to identify signals in individual storms.

Hiroyuki Murakami, a scientist at the Geophysical Fluid Dynamics Laboratory at NOAA, likened the link between climate change and hurricanes to the link between smoking and cancer.

“When you look at the individual person, it’s really difficult to say that this person got lung cancer because they smoke a lot, because there are many people who smoke a lot but still they don’t get cancer,” Murakami said. “This is really similar. I think that probably there are some storms affected by climate change, but it’s really difficult to say that this storm is only attributable to climate change.”

That said, scientists are starting to see some trends underway. With rising average temperatures, oceans are warming. That means when hurricanes do form, they can be stronger.

“We’ve identified that global warming, climate change, can intensify storm mean intensity,” Murakami said. “In terms of storm intensity, it’s really simple: The source of energy for a tropical cyclone is warm ocean evaporation. When we get a much warmer ocean surface, it will lead to more evaporation to energize a tropical cyclone.”

According to a study last year in the Proceedings of the National Academy of Sciences, tropical cyclones have grown more intense between 1979 and 2017. “The results should serve to increase confidence in projections of increased [tropical cyclone] intensity under continued warming,” the authors wrote.

As for the frequency of hurricanes, that’s more complicated, and it’s not clear how climate change will alter the number of major storms that do occur. “A lot of models project a decreasing tropical cyclone number, but still we don’t know yet why climate models show decreasing tropical cyclones in the future,” Murakami said.

He pointed out that even in 2020’s extremely active season, only a couple of the tropical storms turned into major hurricanes. So climate change doesn’t necessarily mean more hurricanes, but a growing proportion of those that occur will likely be more powerful.

Perhaps the most concerning impact of climate change on tropical storms is that it is worsening the most destructive elements of these events. It’s not the wind that usually does the most damage during a hurricane, but flooding. That’s why changes in precipitation patterns can be a major concern.

“A difference between 1 to 2 inches of rainfall is the difference between whether your house gets flooded or not,” said Stony Brook’s Reed.

That flooding in the wake of hurricanes is caused primarily by rainfall and storm surges. Winds from tropical storms can push coastal water inland, creating storm surges. Melting ice and warming oceans due to climate change are causing sea levels to rise, so when storm surges do occur, they reach greater depths and further inland, causing more destruction.

As mentioned earlier, warmer air can hold onto more moisture. So when air heats up over the ocean, it mobilizes more water for rainfall. This warming has led to a rise in extreme rainfall events and increased the deluge from tropical storms.

There’s also some evidence that the movement of tropical cyclones like hurricanes is slowing down due to climate change. That means they are spending more time in a given area, dispatching more rainfall over a smaller space and increasing flood risks.

Another climate signal emerging in recent hurricanes is rapid intensification, which NOAA defines as a gain of 35 mph or more in wind speed over 24 hours. Such storms spool up quickly and can catch forecasters off-guard, making it hard to plan evacuations. A 2019 study in the journal Nature Communications found that the number of Atlantic tropical storms that have rapidly intensified increased significantly between 1982 and 2009, in part due to warming caused by humans. Climate models show that this pattern of rapid intensification will increase as average temperatures rise.

Some of these effects were visible in recent storms like 2017’s Hurricane Harvey, which drenched Houston in more than 50 inches of rain as it slowed down over the city. Hurricane Laura this year also rapidly surged from Category 2 strength to Category 4 strength as it blasted the Gulf Coast with winds topping 140 mph.

“The rapid intensification was very clear in Hurricane Laura, but still weather forecast models could not predict the timing of the intensification,” Murakami said.

President Trump tours damage from Hurricane Laura in Lake Charles, Louisiana, on August 29.
Alex Brandon/AP

Scientists have also begun to tease out the specific ways that climate change worsened recent storms, constructing counterfactuals to compare effects with and without climate change. Reed’s research group at Stony Brook University estimated that Laura dispatched about 10 percent more rainfall due to climate change.

His team conducted a similar assessment for Hurricane Florence, which struck North Carolina in 2018. Scientists estimated that climate change increased the storm’s rainfall by 50 percent. Another team of researchers found that sea-level rise since 1970 caused Florence to flood an additional 11,000 homes than would have been inundated with constant ocean levels.

But there are other factors that can change tropical storm patterns over time. Air pollution, dust clouds, the stability of the upper atmosphere, and the relative warmth of the Atlantic Ocean compared to the Indian and Pacific Oceans can all influence the intensity and frequency of hurricanes. Climate change is certainly a growing factor in hurricane trends, but it’s important to account for other variables, too.

We’re still not doing enough to reduce the looming disaster risks

The fact that so many human elements are driving the growing risk of these disasters means that humans can alter these variables to reduce ongoing and potential destruction.

That will require a concerted suite of strategies, from cutting greenhouse gases, to more resilient infrastructure, to controlled burns of forests, to retreating from high-risk fire and flood zones. It will take time. And it will also take a more sophisticated understanding of rising average temperatures and their myriad ripple effects. Otherwise the disasters of 2020 will become more common and more dangerous.

“All of these things are coming together to make an extreme year, and we need to better understand the factors and how each of them affected it,” Kapnick said. “The question we need to be asking is, what is the climate risk? What is it today? What was it in the past? And what is it in the future?”

Yet at the same time, greenhouse gas emissions show little sign of reversing course. Roughly 40 percent of the US population lives in a coastal county and people are continuing to build in coastal areas facing inundation from rising seas. Homes are still being planned and built in fire-prone regions as residents get priced out of safer areas. Based on recent trends, California will have 645,000 homes in “very high” fire risk areas by the middle of the century.

Even as the dangers of climate change become more vivid, humanity continues to lurch toward them.


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