Climate change driving worsening Sierra wildfires

Editor’s note:

On Monday, July 29, Valley Voice reporter Dave Adalian – on behalf of EarthSky.org – talked with meteorologist and climatologist Daniel Swain. Swain – called the “Carl Segan of weather” by Stanford Magazine – holds both a doctorate in Earth system science from Stanford University, as well as a degree in atmospheric science from the University of California, Davis.

He studies hydroclimate volatility and wildfire dynamics as a climate scientist at the Institute of the Environment and Sustainability at the University of California, Los Angeles. He also works as a research fellow at the Capacity Center for Climate and Weather Extremes at the National Center for Atmospheric Research and at the Nature Conservancy.

Watch the entire EarthSky.org video interview here:

Climate Deniers Are Not Going to Like This

Wildfires in the Sierras have gotten more frequent and more destructive during the past few decades. According to the National Oceanic and Atmospheric Administration (NOAA), the number of large fires in the Western US doubled from 1984 to 2015. Climate change, the NOAA says, is the cause. Climate-change induced drought and long periods of heat – exactly like the heatwave that just ended here – drove wildfires to new extremes of destruction from 2020 to 2022.

The NOAA and other government agencies both here and around the world agree: Human activity is to blame.

The same fast-paced warming of the atmosphere and oceans also drove the two back-to-back wet years that followed, this year and last. Ironically, those extreme weather events prevented a typical California summer wildfire season, said climate expert Daniel Swain.

Human-driven climate change is an established phenomenon. Yet Earth’s climate is constantly changing from day to day, year to year, and on longer scales, and it always has. That can lead to confusion, since it might seem like the recent extreme changes in the climate are just regular background climate variation. Swain says it’s not, and there’s overwhelming evidence to back him up.

“And so we sometimes hear, ‘Well, the Earth’s climate is always changing.’ Of course, that’s true,” he said. “But what we do see at this point in time is a climate that over the course of decades is changing at a pace that is astonishingly fast from a longer-term geological Earth history perspective, so fast even that we can detect these changes, even notice them qualitatively over the course of an individual human lifespan. I can, and I’m not all that old.”

Anyone older than 25 or 30 years witnessed these changes rapidly unfolding.

California and the West Have Always Been Fire-Prone

The other thing anyone who’s spent a lifetime in California – especially in the Central Valley – knows is that our state is drought-prone. It’s always been this way and always will be. Annual dry spells are why we have wildfires here and lots of them.

“In California, it’s quite dry on average between June and August certainly, and maybe even into September or October in a lot of years,” Swain said. “So there’s this built-in annual drought that’s just a typical part of the climate in this part of the world. So you always have drier vegetation by the end of summer than you do at the beginning.”

We’re used to it. Yet things are changing around us in not-so-obvious ways. Those changes make fires more threatening.

“But what we see in a warming climate is that the peak seasonal dryness is even drier than it used to be and lasts for a longer period, into fall in many years, which is a problem in California specifically, because we have these what are known as offshore winds, these hot, dry winds that blow from land to sea and are responsible for a lot of California’s major wildfire activity,” Swain said. “So all this put together means that vegetation, not just in California, but in many of these fire-prone and even some not-so-fire-prone regions globally, is getting drier, if not on average, at least in the extreme. So the driest dries get drier, and that’s precisely when you see these huge spikes in wildfire activity.”

Today – Friday, August 2 – more than 500,000 acres of wildlands are burning across California. That number includes the 92,000-acre 2024 SQF Lightning Complex Fire in Sequoia National Forest in Tulare and Kern counties. That group of fires, which began July 13, is 49% contained after more than two weeks of firefighters’ efforts.

Drier Vegetation Driving Hotter, Meaner Wildfires

A changing climate is why the battle against wildfire is growing increasingly more difficult.

“So we see, yes, hotter, drier air in a lot of cases that facilitates fire spread,” Swain said. “But more importantly, in a lot of cases, more extremely dry vegetation that supports more intense fire activity, faster spreading fire activity, and more extreme fire behavior, all of which cause problems for both ecosystems and for the humans who live within them or adjacent to them.”

While a cycle of annual dry spells is normal here, as is the resulting drier vegetation, it isn’t the norm for most of the rest of the world. Yet many regions with varying ecosystems are seeing a similar explosion in wildfires.

“In places like California and the American West, and even in places that have quite different climates, like the Arctic, the boreal forests, we’re seeing these extreme fires,” Swain said. “Up in Canada right now, and up in Siberia – and as well as places like the Mediterranean and southeastern Australia and coastal Chile, and places that have wildfire-prone environments in South America, places like this – there is this common thread that we are seeing more severe wildfire activity in these regions.”

And these fires are far more destructive than they recently were. These fires even generate their own weather.

“By more severe, I do mean both larger fires, fast-moving fires, but also literally more intense fires, fires that burn at an actual higher temperature, which means that they have greater effects on the landscape,” Swain said. “They have greater potential to evade control, and greater potential to essentially develop these sorts of exotic fire behaviors that you sort of mentioned, generating their own weather in the form of for example, pyrocumulonimbus clouds or fire-generated thunderstorm clouds, essentially, as we have seen on many, many recent fires.”

The Expanding Atmospheric Sponge Effect

Around the globe – and right in our cut of the Sierra Nevada – it isn’t merely hot and arid winds causing the worldwide increase in wildfire. It’s something more subtle and insidious. Something to do with the fuel the wildfires burn.

“The main connection is actually a little bit indirect. It’s through the modification of the vegetation. So hotter, drier conditions themselves, atmospherically, can contribute to more extreme fire behavior. This is true, and this is an additional effect.” Swain said. “But in many places, the key influence is this increase in what we call “demand,” or the quality of the thirstiness of the atmosphere.”

The warmer the atmosphere becomes, the more water it can carry. This is why storms are now more severe, gaining energy from warming oceans before they hit land. In areas like ours, it means the drier, hotter air sucks more moisture from plants and the ground they grow on. Swain has given the phenomenon a name.

“I sometimes call it the expanding atmospheric sponge effect because it kind of gives you this very visual metaphor for an atmosphere that wants to absorb more and more water out of a landscape,” he said. “So evaporating water faster out of soils, in some cases inducing faster and more aggressive transpiration from plants. Because of this, that means that even places that see a fair bit of precipitation, as is the case in Northern California, can see much faster rates of water loss and drying of the vegetation.”

How We Fight Fires Makes them Worse

Wildfires these days are far more frequent and dramatically more damaging than they’ve been in the past. Swain said the changing climate isn’t the sole reason for this. It’s largely a result of how we react to wildfires.

“In some places we’ve suppressed a lot of the natural fire activity that otherwise would have occurred historically over the past century or so for a variety of reasons,” he said. “And that means that the ecosystems that have developed – ecosystems that were quite fire adapted that needed some amount of fire for their health and for their stability and to prevent some of the fuel buildup that contributes to some of these catastrophic fires – we got rid of that, and so we’re sort of reaping the consequences.”

The growing human population of the last century is also a contributing factor.

“And then also we have more people across all of these regions globally who live in high-risk fire zones,” Swain said. “So compared to 50 or 100 years ago, simply more people per square mile are in places that are prone to wildfires. So both of those are part of it.”

Yet these factors cannot alone account for the rapid and extreme growth in the number of wildfires around the planet.

“But we’re also seeing this increase in extreme wildfires in places where nobody lives, in very remote areas, so that’s not a population density problem. And in places where we haven’t had those policies to reduce fires relative to what they would have been 100 or 200 years ago,” Swain said. “There is something else going on here. And it turns out that the overarching theme is climate change.”

Humanity is Not Doomed

Human-driven climate change is responsible for the worsening weather events occurring around the world. Each year, temperatures are hotter than the last. The seas are rising. The situation seems hopeless. But it’s not.

Swain believes the knowledge we’ve gained from his work and that of other climate scientists allow us to slow and eventually stop the extreme change. It’s going to take effort on all levels if we’re to succeed.

As a recognized world authority on how climate change drives weather extremes, Swain has a particular approach he finds effective. And he recommends persistence.

“And so I think a lot of what I try and do is work with folks all the way from the individual level to the federal government and executive branch level to really think about how to incorporate the knowledge that we have about how extreme events, extreme weather events, and extreme sort of weather adjacent events …  are changing and how we can improve our resilience to these sorts of things by active intervention and not just shrugging our shoulders and saying, well, it’s all going to hell. Well, not a lot we can do about it,” he said.

So are we helpless in the face of an increasingly chaotic environment? No.

“Because the answer is there is a lot we can do about how the events affect us,” Swain said.

The truth about humanity’s undeniable hand in causing global climate change should be seen as a good thing, Swain said. Since we’re the root cause of the problem, that means it can be fixed, provided we are willing to do the work.

“But the good news is that people are now aware of this and we’re trying really hard to bend that curve. We have not done it yet, don’t get me wrong. So we have not yet turned the ship, but we could,” Swain said. “What if it really were the sun? Wouldn’t that be scarier? Because what would we do about that? If this were the sun sending the earth more energy, I’m not sure that we would be able to solve the problem.”