Drought and heat, worsened by humans, help fuel California fires

LOS ANGELES — It may take weeks to determine the spark that touched off each of the firestorms that consumed hunks of more than a dozen California communities this week.

But the consensus in the scientific community is that the conditions that cleared a path for the tsunami of flame were made by humans. Decades of aggressive firefighting left too much fuel on the ground. And more than a century of carbon emissions exacerbated the state’s drought and the record high temperatures that baked brush and timber to an explosive dryness.

The damage from the wind-driven flames — which destroyed thousands of buildings and killed at least 17 people — is also more grievous because of another man-made initiative: building more and more homes in hilly communities adjacent to brush and woodlands.

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California's wildfire damages
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California's wildfire damages
Winemaker Pierre Birebent inspects a melted wine bottle among the burned out remains of the Signorello Estate Winery in Napa, California on October 11, 2017. More than 200 fire engines and firefighting crews from around the country were being rushed to California on Wednesday to help battle infernos which have left at least 21 people dead and thousands homeless. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Bicycles destroyed by wildfires are seen in Santa Rosa, California, on October 11, 2017. The toll from Northern California's ranging wildfires continued to grow as officials said the fires destroyed up to 2,000 structures and killed at least 17 people. / AFP PHOTO / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
A car destroyed by wildfires is seen in Santa Rosa, California, on October 11, 2017. The toll from Northern California's ranging wildfires continued to grow as officials said the fires destroyed up to 2,000 structures and killed at least 17 people. / AFP PHOTO / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
Homes destroyed by wildfires are seen in Santa Rosa, California, on October 11, 2017. The toll from Northern California's ranging wildfires continued to grow as officials said the fires destroyed up to 2,000 structures and killed at least 17 people. / AFP PHOTO / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
A car destroyed by wildfires si seen in Santa Rosa, California, on October 11, 2017. The toll from Northern California's ranging wildfires continued to grow as officials said the fires destroyed up to 2,000 structures and killed at least 17 people. / AFP PHOTO / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
SANTA ROSA, CA -OCTOBER 10: A toy car is left scorched in a neighborhood destroyed by fire near Cardinal Newman High School on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
SANTA ROSA, CA -OCTOBER 10: A man retrieves coins from the ruins of his house in a neighborhood destroyed by fire near Cardinal Newman High School on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
SANTA ROSA, CA -OCTOBER 10: A statue of Jesus praying with arms raised is seen among fire damaged buildings at Cardinal Newman High School on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
Burned wine barrels are seen at a destroyed Paradise Ridge Winery in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
A bubbling underground river of wine flows away from a burned Paradise Ridge Winery in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Paradise Ridge Winery is seen burned to the ground in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Burned wine barrels are seen at a destroyed Paradise Ridge Winery in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
TOPSHOT - A destroyed Journeys End Mobile Home Park is seen in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Owner Rene Byck looks over remains of his Paradise Ridge Winery in Santa Rosa, California, on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
A wild turkey wanders through a burned brush in Santa Rosa, California, on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Mike Reynolds of the housing and community services department for the city of Santa Rosa places a red-tag sign on a burned down Arby's restaurant in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
GLEN ELLEN, CA - OCTOBER 10: A view of the remains of multiple homes that were destroyed by the Nuns Fire on October 10, 2017 in Glen Ellen, California. Fifteen people have died in wildfires that have burned tens of thousands of acres and destroyed over 2,000 homes and businesses in several Northen California counties. (Photo by Justin Sullivan/Getty Images)
GLEN ELLEN, CA - OCTOBER 10: A view of the remains of a home that was destroyed by the Nuns Fire on October 10, 2017 in Glen Ellen, California. Fifteen people have died in wildfires that have burned tens of thousands of acres and destroyed over 2,000 homes and businesses in several Northen California counties. (Photo by Justin Sullivan/Getty Images)
The entrance to an Arby's restaurant is seen after burning down in Santa Rosa, California on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
The remains of an Arby's restaurant is seen after burning down in Santa Rosa, California, on October 10, 2017. Firefighters encouraged by weakening winds were battling 17 large wildfires on Tuesday in California which have left at least 13 people dead, thousands homeless and ravaged the state's famed wine country. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
SANTA ROSA, CA - OCTOBER 10: A neighborhood is destroyed by fire in the area of Foxtail Court, on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
SANTA ROSA, CA - OCTOBER 10: A neighborhood is destroyed by fire in the area of Foxtail Court, on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
SANTA ROSA, CA -OCTOBER 10: A neighborhood is destroyed by fire in the area of Foxtail Court, on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
SANTA ROSA, CA -OCTOBER 10: A neighborhood is destroyed by fire in the area of Foxtail Court, on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
SANTA ROSA, CA - OCTOBER 10: A man looks for something to salvage in a neighborhood destroyed by fire in the area of Foxtail Court, on October 10, 2017 in Santa Rosa, California. In one of the worst wildfires in state history, at least 1,500 homes have burned and 11 people have died as more than 14 wildfires continue to spread in eight Northern California counties. (Photo by David McNew/Getty Images)
CALIFORNIA, USA - OCTOBER 10: The remains of a damaged buildings are seen after a wildfire moved through the area in Santa Rosa and Napa Valley in California, United States on October 10, 2017 leaving at least 10 people dead and destroying homes and businesses in their path. (Photo by Tayfun Coskun/Anadolu Agency/Getty Images)
CALIFORNIA, USA - OCTOBER 10: The remains of a damaged buildings are seen after a wildfire moved through the area in Santa Rosa and Napa Valley in California, United States on October 10, 2017 leaving at least 10 people dead and destroying homes and businesses in their path. (Photo by Tayfun Coskun/Anadolu Agency/Getty Images)
CALIFORNIA, USA - OCTOBER 10: The remains of a damaged buildings are seen after a wildfire moved through the area in Santa Rosa and Napa Valley in California, United States on October 10, 2017 leaving at least 10 people dead and destroying homes and businesses in their path. (Photo by Tayfun Coskun/Anadolu Agency/Getty Images)
SANTA ROSA, CALIFORNIA - OCTOBER 9: The Estancia Apartment Homes on Old Redwood Hwy. were completely destroyed on October 9, 2017 in Santa Rosa, California. (Brian van der Brug / Los Angeles Times via Getty Images)
SANTA ROSA, CALIFORNIA - OCTOBER 9: A Fountaingrove Village homeowner surveys her destroyed home she and her husband have owned for 4 years in Santa Rosa on October 9, 2017 in Santa Rosa, California. (Brian van der Brug / Los Angeles Times via Getty Images)
SANTA ROSA, CALIFORNIA - OCTOBER 9: A police officer looks over the destruction of a home in a neighborhood off Fountaingrove Parkway near the Hilltop in Santa Rosa on October 9, 2017 in Santa Rosa, California. (Brian van der Brug / Los Angeles Times via Getty Images)
NAPA, CA - OCTOBER 09: The remains of the fire damaged Signarello Estate winery after an out of control wildfire moved through the area on October 9, 2017 in Napa, California. Tens of thousands of acres and hundreds of homes and businesses have burned in widespread wildfires that are burning in Napa and Sonoma counties. (Photo by Justin Sullivan/Getty Images)
NAPA, CA - OCTOBER 09: The remains of the fire damaged Signarello Estate winery after an out of control wildfire moved through the area on October 9, 2017 in Napa, California. Tens of thousands of acres and hundreds of homes and businesses have burned in widespread wildfires that are burning in Napa and Sonoma counties. (Photo by Justin Sullivan/Getty Images)
Resident Louis Reavis views his burned classic Corvette at his home in Napa, California on October 9, 2017, as multiple wind-driven fires continue to whip through the region. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
GLEN ELLEN, CA - OCTOBER 09: Burned cars sit idle after an out of control wildfire moved through the area on October 9, 2017 in Glen Ellen, California. Tens of thousands of acres and dozens of homes and businesses have burned in widespread wildfires that are burning in Napa and Sonoma counties. (Photo by Justin Sullivan/Getty Images)
GLEN ELLEN, CA - OCTOBER 09: The remains of fire damaged homes after an out of control wildfire moved through the area on October 9, 2017 in Glen Ellen, California. Tens of thousands of acres and dozens of homes and businesses have burned in widespread wildfires that are burning in Napa and Sonoma counties. (Photo by Justin Sullivan/Getty Images)
The William Hill Estate Winery sign is seen partially burnt in Napa, California on October 9, 2017, as multiple wind-driven fires continue to whip through the region. / AFP PHOTO / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
NAPA, CA - OCTOBER 09: Burned out wine bottles sit on a rack at the fire damaged Signarello Estate winery after an out of control wildfire moved through the area on October 9, 2017 in Napa, California. Tens of thousands of acres and hundreds of homes and businesses have burned in widespread wildfires that are burning in Napa and Sonoma counties. (Photo by Justin Sullivan/Getty Images)
Damage from the Napa fire is seen on a destroyed goat dairy farm on October 10, 2017 in Carneros a town just outside of Napa, California. Firefighters battled wildfires in California's wine region on October 10 as the death toll rose to 15 and thousands were left homeless in neighborhoods reduced to ashes. / AFP PHOTO / Amy Osborne (Photo credit should read AMY OSBORNE/AFP/Getty Images)
Grape vines damaged by heat from wildfires are seen at a vineyard in Santa Rosa, California, on October 11, 2017. The toll from Northern California's ranging wildfires continued to grow as officials said the fires destroyed up to 2,000 structures and killed at least 17 people / AFP PHOTO / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
SANTA ROSA, CA - OCTOBER 11: Some houses burned and some did not. Aerial view of the damage caused by wildfire that destroyed the Coffey Park neighborhood on October 11, 2017 in Santa Rosa, California.(Photo by Marcus Yam / Los Angeles Times via Getty Images)
SANTA ROSA, CA - OCTOBER 11: An American hangs outside a home burned down by wildfire that destroyed the Coffey Park neighborhood on October 11, 2017 in Santa Rosa, California.(Photo by Marcus Yam / Los Angeles Times via Getty Images)
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Thousands of residents in Northern California remained out of their homes Tuesday because of evacuation orders, and the smoke from fires at the north end of San Francisco Bay was so thick that people 50 miles away, in Oakland and nearby communities, wore bandanas and white surgical masks.

“It’s very clear that the increasingly hot summers are the product of climate change,” said Daniel Swain, a climate scientist at the University of California, Los Angeles. “And it’s clear that human influence has an impact on the climate system in the American West and more broadly. That increases the risk of fire and the overall acreage burned when we get these conditions.”

Fires that would have once leapt from tree to tree and ridge to ridge now increasingly bound across rooftops and from one subdivision to the next. “With all this vegetation in what is known as the wildland-urban interface, it can be particularly dangerous,” Swain said.

A paper published by the National Academy of Sciences in 2012 found that the area expected to burn from 1984 to 2015 had nearly doubled — increasing by 4.2 million hectares — over the burn total that would have been expected in the absence of warming caused by humans. That’s an additional burn area 30 percent larger than the state of Maryland.

Man-made “climate change has emerged as a driver of increased forest fire activity and should continue to do so,” as long as fuels for the fires are not reduced, according to the same paper, written by John T. Abatzoglou and A. Park Williams of the University of Idaho.

So-called greenhouse gases like carbon dioxide have trapped heat in the earth’s atmosphere and led to higher temperatures. That has exacerbated droughts in places like California, which saw below average rainfall in 10 of the 11 seasons before 2016. (Rainfall is measured from July 1 to June 30 the following year.)

Last year’s wetter weather was substantial enough to produce robust grass and brush growth. In a process called “carbon dioxide fertilization,” vegetation tends to increase “simply through the intake of increasing atmospheric CO2,” according to a 2015 paper in the Bulletin of the American Meteorological Society that examined increased fire activity in California.

But the fresh vegetation from the winter of 2016 was soon visited by the summer’s unprecedented heat, symbolized by the September day that had San Francisco sweltering in 106-degree temperatures, an all-time high in the era of modern record keeping.

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California wildfires turn skies over Disneyland orange
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California wildfires turn skies over Disneyland orange
Not the usual blue skies over #Disneyland today...feels eerie #fire #canyonfire2 #sleepingbeautycastle #smoke #disneylandresort
It's raining ash at Disneyland . . . #dca #disney #disneycaliforniaadventure #disneyland #fire #ash #sky
@Disneyland looking surreal with #canyonfire2 burning in the distance https://t.co/gaVpOrwXw1
My thoughts are with everyone affected by the fires in Anaheim Hills right now. This photo and video were taken by me, no filter used whatsoever.
#Canyon2Fire over #CaliforniaAdventure #Disneyland #abc7eyewitness #ktla #kcal #southerncalifornia #california #disneyland #anaheim #anaheimhills #smoky #photography #photographer #iphoneography #momentsbykerry.com #californiafireseason
Fire skies as seen at #Disney #dca #california #fire https://t.co/OzS5rXcczX
#Disneyland
#Canyon2Fire over #CaliforniaAdventure #Disneyland #abc7eyewitness #ktla #kcal #southerncalifornia #california #disneyland #anaheim #anaheimhills #smoky #photography #photographer #iphoneography #momentsbykerry.com #californiafireseason
The view from Disneyland of the #anaheimhills fire. https://t.co/S7adGlzxU5
Carbon Canyon Fire creating some seriously spooky spectating at Disney’s California Adventure! 🖤🎃 #NOFILTER
The fires in West Anaheim are making the park look real creepy #Matterhorn #Disneyland
Smoky skies over #Disneyland #fire https://t.co/rLoRr1lF2G
#Canyon2Fire over #CaliforniaAdventure #Disneyland #abc7eyewitness #ktla #kcal #southerncalifornia #california #disneyland #anaheim #anaheimhills #smoky #photography #photographer #iphoneography #momentsbykerry.com #californiafireseason
Disney’s California Adventure is on fire 🔥 https://t.co/xqZ2jbSMJM
🎼"Like Fire, Hellfire" • • • • • • • • • • • #disney #disneyland #fire #dragon #lego #nofilter #picoftheday #adventure #actor #actorslife #halloween #spooky #halloweentimeatdisneyland #nature #potd #followforfollow #goodevening
Forest fires causing the sky at Disneyland and DCA to be filled with smoke. Ash raining from the sky. https://t.co/oWWZPOTnx4
A bug eyes view from Bugs land . . . #disneycaliforniaadventure #disney #disneyland #dca #fire #sky
The sky makes me feel like I'm on another planet... Thoughts and prayers go out to anybody affected by the fire in Anaheim Hills. . . . . #disneyland #starwars #pathofthejedi #tomorrowland #disneyannualpassholder #disneylandap
There's a brush fire 🔥 in the area causing a red sun ☀️hope everyone is ok. #nofilter #california #disneyland #californiaadventure
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The summer furnace created a double whammy: Vegetation and trees that had dried out and become diseased during the long drought did not have time to recover, and both the older growth and the new vegetation from the recent rainy season were desiccated by months of relentless sun.

Increases in temperature and vapor pressure from 2000 to 2015 “significantly enhanced fuel aridity across western US forests [and] ... contributed to 75 percent more forested area experiencing high fire-season fuel aridity,” the University of Idaho researchers found. They concluded that that led to nine additional days per year of “high fire potential.”

Yet the magnitude of the burning sweeping the western U.S. is out of proportion to what would be expected, given the level of drought and warming alone, according to a paper by the National Academy of Sciences.

Climate and wildfire had been in relative sync for the last 3,000 years, with a few anomalous periods, according to the research, led by Jennifer R. Marlon, of Yale’s School of Forestry and Environmental Studies.

Man’s activity at first appears to have increased fires — with sparks from railroads touching off a large number of fires in the 1800s, for example, before the railroad companies began to spend more time clearing safety zones around their tracks. Then man’s habits turned and began to reduce the volume of flames. For one thing, livestock grazed away much of the fuel. For another, the early 1900s brought a public campaign for fire suppression after a series of deadly blazes. The U.S. created the national Forest Service, whose founding mission was to suppress fire on public lands.

“Consequently, there is now a forest ‘fire deficit’ in the western United States attributable to the combined effects of human activities, ecological, and climate changes,” Marlon’s paper concludes. Even the large fires at the end of the last century and the start of this one have not stopped the growth in this deficit — the amount of land that nature would tend to burn, in man’s absence.

Those natural forces show no sign of slowing, nor does the drive to put fires down as quickly as possible, saving homes and lives but potentially preserving fuel that will pose a greater threat in the future.

Andrew Blankstein contributed to this story. 

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