Earth is a fire planet. Ever since the first plants appeared — and provided fuel — more than 420 million years ago, fire has flourished in Earth’s oxygenated atmosphere. Some scientists even think that long before humans, fire carved out entire landscapes, clearing dense forests to make way for grasslands.
In recent years, the fingers of flame have extended their reach over more of the Earth’s surface. Wildfires are occurring more often and becoming more severe, a perplexing change in fire patterns that threatens to transform ecosystems, reduce biodiversity and even alter climate. To stamp out the flames, researchers have to understand why fire is spreading and figure out how to fight fire with science.
Fire has many faces. It helps some ecosystems thrive but destroys others. It helps people clear land but can also destroy homes and take lives. Sometimes useful, sometimes destructive, fire is always unpredictable — and that makes it a difficult subject. “Understanding fire is a science, and until now, the science of fire hasn’t been properly recognized,” says ecologist David Bowman of the University of Tasmania in Hobart, Australia.
But now, researchers from different disciplines are beginning to investigate the science of fire. Some ecologists monitor forest fires burning today, using satellite images to discern how much forest is aflame and how severe the fires are. Other scientists track how forests regenerate after fires. And geologists reach into the past, using remnants of long-cooled forest fires for clues about how fire shaped the ancient Earth.
“Everyone in their respective fields had some knowledge about fire. But now we’ve got to come together to map out the role of fire in the Earth system,” Bowman says.
The “Earth system” is the sum of the planet’s physical, chemical, biological and social parts, processes and interactions. Fire can shape landscapes, shift climate and even change processes such as the carbon cycle — blazes have impacted the planet for eons. But now, people could be shifting the balance in a new direction.
Bowman and other fire experts reviewed recent fire research in the April 24 Science. The latest work illuminates the complicated role that fire plays on Earth and highlights the interactions among climate, fire and humans.
People are playing more of a role in fire than ever before, the research reveals. In the tropics, fires are routinely started to clear forestland for agriculture or pastureland. And as more homes are built farther into the wilderness, on land where fire used to roam free, the natural patterns of fire are being suppressed.
“There’s an emerging need to look at fire at the intersection between biology, ecology and society,” Bowman said at a teleconference coinciding with the paper’s online release.
New research suggests that, while fires may burn locally, their consequences spread globally. When forests blaze, carbon stored in vegetation escapes into the atmosphere as carbon dioxide, a greenhouse gas. Added up, the fires that burn all over the world could be a significant source of atmospheric carbon, even contributing to climate change.
At the same time, changes in climate could make landscapes more prone to burning. Droughts can set the stage for fires to burn in places where they never used to. And when the natural patterns of fire change, ecosystems change. Fire may kill but not completely destroy trees. “That means there’s more fuel waiting for the next fire,” says Eric Kasischke of the University of Maryland in College Park, an ecologist who studies fire and climate in boreal forest ecosystems.
Scientists don’t have a complete understanding of fire in the Earth system yet. But as researchers pull together the threads of fire science, integrating different areas of research, a new appreciation of fire’s global impacts is emerging.
Fire watch
Some ecosystems thrive on fire, such as arid savannas that burst into flame easily, burn often, but soon regenerate. For other ecosystems, fire is a rare and serious event. Ancient forests take thousands of years to grow to maturity and can be destroyed by fire in a matter of hours. The challenge is figuring out the role of fire in each ecosystem, Kasischke says.
Using satellite data, Kasischke and his team monitor fires in boreal forests in Alaska and Canada, figuring out how much forest has burned and tracking the forests as they recover.
In the past, fire visited these forests infrequently. But in recent years, fire has picked up the pace. “Between 1959 and 1983 we saw three big fires — that’s about once a decade,” Kasischke says. “In the last 25 years, the frequency of these big fires has doubled.”
Increasing fire frequency is disastrous for slow-growing forests. Black spruce trees can take 30 years to mature and produce seeds. If the forest burns before the trees mature, there won’t be any black spruce seedlings to regrow the forest. This opens the door to more quickly maturing species, which can soon dominate the ecosystem, Kasischke says.
Putting out small fires can worsen the problem. That’s because small fires — which are low-lying and burn around the mature trees without harming them — clear brush and leaf litter, potential fuel for bigger fires, from the forest floor.
Unfortunately, when homes and people lie in the fire’s path, letting even a small fire run its course could be dangerous. So humans suppress the small fires, and brush builds up, providing extra fuel that could turn the next fire into a really big, devastating one, Kasischke says.
Setting controlled fires to mimic natural ones is a strategy already used in some places. Yet even these controlled fires can sometimes get out of hand. “There are lots of factors that turn a small fire into a big one, and we don’t understand all of them yet,” Kasischke says.
No one puts out the fires in the remote boreal forests that Kasischke and his team monitor, and people rarely start them. Along with the increasing number of fires in these forests, fires in recent years are more likely to have been severe. “2004 and 2005 were our biggest fire years ever,” says Kasischke, noting that 2004 was particularly warm and dry.
Conflagration climates
Fire is linked to changing weather patterns, he and other researchers say. Warmer weather dries out the forest, turning it into a tinder box. In summer of 2004, a high pressure weather system sat over the region that Kasischke monitors for weeks —weather that lengthened the fire season, he says.
A 2006 paper in Science investigated the consequences of an extended fire season. Anthony Westerling of the University of California, Merced and his colleagues compiled a database of wildfires in the western United States since 1970 and compared the fire records with climate data. Since the mid-1980s, increased temperatures meant that less snow fell and that snow melted earlier. Those changes correlated with increases in wildfires, which burn large swaths and last for long times. Unusually warm springs and longer summers dried out vegetation and provoked more frequent fires of all sizes, the researchers report.
“We’re interested in understanding what controls fire frequency and severity, and learning what makes some ecosystems more resilient to fire,” says Jennifer Balch of the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. She and her colleagues investigate fire in the Amazon’s tropical forests.
Big forest fires used to rage through the rainforest about every two centuries. Now, it’s more like every two decades, Balch says. A major reason is that fire is a quick, easy and cheap way to clear forest for agriculture and livestock, and is a method widely used in the Amazon, she says. But fire is a fickle friend. It can easily get out of control and end up destroying more forest than intended.
To investigate the effects of fire, Balch and her team, with permission from farmers, deliberately burn blocks of Amazonian forest. As the forest burns, the scientists watch to see which types of vegetation burn and which don’t, and measure how such factors as temperature, humidity and amount of leaf litter affect fire intensity — its temperature and duration. After the fire, the researchers track which types of vegetation bounce back most quickly.
“Repeated fires alter the composition of the forest by favoring species with specific traits that help them regenerate after a fire,” Balch says. Often, the species that take over the ecosystem are also easy burners, such as grasses that sprout once the trees have been cleared. When fire has raged through a forest once, it’s more likely to return, the researchers say.
The team also calculates the amount of carbon dioxide the burning releases into the atmosphere. In the past, scientists have assumed that any carbon released from forest fires would be balanced by vegetation taking in carbon as the forest regenerates. But as more forest burns and less forest regenerates, changes in vegetation could alter ecosystems and biogeochemical cycles on a global scale.
“Our calculations show that forest fires could account for one-fifth of the greenhouse gases produced by humans,” Balch says.
Besides releasing heat-trapping carbon dioxide, fire contributes to climate change by releasing aerosols, or black soot. Soot can warm the atmosphere by absorbing solar radiation.
Fire is such a large source of carbon that it should be incorporated into predictions of climate change, researchers note. “The Intergovernmental Panel on Climate Change should take fire into account,” Balch says.
The most recent IPCC report, released in 2007, does acknowledge fire as a source of atmospheric carbon, says Chris Field, an ecologist at the Carnegie Institution for Science at Stanford University, who contributed to the report.
“Fire has not been ignored. But we just don’t have good models to incorporate fire into climate change,” Field says. “The IPCC is working hard on coming up with better models right now.”
Ancient burns
Today’s climate changes are not the first Earth has endured, and some researchers think fire may be connected with past climate change, too. Jennifer Marlon of the University of Oregon in Eugene and her colleagues use charcoal deposits as records of ancient wildfires, finding that fire has fluctuated with climate for thousands of years. Studying changes in fire regimes over long time periods puts recent increases in the number, frequency and severity of fires in context.
Over many years, charcoal, which is burned vegetation, washed into lakes, settled and was eventually incorporated into layers of sediment. Marlon and her colleagues take cores of the sediment and analyze the charcoal content layer by layer, getting a snapshot of forest fires at different times.
In a 2008 paper in Nature Geoscience, Marlon and her colleagues analyzed lake-bottom sediment cores from around the world to reconstruct fire’s history over the past 2,000 years. The data show a strong link between fire and climate, with reduced fire in cold intervals and increased fire in warm intervals, regardless of whether humans were present.
From A.D. 1 to 1750, wildfires decreased in number, the researchers reported. “Earth went through a cooling period at that time, and fires declined along with the temperatures,” Marlon says.
Between 1750 and 1870, fires increased. Settlers in the Americas and Australia slashed and burned to clear forest quickly and make land available for agriculture or animal grazing, Marlon says.
After 1870, fire activity hit a lull. “At that time, enough land was cleared that the slash-and-burn methodology slowed down,” Marlon says. And as people built homes on previously uninhabited land, wildfires were no longer allowed to rage as they once did. But in remote areas, the number of wildfires remained high and increased as climate warmed, she says.
More information on the patterns of ancient fires comes from the trees themselves. When low-lying understory fires rage through a forest, some trees get damaged, picking up telltale fire scars, says Thomas Swetnam of the University of Arizona in Tucson. By looking at the location of the scars in relation to tree rings, researchers can figure out when fires have occurred.
“We sample living and dead trees that were injured by fire,” Swetnam says. “Using fire scars, we can determine the year and the season of the fire.”
By scaling up the analysis from trees in single forests to whole regions and even continents, the researchers can get a global record of fires over time.
Swetnam and his colleagues report that fire is intimately connected with climate, with particularly bad fire seasons in drought years. “This is when we see many fires burning at one time,” Swetnam says. Tree-scar records show that fire can also be impacted by El Ni±o and La Ni±a. These oscillations in ocean temperature trigger changes in atmospheric conditions that shift weather patterns in different parts of the world.
La Ni±a brings drought to some parts of the United States, contributing to a spike in fire frequency, especially in the western states. El Ni±o brings wet weather and flooding — but that doesn’t mean the forests are safe. “That reduces fire activity and gives fuel a chance to build up, meaning that the next year of fires will be even worse,” Swetnam explains. “Weather patterns swing between El Ni±o and La Ni±a every two to seven years. We see the frequency of severe fires fluctuate on a similar range.”
Beginning in the 20th century, though, “we’re seeing a major shift in the fire regime,” Swetnam says. “Fire is increasing, and now it’s feeding back into climate.” But by collecting information about how fire patterns have varied through history, the researchers hope to gain a clearer understanding of the relationship between changes in climate and changes in fire regimes.
Fire patterns are still not well understood, but the latest research is showing that they follow a cycle that human activity is changing. Fully elucidating the role of fire in the Earth system will require figuring out how fire affects different ecosystems and how these impacts add up.
“There’s no basic ecology textbook that describes how fire changes ecosystems or climate,” Balch says. “But we’re accumulating more information — and we’re getting closer.”
Solmaz Barazesh is a science writer based in Baltimore.
