A wildfire is an uncontrolled fire that burns in wildland vegetation such as forests, shrublands, or grasslands. Due to climate change, a preponderance of fuels in western forests, and an increase in the population living in proximity to high wildfire hazard areas, wildfires have become increasingly destructive in recent years. This explainer discusses the context and consequences of wildfires in the United States, including relevant trends; primary causes; and the impacts on the economy, the environment, and human health. While we will consider trends, causes, and impacts of fire throughout the United States, a substantial portion of this explainer will focus on western wildfires, due to the magnitude of wildfire impacts in the American West.
Over the past several decades, wildfire activity, damages attributed to wildfires, and costs of managing wildfires have increased substantially in the United States. While the average annual number of fire ignitions in the US declined by approximately 780 fires per year between 1991 and 2020, the annual area burned in fires increased by roughly 192,000 acres per year during the same time period. Areas burned in wildfires increased particularly sharply in the western United States, where the annual average area burned in large forest fires grew by about 1200 percent between the 1970s and the 2000s. Three of the top five years for burned acreage in the United States since 1960 have occurred since 2015.
More wildfire ignitions occur in eastern states than in the western states. However, eastern fires are typically smaller than those in the West. In 2020, over 33,000 fires burned approximately 700,000 acres in the east, while almost 26,000 wildfires burned approximately 9.5 million acres in western states.
Across the 11 contiguous western states, nearly fifty percent of all land area is owned and managed by the federal government. However, because wildfires burn through wildland vegetation, they disproportionately burn on federal lands, and the federal government plays a significant role in managing them in the West. In 2020, more than seventy percent of all area burned by wildfires in the United States was on federal land.
See the infographic below for a snapshot of wildfire trends in the United States. Click here to view larger.
US Wildfire Trends
With recent increases in wildfire activity have come an increase in damages due to wildfire. The prevalence of wildfire smoke has increased substantially since the mid-2000s, and wildfires now cause approximately 25 percent of Americans’ total exposure to harmful fine particulate matter known as PM2.5. Smoke exposure—which can affect many people as wildfire smoke spreads across a wide area—can cause and exacerbate health problems. More details about the impacts of smoke are outlined in a later section of this explainer.
As well, wildfires have become increasingly destructive to private property. In 2020, almost 18,000 structures burned in US wildfires, including almost 10,000 homes. Fifteen of the twenty most destructive fires in California state history as of July 2021, in terms of structures of destroyed, have occurred since 2015.
Increased damages due to wildfire owe in part to an increase in the number of homes, businesses, and other structures in wildfires’ paths. The past several decades have seen a large increase in the number of US homes in the area known as the wildland-urban interface, where people live close to large areas of wildland vegetation. Over the past two decades, the number of residential homes in the WUI has grown by about 350,000 homes per year, and as of 2021, roughly 49 million homes were in these areas. The eastern United States contains the majority of WUI areas, although California, Oregon, Washington, and Colorado all have large swathes of WUIs.
Both increases in wildfire activity and the increase in homes and business exposed to wildfire activity has led to an increase in the costs of managing wildfires. Fire suppression costs have almost quadrupled since 1985.
Causes and Risk Factors
Limiting Factors for Fire
To start and spread, wildfires need both an ignition source and fuels. “Fuel” is anything that can burn in a fire. Grasses, trees, pine needles, leaves, and wooden structures are all types of fuel. The availability and aridity of fuel are both important factors limiting how wildfires start and spread. Fuel availability refers to the sheer amount of available organic matter for a wildfire to burn. Fuel aridity refers to how dry the fuel is. When fuels are abundant and dry, fires burn more intensely and spread more quickly.
Fuel availability is determined in large part by ecosystem processes and management practices. Climate and ecological factors determine how much vegetation grows in an area. Disturbances such as wildfires or active management such as mowing, thinning, and prescribed burns can remove fuels, while exclusion of wildfire from the landscape allows fuels to accumulate.
Fuel aridity is largely determined by factors related to climate, including temperature, precipitation, and humidity. Lighter snowpacks, higher summer temperatures, and lower humidity contribute to drier conditions in which fuels are more apt to burn.
As the climate has changed, fire seasons around the world have grown longer. In the United States, fire seasons in western states have been especially affected. According to a 2020 paper, the period from 2000-2018 was the driest 19-year span southwestern North American has experienced since the late 1500s, and the second driest since 800 CE. As well, spring is arriving earlier in the western US, resulting in earlier snow melt each year.
As a result of these trends, there are now longer periods each year in which fuels are dry and primed to burn. Abatzoglou and Williams (2016) found that fuel aridity has increased across the western United States and that fire seasons are now nine days longer on western forestland on average. A substantial portion of the recent increase in wildfire activity in the western United States can be attributed to increasingly arid conditions caused by climate change. Abazoglou and Williams found that between 1984 and 2015, cumulative forest fire area was twice as large as would have been expected in the absence of climate change.
In addition to creating conditions that enable fires to start and spread more easily, climate change has been linked to an increase in lightning strikes, which is a primary source of wildfire ignitions.
Wildfires and climate change can form a feedback loop. As wildfires burn, they release CO₂ and other greenhouse gases stored in soil and organic matter into the atmosphere. This in contributes to further climate change, which increases wildfire activity.
In some places, wildfires were historically not only limited by the climate and fuel aridity, but by the sheer availability of fuels. This was particularly true in dry forests of the western United States, which historically burned relatively frequently. These repeated fires removed fuels and created a varied landscape, which limited fire spread and, in many cases, fire intensity. However, since the late nineteenth century, both due to the cessation of traditional indigenous cultural burning practices and due to the start of decades of aggressive fire suppression by federal and state land management agencies, fire has been largely excluded from many ecosystems. As a result, availability of fuels no longer limits fire spread in many dry western forests, which are now often dense with vegetation. When fires do occur in these forests, they are more likely to spread to the forest canopy, where they burn at higher intensity and are more hazardous and harder to contain.
However, while the exclusion of fire is an important factor contributing to wildfire where fuels had been historically limiting—namely, in dry western forests like the ponderosa pine forests of northern California, Idaho, and Montana—this is not the case for all western forests. In the wet and dense forests of western Oregon and western Washington, fire has historically been limited more by low fuel aridity than fuel availability. Therefore, climate change probably played a greater role in contributing to Oregon’s unprecedented 2020 fire season than did historical fire exclusion.
On the other hand, in the fast-growing chaparral shrubland ecosystems of southern California, fires now occur more frequently than they did historically due to an increase in ignition sources. More frequent fires threaten the health and survival of these chaparral ecosystems, since plant communities may not have time to recover between fires. In chaparral ecosystems, fewer fires are needed rather than more. Therefore, while historical fire exclusion and an increase in the prevalence of fuels are important factor contributing to increasing wildfire hazard in some parts of the western United States, their relevance varies by location.
Fire needs not only fuels, but an ignition source to start and spread. Between 2016 and 2020, an average of 88 percent of US wildfires were started by humans due to sparks from power lines, vehicles, and other equipment; improperly contained campfires; arson; and other ignition sources. In addition to human causes, wildfires can also be caused by natural occurrences such as lightning strikes; for example, the 2020 Pine Gulch Fire—the largest wildfire in Colorado history—was caused by a lightning strike igniting drought-stressed vegetation.
Up until a certain population density, the risk of a fire starting increases with the number people on the landscape. Research has found that human ignitions have tripled the length of wildfire season—while thunderstorms tend to occur most frequently in the summer, human-caused ignitions occur in other seasons as well. Human-caused fires also tend to be more destructive than naturally ignited fires, in part because they are more likely to occur near homes.
Notably, while fires started by utilities (such as those sparked by power lines) are relatively rare, they are also disproportionately hazardous. California’s 2018 Camp Fire, which was the state’s deadliest and most destructive, was ignited by electrical transmission lines.
The Wildland-Urban Interface
The wildland-urban interface (WUI, pronounced “Woo-E”) is the land where homes and businesses coexist with wildland vegetation. Development in the WUI is especially susceptible to wildfire damages, due to proximity to fuels, and can create significant costs for forest managers who must suppress and battle fires they otherwise would have left alone.
As the US population has expanded and housing prices have increased in metropolitan areas, the WUI has expanded as well. According to a 2018 study, the WUI grew by 41 percent from 1990 to 2010. In 2020, an estimated 33 percent of all US homes were located in the WUI, 4.5 million of which are at high or extreme wildfire risk. Most WUI areas in the United States are in eastern states, where wildfires are less problematic; however, housing growth in the WUI in wildfire-prone areas has been strong. In addition to increasing wildfire activity in the western United States, expansion of housing in the wildland urban interface over the past several decades has contributed to increasing fire damages and increasing fire management costs.
Effects of Wildfires
The effects of wildfires can generally be grouped into three categories: economic, environmental, and human health related. In 2020, wildfires in the United States caused approximately $16.5 billion in damages to structures and management costs. Indirect damages, including indirect economic consequences, impacts on human health, and impacts to ecosystems and ecosystems services, are likely considerably higher, with long-lasting repercussions.
Damage, Loss, and Risk to Property: Wildfires can cause immense direct damage to structures and property, and fires that destroy thousands of homes have become a regular occurrence. In 2020, wildfires destroyed almost 18,000 structures, 54 percent of which were homes. California has been notably hard-hit by destructive fires. A 2020 study found that in 2018, wildfires caused a total of almost $28 billion in capital losses in California, including damage to both homes and businesses. As homes in high-risk areas essentially become uninsurable due to persistent year-to-year threats, it is becoming more difficult for some homeowners to find affordable insurance.
Economic Activity: Fires can disrupt economic activity by destroying physical capital, interrupting transportation networks, impacting workers’ health and hours worked, and decreasing tourism and recreation. Of the nearly $150 billion in damages estimated to have been caused by California’s 2018 wildfires, approximately 60 percent were indirect economic losses caused by disruption to economic activity. Wildfire smoke can also lead to lost productivity far from a fire’s location; a 2020 working paper found that wildfire smoke led to annual declines in labor market activity worth $70 billion on average between 2006 and 2015.
Displacement: Wildfires have the ability to displace hundreds of thousands of Americans each fire season. These can be short-term displacements from emergency evacuation, or long-term displacements when homes are destroyed. Those living in the WUI are most affected, as this is where fires are most likely to threaten homes. Those without fire insurance—often renters and low-income households—are also heavily affected, as they are less able to return and rebuild. Rising insurance premiums also decrease people’s ability to recover from displacement.
Energy Availability: Efforts to prevent wildfires have recently begun to disrupt energy systems. In California and Oregon, electric utilities have on several occasions responded to hot, dry, windy conditions by shutting off power to hundreds of thousands of households and businesses to reduce fire risk. These power shutdowns may be prudent when fire danger is very high, and they have shown to be effective in reducing the likelihood of ignitions. Nonetheless, they are extremely costly, causing businesses and schools to close, perishable foods to spoil as refrigerators turn off, and medical devices to fail without power. One study estimated that a shutoff event in October 2019 cost California’s economy up to $2.5 billion. While so far power supply shutoffs have primarily been a Californian phenomenon, Portland General Electric cut power to more than 5,000 Oregonians in September 2020 due to wildfire weather conditions.
Human Health Impacts
Fatalities: Wildfires cause human fatalities both directly (when people are unable to escape a blaze, or when firefighters are killed while containing a fire) and indirectly (particularly due to the health effects of smoke inhalation).
Smoke: The smoke emitted by wildfires causes significant negative human health impacts, not only near a fire but hundreds or even thousands of miles away. Wildfire smoke is made up of gases and fine particulate matter (soot), which can be very hazardous to health, especially for older people and children. Exposure to fine particulate matter—known as PM2.5, because the particulates are smaller than 2.5 micrometers in diameter—is linked to a variety of lung and heart problems. In recent years, wildfires have contributed 25 to 50 percent of PM2.5 exposure in the United States, depending on the region. This smoke is estimated to kill approximately 500 elderly people in the United States each year, on average.
Mental Health: A study conducted after California's 2018 Camp Fire found that direct exposure to the wildfire significantly increased the risk for mental health disorders such as post-traumatic stress disorder (PTSD) and depression. Another study conducted in Arizona a year after the Wallow Fire documented significant psychological stress due to the dramatic change to the landscape and the financial burdens the wildfire caused. Notably, the Arizona study found that households with an annual income greater than or equal to $80,000 suffered less psychological distress.
Watersheds: Wildfires can impact water quality and water supply within watersheds. After fires, soils may be less able to absorb water. Heavy storms that occur directly after fires can therefore be especially dangerous. For example, in January 2018, directly following the nearby Thomas Fire, a storm struck Montecito, California, resulting in landslides that killed 23 people. Burned watersheds can also see diminished water quality, which can threaten community water supplies. In communities through which fires have burned, water supplies have been contaminated with toxic chemicals that may affect drinking water for years.
Ecosystems: While many ecosystems need fire to thrive, wildfires outside historic fire severity levels or fire return intervals may be ecologically harmful. If burned too often, ecosystems may not have time to recover in between. Severe wildfires can also have a large effect on old-growth trees—in 2020, California’s Castle Fire destroyed 10 percent of the world’s mature giant sequoias.
Carbon Emissions: By burning through carbon-rich forests and grasslands, wildfires can emit an enormous amount of carbon dioxide, methane, and carbon monoxide into the atmosphere. While wildfires are essential for ecosystem function in many western ecosystems, recent increases in western wildfire activity have yielded increased emissions. In 2020, wildfires in California alone emitted an estimated 112 million metric tons of carbon dioxide—roughly the same annual emissions as 24.2 million cars. Because they emit so much carbon dioxide, wildfires and climate change can enter into a feedback loop; climate change creates more intense wildfires, which then emit more greenhouse gases, which then makes climate change worse.