Wildfire Trends in the United States
Wildfires are not a new phenomenon, but in many regions of the United States, particularly the western states, they have become larger, longer-lasting, more frequent, and more destructive in terms of lives lost and economic costs. In 2017 in California, 9,000 wildfires burned more than one million acres of land. That same year, fire seasons in Washington and Oregon extended nearly three weeks longer than ever recorded. In August of 2018, less than a year after the Thomas fire grew to become the largest fire in California’s history, the Mendocino Complex fire broke that record. Recent research has helped clarify the confluence of factors that is driving these destructive trends, including climate change, increased settlement along the wildland-urban interface, the spread of invasive species, and an antiquated “zero-tolerance” fire-management paradigm that reigned for nearly a century and is only now being replaced by more evidence-based strategies. Although the mechanics of how wildfires start and spread are well established, it’s crucial to understand how a changing climate will influence these mechanics, how important fire is to the health of many ecosystems, and how we can learn to coexist with fire in the future.
What is a wildfire?
Wildfires are unplanned and uncontrolled fires that originate in wildland areas but which sometimes spread to populated areas where they can consume not just natural but also human resources.
Wildfires differ from “prescribed burns”—wildland fires that are purposefully ignited and controlled for forest management purposes.
How do wildfires start?
- Fires require:
Fuel (typically wood or grasses, with smaller and drier burning more easily, but “green” fuels sometimes feed flames with flammable oils)
Oxygen (most fires require an atmospheric oxygen level of at least 16 percent; concentrations are around 21 percent at sea level, so oxygen is rarely a limiting factor even at high elevations)
Heat provides the energy for initial ignition and for spreading fire to surrounding fuel.
- Between 2001 and 2012, 85 percent of U.S. wildfires were human-caused (e.g., from arson or improperly extinguished campfires)—primarily in areas where human-driven development abuts wildlands:
Perhaps unsurprisingly, July 4th is the day with the most human-started fires.
During the same period, however, 60 percent of the total acreage burned was from naturally ignited fires (e.g., from lightning).
This means that while people are the leading cause of wildfires, naturally occurring wildfires are responsible for most of the area burned—in part because firefighters lack easy access to the deep wildlands, where many lightning ignitions occur.
Human ignited fires tend to threaten property that needs to be saved, generating a higher need for suppression compared to naturally occurring fires, which in many cases are monitored but minimally suppressed.
Lightning storms, which peak in the summer months when fuels are driest, also often cause multiple simultaneous fires, an increased “fire load” that can overwhelm suppression response capacity.
When is “wildfire season”?
Wildfire season varies from place to place, based on regional weather patterns and topography, and is defined as the period of time in a given area when environmental conditions are optimal for wildfires to occur.
Typically, the season aligns with times when dry fuels are abundant, temperatures and winds are high, and humidity is low.
But human factors are key as well: A 2017 study showed that the increase in human-caused ignitions have tripled the length of the average fire season between 1992 and 2012, compared to the lightning-caused wildfire season.
Is climate change causing an increase in wildfires?
- Climate change is already contributing in some ways to the increased destructive power of wildfires and is projected to have a growing impact in the future. Factors include:
Temperature. In the western United States, higher temperatures and altered precipitation patterns associated with climate change are predicted to contribute to drought and earlier and longer wildfire seasons—and appear already to be contributing to drier fuels.
A 2016 study found that human-caused climate change was responsible for more than half the documented increases in fuel aridity since the 1970s and for doubling the cumulative forest fire area since 1984.
The Intergovernmental Panel on Climate Change projects that by 2050, summer temperatures in the U.S. West will increase between 2 and 5 degrees Celsius—a worrisome prediction given that a 2012 wildfire risk assessment by the U.S. Forest Service concluded that a temperature increase of just 1.6 degrees Celsius would at least double the area burned in most western states.
A 2013 study projected that by mid-century, the U.S. wildfire season would be about three weeks longer, up to twice as smoky, and would burn more acreage in the western states than it did, on average, between 1980 and 2004.
Natives & Invasives. Climate change is fostering the spread of native insects in some regions, which can weaken or kill vegetation and produce more fuel for wildfires to consume. Warming temperatures are also likely to offer an advantage to some invasive, highly flammable plants, including cheatgrass.
Lightning. A 2014 study projected that lightning, the leading cause of wildfires in terms of area burned, will increase by approximately 12 percent for every degree Celsius of global warming.
- Despite these important impacts, human activities remain the major driver of extended fire seasons and increased wildfire risks.
What factors affect how wildfires spread?
Dry and plentiful fuels, particularly small twigs and needles, are highly flammable and facilitate spreading.
High winds, which deliver more oxygen to wildfires, increase their temperature and spread flames and flaming debris.
Topographic slopes also increase the ability of fires to spread. (Wildfires tend to climb uphill, as heat rises and dries out vegetation.)
Different types of fires progress differently.
Surface fires usually do the least damage and are easiest to put out.
Crown fires in the canopy of forest and shrubland ecosystems burn hotter and often create additional “spot fires” miles away as flaming debris is carried by the wind.
Subsurface fires, or ground fires, burn organic material within soil. These progress slowly but can be difficult to fully suppress.
It is not uncommon for multiple fire-types to occur simultaneously. Often, crown fires in forests begin at the surface, and progress upward via “ladder fuels” such as low branches, tall shrubs, and young trees.
Structures built of flammable materials, including wooden patios and decks, can accelerate the spread of wildfire in populated areas.
Wildfire suppression strategies
While fully extinguishing a wildfire is usually the ultimate goal, firefighters often focus first on “suppression”—that is, working to limit a fire’s ability to spread. Common suppression techniques include:
Creation of firelines or firebreaks: A process by which strips of land are partially or fully cleared of fuel preemptively, to block a fire’s progression.
Application of water and fire retardant: These are dropped onto a wildfire from the air to increase fuel moisture content and reduce the fire’s access to oxygen. Fire retardant is usually a mixture of water and additives that encourage the retardant to stick to fuel longer before evaporating. (The potential harmful environmental impact associated with flame retardants is an area of active debate and scientific research.)
Ignition of controlled fires: These purposefully ignited burns help widen firelines and preemptively consume fuels so they are not available to the wildfire.
Should we always try to extinguish wildfires?
- Not in every case. For more than a century, it has been common practice to suppress wildfires whenever and wherever possible. Scientists now know that in areas not inhabited by people, it is sometimes advantageous to let wildfires fires burn.
- When every wildfire is suppressed, forests become denser, more tightly packed with fuel, and much more flammable—making even more destructive fires possible in the future.
- Fire is also an integral and natural part of many ecosystems.
- Fire germination. For some plants in fire-prone areas, fires are a critical growth catalyst, opening waterproof seed pods to let in moisture and spur germination. Some plant species actually require open flames for germination to occur, while others need exposure to the hot gases found in smoke or the chemicals produced by partially charred wood.
- Fertilizer. The wood-ash produced by wildfires acts as a natural fertilizer. It is rich in potassium, calcium, and magnesium, which stimulate the fruiting and flowering process for many plants. The carbonates in wood ash can also help neutralize overly acidic soil.
- Opening the canopy. Crown fires clear away vegetation and branches high in the canopy, exposing the forest floor to sunlight and rainfall so new growth can occur.
- Researchers recognize that, under the right conditions, it can be beneficial to ignite “prescribed” fires, which, along with selective logging or “thinning,” can provide ecological benefits while also reducing wildfire risks.
- A study of the 2006 Tripod Complex wildfires in Washington state showed that in areas where prescribed burns and thinning had occurred, more than 57 percent of trees survived, while only 14 percent survived in areas with no prior preventative treatments. While these tactics seem promising, vegetation regrows quickly, and thinning and prescribed burns would need to be carried out regularly.
Can scientists predict when and where wildfires will occur?
Not reliably, since ignition is often caused by unpredictable human activities or lightning strikes. But once a wildfire starts, scientists have sophisticated computational models that integrate information about fuel type, fuel location, topography, and weather to predict how it will behave and spread.
These models help fire managers determine which wildfires must be suppressed, and which can or should be left to run their course. They also inform decisions about where firelines should be built and which communities are most at risk.
The accuracy of fire models continues to improve as computing capacity increases, and real-time satellite data is more seamlessly integrated.
How much do wildfires cost?
- In 2017, federal suppression costs for U.S. wildfires exceeded $2 billion for the first time. A 2017 report by the National Institute of Standards and Technology concluded that the annual U.S. economic burden of wildfires was between $71 and $348 billion, including local, state, and federal suppression costs.
- Cost estimates vary widely depending on whether they include prevention and preparedness costs and indirect losses such as impacts on tourism, housing value, supply chains, and health impacts.
The health impacts of wildfires
Wildfire smoke can linger in the atmosphere long after the fire has been controlled, and can contain carbon monoxide, lung-damaging particulates, and toxic, volatile organic compounds.
Air pollution from wildfire smoke is usually measured by the amount of fine particulate matter in the atmosphere.
According to the U.S. Environmental Protection Agency, fine particulate pollution is unhealthy once the daily average concentration exceeds 35 micrograms per cubic meter. Particulate levels during and after a wildfire often exceed that limit many times over.
The health effects of exposure to wildfire smoke depend on the duration of exposure and timing of exposure (how much the smoke has dissipated). The effects can range from irritated eyes and coughing to serious respiratory problems and a worsening of chronic heart and lung conditions.
People with asthma, other preexisting respiratory conditions, or cardiovascular disease are especially at risk from wildfire smoke exposure, as are the elderly, pregnant women, children, and cigarette smokers.
LAST UPDATED August 9, 2018
SciLine is an independent, nonpartisan, philanthropically supported service hosted by the nonprofit American Association for the Advancement of Science (AAAS). For more information visit our About SciLine page or write to us at email@example.com
key references for those who want to dig deeper
Wildfire Behavior, Prevention, Prediction, & Mitigation:
- The National Interagency Fire Center created a communicator’s guide for wildland fire. Chapter two, Wildland Fire Overview, gives a detailed introduction to fire dependent ecosystems, fire behavior, and the effects of fire.
- In 2010, the U.S. Department of Agriculture published The 2010 Wildland-Urban Interface of the Conterminous United States, which outlines the increase in population along the wildland-urban interface and what that means for wildfire management.
- A 2017 paper in the Proceedings of the National Academy of Sciences (PNAS), Human-started wildfires expand the fire niche across the United States, found that wildfires ignited by people accounted for the vast majority of wildfires and tripled the length of the fire season when compared to lightning wildfire season.
- In 2017, the National Academies of Science, Engineering, and Medicine organized a workshop called A Century of Wildland Fire Research: Contributions to Long-term Approaches for Wildland Fire Management. The proceedings of a workshop which examined the last century of wildland fire research in the context of the recent, rapid increases in extreme and hazardous fires as well as the occurrence of fire as an integral part of the landscape.
- The Canadian Journal of Forest Research published a 2010 paper, Fuel treatments reduce the severity of wildfire effects in dry mixed conifer forest, Washington, USA, that demonstrates the benefits of prescribed burning and thinning for the protection of forests from future destruction from wildfires.
Climate Change & Wildfires
- A 2013 study in the journal Atmospheric Environment, Ensemble projections of wildfire activity and carbonaceous aerosol concentrations over the western United States in the mid-21st century, used 15 internationally recognized climate scenarios, decades of historical meteorological data, and past fire activity to predict what wildfire activity in the western United States would be like in the middle of this century.
- A 2014 Science paper, Projected increase in lightning strikes in the United States due to global warming, constructed a proxy based on both the energy available to cause air to rise and on the precipitation rates in order to model the frequency of lightning strikes in the United States. The authors then applied the proxy to 11 climate models and found that strikes would increase 12 ± 5% per degree Celsius of global warming.
- In 2012, the U.S. Department of Agriculture published a general technical report, Effects of Climatic Variability and Change on Forest Ecosystems. The concluding chapter by the U.S. Forest Service, Risk Assessment for Wildfire in the Western United States, assesses the risk of wildfires in response to climate change in relation to fire frequency, intensity, severity, and area burned.
- A 2016 PNAS paper, Impact of anthropogenic climate change on wildfire across western US forests details the impact human-driven climate change has had on increased fuel aridity during fire season.
Economic and Health Impacts of Wildfires:
- In 2017, the National Institute of Standards and Technology released Special Publication 1215, The Costs and Losses of Wildfires: A Literature Survey. The report comprehensively outlines the various costs of wildfire management and wildfire-related losses, including costs related to intervention and prevention, as well as the economic burden of direct and indirect losses.
- The Federal Emergency Management Agency has created a fact sheet for the general public covering some quick tips for how to prepare and respond to a wildfire safely.
- Wildfire Smoke: A Guide for Public Health Officials was compiled and published by the U.S. Environmental Protection Agency developed in part as a result of a workshop held at the University of Washington under the auspices of the EPA. It provides an introduction for public health officials to the health impacts of wildfire smoke and offers advice on how to prepare and respond.