Wildfires are burning more land than they did a generation ago, and the reasons are layered: a warming climate that dries out vegetation faster, decades of fire suppression that left forests overgrown, invasive grasses that act as kindling, and millions of new homes built in fire-prone areas. In the western United States alone, the wildfire season has stretched from five months in the 1970s to roughly seven months today. None of these factors work in isolation. They compound each other in ways that make large, destructive fires increasingly common.
The Numbers Tell a Clear Story
Data from the National Interagency Fire Center shows a striking shift. In the late 1980s and early 1990s, annual acreage burned in the U.S. typically ranged from 1.5 to 3 million acres. By the 2000s, years topping 7 million acres became routine. The five-year stretch from 2015 to 2020 included three years where more than 10 million acres burned. The year 2020 alone saw 10.1 million acres go up in flames.
The number of individual fires hasn’t necessarily increased. In fact, the annual count has often hovered between 50,000 and 70,000 in recent years, compared to 80,000 or more in some years during the 1990s. What has changed is the size and intensity of those fires. Fewer fires are burning far more land, which points to conditions that allow fires to grow rapidly once they start.
Hotter, Drier Air Pulls Moisture From Fuel
The single biggest driver behind more destructive wildfires is a warming atmosphere. Higher temperatures don’t just raise the risk of fire in a vague, general way. They work through a specific mechanism: the hotter the air, the greater its capacity to pull moisture out of soil, grass, shrubs, and trees. Scientists measure this as vapor pressure deficit, essentially the gap between how much moisture the air could hold and how much it actually contains. As that gap widens, vegetation dries out faster and ignites more easily.
Vapor pressure deficit has been closely linked to forest area burned across the western United States. When the atmosphere is especially thirsty, even living vegetation can become flammable. This is why fires in recent years have torn through green forests that would have resisted burning in cooler decades. Average temperatures in the western U.S. have risen roughly 2°F since the 1970s, and that seemingly small shift has a large effect on how quickly fuels dry out during summer.
Earlier Snowmelt Extends the Danger Window
Warming doesn’t only affect summer. It changes when spring arrives. Across the mountainous West, snowpack is melting earlier, which means soils and vegetation begin drying out weeks sooner than they used to. Fire is advancing to higher elevations as warming, drier summers, and earlier spring snowmelt combine to push the fire line uphill into forests that historically rarely burned.
This creates a feedback loop: wildfires themselves strip away tree canopy, which exposes the remaining snowpack to more direct sunlight the following winter. Research shows that wildfires are causing earlier snowmelt across the western U.S., and projected warmer winters will make that effect worse. Less snow cover in spring means less moisture feeding into the soil during the critical months before fire season.
A Century of Fire Suppression Built Up Fuel
For most of the 20th century, U.S. fire policy focused on putting out every fire as quickly as possible. That approach seemed logical, but it had an unintended consequence: forests that evolved with periodic fire grew unnaturally dense. Undergrowth, dead wood, and small trees accumulated on the forest floor for decades without being cleared by the low-intensity burns that once cycled through naturally every 10 to 30 years.
The result is forests packed with fuel. When a fire does start in these overgrown areas, it doesn’t creep along the ground the way a natural fire would. It climbs into the canopy and becomes a high-intensity blaze that’s far harder to control and far more destructive to the ecosystem. Many of the catastrophic fires in California, Oregon, and Colorado in recent years burned through forests that hadn’t seen fire in 80 or 100 years.
Invasive Grasses Create a Fire Cycle
In the rangelands of the Great Basin, covering parts of Nevada, Utah, Idaho, and Oregon, a different kind of fuel problem has taken hold. Cheatgrass, a non-native annual grass introduced from Eurasia, has invaded millions of acres of sagebrush habitat. It grows fast in spring, dies early in summer, and dries into dense, continuous mats of dead plant material right when temperatures peak.
Native shrublands have natural gaps between plants that slow fire spread. Cheatgrass fills those gaps, creating an unbroken carpet of fine fuel. Fire return intervals in cheatgrass-dominated areas are now two to four times more frequent than historic levels. And each fire makes the problem worse: sagebrush takes decades to regrow, but cheatgrass rebounds within a single season. The result is a self-reinforcing cycle where fire converts native shrubland into grassland, which then burns again. Soil disturbances from livestock grazing have accelerated the invasion by giving cheatgrass a foothold in areas where native plants once dominated.
Insect Outbreaks Leave Forests Vulnerable
Bark beetles have killed billions of trees across western North America over the past two decades, aided by warmer winters that allow beetle populations to explode. The relationship between dead trees and fire is complex, but the basic physics are straightforward. Dead pine trees in the initial stage after a beetle attack, when they still hold dry red needles, have higher ignition potential and can burn more intensely. As those needles eventually fall, surface fuel loading increases as branches, foliage, and eventually entire trunks decompose on the forest floor.
High levels of tree mortality can rapidly reshape the fuel structure of a forest, creating conditions that foster more intense and severe fires. In California’s Sierra Nevada, the drought of 2012 to 2016 killed an estimated 129 million trees, leaving vast stretches of mountainside covered in standing dead timber.
More Homes in Fire-Prone Areas
Even if the same number of acres burned every year, wildfires would cause more damage now simply because more people live in their path. The wildland-urban interface, the zone where homes meet or intermingle with undeveloped wildland, has expanded dramatically. Between 1990 and 2020, the number of homes in this zone increased by 47 percent. Even in the most recent decade, 2.6 million new homes were built there.
This growth has two effects. First, more structures and more people are directly at risk. Second, the presence of homes changes how firefighters respond. Resources that might otherwise be used to manage a fire’s spread get redirected to structure protection, which can allow the wildfire itself to grow larger. Homes also introduce ignition sources: power lines, vehicles, equipment, and landscaping that burns easily. The expansion of development into fire-prone landscapes is one of the clearest reasons wildfires cause more destruction today than in previous decades, even when the total acreage burned is comparable.
What the Trend Looks Like Going Forward
The United Nations Environment Programme projects that extreme wildfires globally will increase by up to 14 percent by 2030, 30 percent by 2050, and 50 percent by the end of the century. These projections are driven by continued warming and land-use patterns that push more people and more flammable vegetation into close contact.
None of the underlying factors are reversing. Temperatures continue to rise. Development in fire-prone zones continues. Invasive grasses are expanding their range. And the fuel accumulated over a century of fire suppression can’t be removed overnight. Prescribed burns and mechanical thinning are scaling up in many western states, but the pace of treatment lags far behind the scope of the problem. The U.S. Forest Service has estimated that tens of millions of acres of federal forest need fuel reduction work. Current treatment rates cover a fraction of that each year.