Wildfires are burning more intensely and across wider areas than they did a few decades ago, driven by a combination of rising temperatures, over a century of fire suppression, expanding human development, and the fact that people start the vast majority of fires in the first place. No single factor explains the increase. These causes reinforce each other, creating conditions where fires that once would have stayed small now explode into catastrophic events.
People Start Most Wildfires
Of the roughly 1.5 million wildfires recorded in the United States between 1992 and 2012, 84% were started by people. Lightning accounts for the rest. Human ignitions come from power lines, equipment use, campfires, arson, fireworks, and vehicle sparks, among other sources. What makes human-caused fires especially significant is their timing and geography: they tripled the length of the fire season compared to lightning fires alone and dominated an area seven times larger. Lightning fires cluster in remote terrain during summer storms, but human ignitions happen year-round, near roads, towns, and dry grass.
In California, utility infrastructure causes less than 10% of reported wildfires by count, but power line failures are responsible for roughly half of the most destructive fires in the state’s history. A single downed line in dry, windy conditions can ignite a fire that reaches populated areas within hours.
A Hotter, Drier Atmosphere
Rising temperatures are drying out vegetation faster and more thoroughly, making landscapes far more flammable. The key mechanism is something called vapor pressure deficit, essentially a measure of how thirsty the air is. When the air is hot and dry, it pulls moisture out of soil, leaves, and dead wood. As background temperatures climb due to climate change, this drying effect intensifies across entire regions.
Research published in Geophysical Research Letters found that across the western United States, the fire potential associated with ordinary weather patterns has increased significantly. In practical terms, this means less extreme weather is now sufficient to start large fires. A moderately hot, dry day that would not have been dangerous 30 years ago can now push a landscape past the ignition threshold. This “lowering of the bar” explains why fire risk is expanding into regions that historically didn’t burn much.
Globally, the fire weather season lengthened by nearly 19% between 1979 and 2013, with statistically significant increases across about a quarter of all vegetated land on Earth. In North America, the trend has been consistent and measurable for decades.
A Century of Putting Out Every Fire
For most of the 20th century, the official policy in the United States was to suppress every wildfire as quickly as possible. That approach succeeded at eliminating smaller, lower-intensity fires, the kind that historically cleared out brush and dead wood every few years or decades. But by removing those natural fires, forests accumulated enormous amounts of fuel: dense undergrowth, dead trees, and deep layers of dry litter on the forest floor.
A 2024 study in Nature Communications described this as the “suppression paradox.” In trying to eliminate all fires, agencies only eliminated the less intense ones. The result was higher fuel loads and what researchers called “suppression-resistant” fires, blazes so intense that modern firefighting resources struggle to contain them. The fires that do escape suppression now burn through decades of accumulated fuel, producing far more heat and moving far faster than they would in a landscape that burned regularly.
Invasive Grasses Are Changing Fire Cycles
Across large parts of the American West, invasive grasses have fundamentally altered how fire behaves. Cheatgrass is the most well-known example. It colonizes the spaces between native shrubs like sagebrush, creating a continuous carpet of fine fuel that dries out weeks earlier than native plants. When it burns, it recovers quickly, providing fresh fuel for the next fire and shortening the interval between burns to a pace that native vegetation cannot survive.
A nationwide analysis published in the Proceedings of the National Academy of Sciences documented regional-scale fire regime changes from at least eight invasive grass species. Some increased fire occurrence by up to 230% and fire frequency by up to 150% in the areas they colonized. This is not a minor ecological nuisance. Invasive grasses are rewriting the fire behavior of entire ecosystems, from Great Basin shrublands to pine savannas in the Southeast.
More Homes in Fire-Prone Areas
The number of homes sitting in or near wildland vegetation has grown dramatically. As of 2020, 44 million homes in the contiguous United States, 32% of all housing, were located in what’s known as the wildland-urban interface: the zone where development meets undeveloped, burnable land. Between 1990 and 2020, the number of homes in these areas grew by 47%, and the total WUI acreage expanded by an area equivalent to the state of Washington.
Even after the fastest growth period slowed around 2010, another 2.6 million homes were built in the WUI during the following decade. This expansion puts more ignition sources near wildland fuels and more property in the path of fires that do start. It also forces firefighting agencies to prioritize protecting structures over managing fire on the landscape, which feeds back into the suppression paradox.
The Numbers Tell Two Stories
National Interagency Fire Center records show a clear upward shift in area burned across the United States. Through most of the late 1980s and early 1990s, annual acreage burned hovered between 1 and 5 million acres. Since 2000, multiple years have exceeded 8 million acres, with 2020 topping 10 million. The year 2024 saw nearly 8.9 million acres burn.
Federal suppression costs reflect this escalation. In 2019, total suppression spending was about $1.6 billion. By 2021, it reached $4.4 billion. Even in a relatively modest fire year like 2023, the bill exceeded $3.1 billion.
Globally, the picture is more complicated. Total area burned worldwide has actually declined by about a quarter since the early 2000s, largely because fires in African savannas have decreased as landscapes there become more fragmented by agriculture. But that overall decline masks sharp regional increases. Eastern Siberia, western North America, and parts of South America have seen burned area rise by more than 40% since 2000. The 2024-2025 fire season brought extreme fire activity to Canadian boreal forests, the Amazon, and Bolivian dry forests. In other words, the global fire problem hasn’t grown uniformly. It has shifted, concentrating in the forests and boreal regions where fire is most destructive and hardest to manage.
Why These Factors Compound
None of these causes operate in isolation. A hotter atmosphere dries out the excess fuel left by decades of suppression. Invasive grasses fill in the gaps that prescribed burns or natural fires once maintained. More people living in fire-prone areas means more ignitions and more pressure to suppress fires rather than let them play their ecological role. Each factor amplifies the others.
The result is a fire environment where the baseline has shifted. Landscapes are drier, fuel loads are heavier, ignition sources are more widespread, and the areas at risk are larger. Fires that start under these conditions grow faster, burn hotter, and resist containment in ways that previous generations of firefighters rarely encountered. The increase in catastrophic wildfires is not a temporary spike. It reflects structural changes in climate, land use, and vegetation that have been building for decades.