Wildfires and Climate Change: What Is Driving Bigger Fire Seasons

Wildfires are a natural part of many ecosystems. Forests, grasslands and shrublands have burned for millions of years, and some plants even depend on fire to reproduce. But in many regions, fire seasons are getting longer, and large, intense fires have become more frequent. This explainer looks at how climate change fits into that picture, what else is going on, and what it means for people living with fire and smoke.

What a fire needs

Firefighters talk about the fire triangle: heat, fuel and oxygen. For landscape wildfires, it helps to think about three ingredients:

1. Fuel: grass, leaves, needles, shrubs, fallen branches and trees.
2. Dry, fire-friendly weather: heat, low humidity, wind and a lack of recent rain.
3. An ignition: lightning, or a human source such as a downed power line, a campfire, equipment sparks or arson.

Climate change does not create sparks, and it does not on its own decide how much fuel is on the ground. Its strongest influence is on how dry that fuel becomes and for how long.

How warming dries the land

Warmer air can hold more moisture. That sounds like it would make things wetter, but in practice it means warm air pulls more water out of plants and soils. Scientists measure this "thirst" of the atmosphere with a quantity called vapor pressure deficit. When it is high, vegetation dries out faster and burns more readily.

Several related changes add up:

• Hotter summers and more frequent heat waves dry out fuels. See Heat waves and health for more on extreme heat.
• Earlier snowmelt in mountain regions means soils and forests start drying out sooner in the year.
• Longer warm seasons stretch the window when fires can start and spread.
• Drought, when it happens, leaves vegetation stressed and dead trees that act as extra fuel.

The IPCC's Sixth Assessment Report concluded that human influence has likely increased the chance of compound events such as hot, dry conditions, and that fire weather has become more frequent in some regions. It projects further increases in fire weather in many regions as warming continues.

Not everywhere, and not every year

Climate change does not mean more fire everywhere. Globally, the total area burned each year has actually decreased in some recent decades, largely because savannas and grasslands in places like Africa have been converted to farmland. Meanwhile, burned area and fire severity have risen in other regions, including parts of the western United States, Canada, Siberia and Australia. Year-to-year swings are also large, influenced by natural patterns such as El Niño and La Niña.

Other factors that matter

Climate is only one part of the story. Researchers emphasize several other drivers:

Factor	How it affects wildfire
Fire suppression history	In some forests that once burned often at low intensity, a century of putting out every fire let fuel build up, setting the stage for more intense fires.
Human ignitions	In the United States, most wildfires are started by people, whether by accident or on purpose. Human ignitions also extend fire seasons beyond the lightning season.
Building in fire-prone areas	More homes in the "wildland-urban interface" means more potential ignitions and more property at risk when fires happen.
Invasive grasses	In some regions, non-native grasses dry out quickly and carry fire into places that rarely burned before.
Land management	Prescribed burning, thinning and grazing can reduce fuel loads; their absence can increase them.

The National Interagency Fire Center publishes US statistics on the number of fires, acres burned and causes. Its figures show large year-to-year variation, which is a reminder to look at long-term trends rather than single seasons.

Fire and the carbon cycle

Fires release carbon dioxide as vegetation burns. In ecosystems that regrow, much of that carbon is eventually taken back up by new plants. The concern is when fires become so frequent or severe that forests do not recover, converting to shrubland or grassland, or when fires burn deep into carbon-rich peat soils in the tropics and the far north. In those cases, carbon that was stored for centuries can be lost for a long time. Northern fires can also speed up thaw of frozen ground; see Permafrost thaw explained.

Smoke and health

For many people, the most direct experience of wildfire is smoke. Wildfire smoke contains fine particles known as PM2.5, small enough to get deep into the lungs. Smoke plumes can travel across continents, so cities far from any fire can see poor air quality for days.

Health agencies advise that people with heart or lung disease, older adults, children and pregnant people are among those most at risk. Practical steps during smoky periods include:

• Check local air quality on AirNow or your national equivalent.
• Stay indoors with windows and doors closed when air quality is poor, and reduce strenuous outdoor activity.
• Use a portable air cleaner with a HEPA filter, or the highest-rated filter your heating and cooling system can use.
• If you need to go outside, a well-fitting N95 or equivalent respirator offers better protection than cloth or surgical masks.

The EPA's wildfires page has more detailed guidance on preparing for smoke. This is general information; anyone with health concerns should follow advice from their doctor and local public health authorities.

Staying safe in fire country

If you live in a fire-prone area, the National Weather Service's wildfire safety page explains red flag warnings and how to prepare. Common advice includes having an evacuation plan and a go-bag, signing up for local emergency alerts, and creating defensible space by clearing flammable material from around your home. Always follow evacuation orders from local officials.

What can be done

Reducing wildfire risk takes action on several fronts at once. Over the long term, limiting warming by cutting greenhouse gas emissions will reduce how much fire weather increases. In the near term, land managers are expanding prescribed burning and cultural burning practiced by Indigenous communities, communities are adopting fire-resistant building standards, and utilities are hardening power lines. For personal steps on emissions, see What can one person do?

The bottom line

Wildfire is natural, but the conditions that make fires large and dangerous are becoming more common in many regions as the climate warms. Climate change works alongside fuel buildup, human ignitions and development patterns, so solutions need to address all of them. Smoke means wildfire is a health issue even for people who never see a flame.