The 2025 Los Angeles wildfires burned over 40,000 acres, destroyed thousands of structures, and caused an estimated $250 billion in damages, making it one of the costliest wildfire events in U.S. history. Globally, wildfire seasons are growing longer and more intense, with the area burned annually increasing by roughly 20% since the 1980s according to NASA Earth Observatory.
A wildfire is an uncontrolled fire that spreads through vegetation, driven by the interaction of heat, drought, and wind. As one of the 12 physical risk hazards in climate risk assessment, wildfire risk is measured using a Fire Weather Index that combines temperature, precipitation, wind speed, and land cover data. Understanding what a wildfire is and what drives fire danger is essential for protecting communities, infrastructure, and natural ecosystems.
What Is a Wildfire?
A wildfire is any uncontrolled fire burning in wildland vegetation, including forests, grasslands, and shrublands. Wildfires differ from prescribed burns (controlled fires set deliberately for land management) in that they are unplanned and uncontained, spreading according to fuel availability, weather conditions, and terrain.
Wildfires can travel at speeds exceeding 20 km/h in extreme conditions, jumping roads, rivers, and firebreaks. Crown fires that reach tree canopies are particularly destructive, generating their own wind patterns that can propel embers kilometers ahead of the fire front. The wildfire definition encompasses everything from small brush fires to landscape-scale infernos that burn for weeks.
The destructive power of a wildfire depends on three interacting elements known as the fire triangle: fuel (vegetation), heat (temperature and drought), and oxygen (wind). Remove any one element and the fire cannot sustain itself. Climate change is amplifying all three, extending fire seasons and expanding the area vulnerable to wildfire.
What Causes Wildfires?
Wildfire causes fall into two categories: ignition sources and the conditions that allow fires to spread.
Natural ignition. Lightning is the primary natural wildfire cause, responsible for roughly 15% of wildfires globally. Lightning-caused fires tend to burn in remote areas and can smolder undetected for days before conditions allow rapid spread.
Human ignition. Human activity causes approximately 85% of wildfires. Sources include power lines, agricultural burning, campfires, discarded cigarettes, arson, and equipment sparks. The wildfire-urban interface, where development meets wildland vegetation, concentrates human ignition sources near highly flammable fuels.
Fire-enabling conditions. Regardless of ignition source, wildfire spread depends on three climate-driven conditions:
- High temperatures dry out vegetation, reducing its moisture content and making it more flammable. Heatwaves create ideal wildfire conditions by desiccating fuels across large areas.
- Drought depletes soil and vegetation moisture over weeks and months, creating a deep fuel bed. Extended drought turns even green vegetation into potential fuel.
- Strong winds supply oxygen to the fire, push the fire front forward, and carry embers ahead of the main blaze. Wind-driven wildfires are the most dangerous and difficult to contain.
How Climate Change Increases Wildfire Risk
Climate change is amplifying every element of wildfire risk. Research from the UN Environment Programme projects a 30% increase in extreme wildfire events by 2050.
Longer fire seasons. Earlier snowmelt and later fall rains extend the window when vegetation is dry enough to burn. In parts of western North America, fire season has lengthened by nearly 80 days since the 1970s.
Hotter and drier conditions. Rising temperatures increase evaporative demand, drying vegetation faster and more thoroughly. Regions that historically had sufficient moisture to resist wildfire are crossing flammability thresholds as temperatures rise.
Increased storm intensity. Lightning frequency increases with atmospheric instability driven by warming. More lightning means more natural ignition events, particularly in boreal forests that contain vast carbon reserves.
How Is Wildfire Risk Assessed?
Wildfire risk assessment uses a Fire Weather Index proxy that combines three NASA NEX-GDDP-CMIP6 climate variables with ESA WorldCover land use data. The formula accounts for the key wildfire causes that determine fire danger:
fire_risk = drought_factor × temp_factor × wind_factor × landcover_modifier
The three climate factors capture the fire weather conditions that enable wildfire spread, while the land cover modifier adjusts risk based on the type of vegetation present:
| Land Cover Type | Modifier | Rationale |
|---|---|---|
| Forest / Shrubland | 1.0 | Highest fuel load, canopy fire potential |
| Grassland | 0.5 | Fast-spreading but lower intensity |
| Cropland | 0.3 | Managed vegetation, seasonal fuel |
| Urban / Water / Bare | 0.1 | Minimal vegetation fuel |
Wildfire risk ratings follow a 5-tier scale:
| Risk Rating | Fire Weather Index | Fire Danger Level |
|---|---|---|
| Low | Less than 5 | Fires unlikely to sustain |
| Moderate | 5-15 | Fires possible under dry conditions |
| High | 15-30 | Active fire spread likely |
| Severe | 30-50 | Rapid spread, difficult containment |
| Extreme | Greater than 50 | Catastrophic fire potential |
Wildfire risk is assessed under SSP2-4.5 and SSP5-8.5 scenarios across baseline, 2030, 2040, and 2050 time horizons. Platforms like Continuuiti assess wildfire alongside 11 other climate hazards for a complete physical risk profile.
Frequently Asked Questions
What is a wildfire?
A wildfire is an uncontrolled fire burning in wildland vegetation. Wildfires spread based on fuel availability, weather conditions, and terrain. The wildfire definition covers everything from small brush fires to landscape-scale infernos that burn for weeks.
What causes wildfires?
Approximately 85% of wildfires are caused by human activity, while 15% are caused by lightning. Fire spread depends on high temperatures, drought, and wind. These three conditions form the fire weather triangle that determines wildfire behavior.
How does climate change affect wildfire risk?
Climate change lengthens fire seasons, creates hotter and drier conditions, and increases lightning frequency. The UN Environment Programme projects a 30% increase in extreme wildfire events by 2050 as warming continues.
How is wildfire risk assessed?
Wildfire risk uses a Fire Weather Index combining drought, temperature, and wind data from NASA NEX-GDDP-CMIP6 with ESA WorldCover land use data. Forest areas have the highest risk modifier while urban areas have the lowest. Risk is rated from Low to Extreme.
Wildfire risk is growing as climate change extends fire seasons and intensifies the heat, drought, and wind conditions that drive fire behavior. Assessing wildfire risk requires combining climate projections with land cover analysis to identify where fire danger is increasing. Locations transitioning from Moderate to High wildfire risk within a single decade need proactive fire management and infrastructure hardening.
