Published on October 31, 2019

This month, in both Northern and Southern California, residents have grappled with destructive wildfires.

The October 2019 fires come after more than 1 million acres were burned a cross the state in both 2017 and 2018. Those were the most destructive years in the state since at least 2008, based on data compiled by the National Interagency Fire Center (NIFC).

At this rate, 2019 could be less destructive than the last two years, both in California and nationwide, but nearly two decades of data show that wildfires — defined by NIFC as any “unplanned” and “unwanted” fire that begins in an undeveloped location — have become more destructive over the years.

How often do wildfires occur and how big do they get?

The number of wildfires nationwide varies widely each year. There were 71,499 recorded in 2017 and 58,083 recorded in 2018. The amount of land burned also varies widely year to year, with more than 10 million acres affected in 2017 (slightly smaller than Massachusetts and Connecticut combined) and 8.8 million affected in 2018.

This variability is even more noticeable when looking at the annual data back to 1983. Comparing 10-year averages shows a few trends.

First, the number of wildfires is lower than levels in previous decades.

Wildfires (Average of prior 10 years)

Wildfires that occurred in each year

NOTE: This data includes all wildland fire excluding fires classified as “prescribed” or planned by government agencies.

The second trend to note: The number of acres burned has increased in recent decades. The average for the last 10 full years (2009 to 2018) was about 7 million acres, slightly larger than the state of Massachusetts. That’s 150% higher than the 10-year average of 2.7 million acres between 1983 to 1992.

Acres burned (Average of prior 10 years)

Acres burned by wildfire in each year

The result is the third trend: The average wildfire is burning a larger area of land. Between 1983 and 1992, the average wildfire affected 43 acres of land, the size of 33 football fields. Between 2009 and 2018, the average fire affected 104 acres, half the size of the National Mall.

Average wildfire size over prior 10 years

Average wildfire size for the year

Where do fires occur?

In terms of the nearly 670,000 of wildfires reported nationally between 2009 and 2018, five states were responsible for 41% of those: Texas (13%), California (12%), North Carolina (6%), Georgia (6%) and Florida (4%).

During that period, 70 million acres (about the size of Colorado) were burned in wildfires. Fifty-four percent of that land burned in just five states: Alaska (18%), California (10%), Idaho (9%), Texas (8%) and Oregon (8%).

The severity of fires differs by state. The average wildfire blaze in Alaska from 2009 to 2018 affected nearly 2,500 acres. However, 14 states had wildfires that affected on average less than 10 acres.

How do wildfires occur?

The NIFC classifies wildfires based on ignition source. The two primary ignition sources are lightning strikes and humans. A wildfire is classified as human-caused and can begin via “debris burning, campfires, arson, discarded smoking products, sparks from equipment in operation, arced power lines, and other means.”

Between 2009 and 2018, 88% of wildfires were classified as human-caused, while lightning strikes were responsible for 12%. However, wildfires caused by lightning are more destructive than human sources. Over those 10 years, 57% of the acres burned in wildfires were caused by lightning.

Wildfire incidents by year

Wildfire acres burned