More than 30 square miles and 130 structures have been destroyed by wildfires in southern California, fueled by Santa Ana winds. This is just one example of the many wildfires that have ravaged the state this year. However, California is not alone in facing this type of disaster.
Approximately 1.2 billion people live in areas that are prone to wildfires, as human development encroaches on fuel-rich wilderness. These areas, collectively known as the wildland-urban interface (WUI), cover 750,000 square miles. A new study published in Science Advances, which used NASA satellite data to track the growth of the WUI from 2000 to 2020, found that the global reach of the WUI increased by 15% from 2000 to 2010 and then surged by 85% from 2010 to 2020. This dramatic expansion, which accounts for 1.44% of the planet’s continental surface, has led to a significant increase in wildfires.
The three countries that witnessed the most significant expansion in wildland-urban overlap were the U.S., Nigeria, and China. However, China accounted for 95.97% of all WUI expansion from 2010 to 2020, driven by the rapid growth of its cities and towns. Over the 20-year period, temperate North America expanded its WUI footprint by 24.8%, central Asia by 21%, and Europe by 10.3%.
“The increase in WUI was primarily driven by the unprecedented expansion of global urbanization,” the authors wrote, “contributing an additional 589,914 square kilometers [227,727 sq. mi.] of WUI.”
While every continent except Antarctica experienced an increase in wildland-urban interface coverage during the study period, vast areas with sparse human populations remained untouched, particularly in Northern Canada, the Australian deserts, the Sahara Desert, and Greenland.
The study’s observations were based on data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the space agency’s Terra and Aqua satellites. Orbiting at a high altitude, these spacecraft were able to capture images with a resolution of 99-ft. patches of ground—remarkably detailed given the distance.
The most dangerous areas identified in the study were not at the fringes of the WUI, where the ground is predominantly urban or wild. This is because ignition sources like cigarette butts, campfires, or downed power wires are less likely to encounter fuel sources in these areas. The greatest risk occurs where 1,300-ft. bands of wildland overlap with 650-ft. bands of urban or suburban development. When wilderness in these areas ignites, the fire can spread quickly and extensively, as demonstrated by the 2023 wildfire in Lahaina, Maui, which was caused by a downed powerline and destroyed over 2,200 structures, killing 115 people. Similarly, the 2018 Camp Fire in Paradise, Calif., also triggered by a downed power line, resulted in the deaths of 85 people and destroyed nearly 19,000 structures. Even relatively contained fires can cause widespread damage, with smoke inhalation leading to respiratory injuries hundreds of miles away from the source.
While flames don’t travel as far as smoke, they do spread—an average of 1.25 to 5.5 miles from their origin within the WUI, according to the study. The MODIS data showed that these high-risk zones experience approximately 1,500 fires per year. At greater distances, communities benefit from fragmentation, as highways, streets, and parking lots create paved fire breaks that separate developments. Fires that reach these areas are often extinguished more quickly due to the ease of access for firefighting equipment via the extensive paved roads.
The intertwining of urban and wildland areas is now too extensive to separate, and Guo and his colleagues do not expect one study to persuade developers to reverse two decades of growth. However, they hope that the increasing number of wildfires over the past two decades will encourage officials to allocate more resources and firefighting assets within the WUIs to prevent future large-scale fires like those in Lahaina or Camp.
As Guo stated: “Governments in countries that have experienced large increases in the WUI since 2000, such as eastern China, the United States, and Nigeria, should use more policy tools and allocate more resources to WUI areas.”