People are flocking to nature’s doorstep, and into wildfire territory.

Homes constructed where human development meets undeveloped wildland are particularly vulnerable to wildfires and other natural hazards (SN: 11/9/23). Nonetheless, people are moving into the wildland-urban interface, or WUI, and rapidly expanding it. From 2000 to 2020, the global footprint of the WUI grew by about 35 percent, reaching a total area that’s roughly the size of Mexico, researchers report November 8 in Science Advances.

The most pressing conflicts between people and nature are concentrated in these areas, says geographer Jianghao Wang of the University of Chinese Academy of Sciences in Beijing. The expansion of the WUI’s total area “raises the likelihood of human exposure to wildfires, posing greater threats to life and property.”

That risk has been on full display in recent weeks. For instance, multiple wildfires have spread through WUI areas in Southern California. These include the ongoing Mountain Fire, which ignited in the Somis area on November 6 and has since destroyed more than 130 properties and forced more than 10,000 people to evacuate.

Using satellite data from 2000, 2010 and 2020, Wang and his colleagues mapped the global spread of the WUI at 30-meter resolution. They found that urbanization had fueled a rapid WUI expansion that had accelerated over time.

Starting from about 1.4 million square kilometers in 2000, the WUI grew by about 5 percent in the first decade and about 30 percent in the following decade, to around 1.9 million square kilometers in 2020. The United States, eastern China and Nigeria saw some of the most extensive WUI growth. “This reality emphasizes the need for policy makers to prioritize wildfire control as more regions become vulnerable,” Wang says.

A subsequent analysis of satellite detections of wildfires revealed that fire activity near the WUI had also changed. The team looked at two periods: From 2008 to 2012 and 2018 to 2022. The number of small wildfires — those burning one square kilometer or less — increased by about 23 percent within one kilometer of WUI areas between those two times.

The findings could help governments develop or refine policies and guidelines to manage wildfire risk, the researchers say (SN: 5/13/20). WUI residents are also advised to be cautious about lighting fires in vegetated areas, lest they exacerbate the hazard for others living on these frontiers of fire.