Rare and highly destructive wildfires drive human migration in the U.S.

Wildfires in the U.S. have dramatically increased in scale and intensity in recent decades, posing a significant threat to human settlements. This increase is attributed to changing wildfire patterns (more acres burned, larger fires, and longer fire seasons) and the growing number of people living in high-risk areas. While the destruction of buildings by wildfires is escalating, the relationship between this destruction and human migration remains poorly understood. This study aims to address this gap by examining the migration effects of the most destructive wildfires in the contiguous U.S. between 1999 and 2020. The researchers hypothesize that wildfire-driven migration occurs through two pathways: (1) direct displacement due to structure loss and (2) indirect effects stemming from changes in residential preferences and capabilities (e.g., altered natural amenities, air quality, economic conditions, insurance access). Existing literature on environmental hazards highlights the variability in climate-migration relationships, influenced by hazard type and socio-economic contexts. This study aims to contribute to this literature by focusing on the effects of wildfires across a spectrum of severity levels, from the most extreme to the less severe, more common events. The study anticipates a non-linear relationship, where migration effects are most pronounced at the highest levels of destruction.

Previous research on environmental hazards indicates that climate-migration relationships are highly variable in both direction and magnitude, depending on the specific hazard, geographic location, and social and economic context of affected populations. Studies on other environmental disasters, such as hurricanes and tsunamis, show a range of migratory responses from immobility to large-scale out-migration, depending on the severity of the event. While some research has explored wildfire-related migration intentions and household relocation decisions, these studies often lack the spatial and temporal resolution needed to accurately assess the impact of wildfires on population mobility. Moreover, there is a significant gap in the literature concerning the simultaneous effects of wildfires on residential mobility and the role of civic decision-making in shaping livability and risk perceptions.

This study analyzes the migration effects of the top 10% most destructive wildfires in the contiguous U.S. (N = 519) between 1999 and 2020. The researchers used a harmonized dataset combining wildfire-related structure loss data from the U.S. National Incident Command System/Mitigant Status Summary Forms (ICS) with migration data from the Federal Reserve Bank of New York/Equifax Consumer Credit Panel (CCP). The ICS data provided counts of structures destroyed, while the CCP data estimated in- and out-migration at the census tract level. The study stratified wildfires based on destruction severity into four groups: (1) the full top decile (14–18,804 structures destroyed), (2) the less destructive portion of the decile (14–257 structures), (3) the more destructive portion (258–700 structures), and (4) the single most destructive event (the Camp Fire, 18,804 structures). The analysis used a difference-in-differences approach, comparing migration patterns in burned census tracts to those in control tracts at varying distances (0–5 miles, 5–25 miles, and 25–50 miles) to account for potential spatial spillover effects. Exact matching (CEM) was used to balance covariates between treatment and control groups, enhancing causal inference. The researchers used regression models to analyze the relationship between wildfire structure loss and migration probabilities, examining both out-migration and in-migration trends.

The study's key findings highlight the non-linear relationship between wildfire severity and migration. The majority of wildfires (84.4%) caused no damage to the built environment. Only the most extreme wildfires (those in the most destructive portion of the top decile, 258+ structures destroyed) were associated with significantly increased out-migration. This effect was most pronounced in the first year following the event and primarily attributed to direct impacts on the built environment (structure loss). Less destructive wildfires showed minimal or no impact on out-migration. The Camp Fire (18,804 structures destroyed) demonstrated an exceptionally large and long-lasting out-migration effect, exceeding the magnitude of other events by a significant margin. Regarding in-migration, no significant changes were observed across different wildfire severity levels except for the Camp Fire, which showed a significant increase in in-migration, interpreted as "recovery migration." The results suggest that immobility is the most common response to wildfires, with population-level migration changes only occurring in response to extremely destructive events.

The findings demonstrate that, during the study period, only exceptionally destructive wildfires directly impacted population-level migration through structure loss and subsequent displacement. The non-linearity observed in the migration response to wildfire severity highlights the disproportionate impact of extreme events. The prevalence of immobility suggests that factors like strong social networks, local environmental amenities, and affordability constraints may outweigh the desire to relocate for many residents in fire-prone areas. The significant in-migration following the Camp Fire underscores the recovery processes after extreme events. The study's results challenge the assumption that wildfire routinely drives widespread migration, emphasizing the need to consider the context-dependent nature of climate-migration relationships and the importance of studying immobility in the face of environmental hazards. The study also highlights potential future research directions on the role of individual adaptations and macro-level structural factors that shape mobility decisions in fire-prone places.

This study provides novel evidence on the relationship between wildfire destruction and human migration. The findings show that only the most extreme wildfires, those causing substantial structure loss, significantly influence migration patterns, primarily through direct displacement. Less destructive wildfires, while common, have limited impact on population mobility. The predominance of immobility emphasizes the complexity of human responses to environmental hazards. Future research should investigate the evolving visibility of wildfire-related migration and explore the experiences of vulnerable populations in the context of increasing wildfire severity.

The study's limitations include the inability to detect residential moves within census tracts, potentially underestimating the magnitude of migratory effects. The CCP data, while providing broad geographic coverage, underrepresent younger and financially disadvantaged individuals, potentially underestimating migration impacts on vulnerable populations. The lack of demographic information in the CCP data prevents the analysis of socio-demographic disparities in migration responses. The focus on the contiguous U.S. excludes wildfires in Alaska and Hawaii.