Multiple hazards and exposure in California: A space-time analysis of temperature, drought, and wildfire

Natural hazards sharing climatic precursors of temperature and precipitation– heat, drought, and wildfires–amplify their individual effects when they occur in the same time and space. Their combined effects pose numerous challenges to urban areas by decreasing resilience of infrastructure, affecting people's health and safety, and undermining economic stability. This study quantifies the temporal and spatial interactions between heat, drought, and wildfires in California's Metropolitan Statistical Areas for the month of July from 1981 to 2022. We aim to address two questions: 1) How are these hazards changing over time and space? And 2) How are these hazards combined exposing the local land and populations? The temporal analysis found each individual hazard is increasing in frequency. The annual average values for maximum temperature and wildfire burn areas measure consistently above their 20th-century average and drought above the 40-year average. Since 2000, each hazard measured higher than their long-term averages for almost every year, underscoring the persistent climate strain on the state. The spatial assessment revealed, from 1981 to 2022 for the month of July, one-third of California has experienced multiple hazard events encompassing temperature, drought, and/or wildfire. Comparison between population exposure and land (% area) exposure reveals that Riverside experienced the highest total population exposed to multiple hazards, over 2 million people, despite less than 30 % of its land exposed to multiple hazards. The results indicate urbanization has further amplified vulnerabilities and comprehensive data regarding multiple hazards and types of exposure are necessary to estimate risk for public health and economy. • One-third of California has experienced multiple hazards over a 40-year period. • Each individual hazard is increasing and exceeding their long-term averages. • Multiple hazard exposures are quantified for each Metropolitan Statistical Area.