Comparing Categorical Fire Risk Maps through 2001-2023

Abstract

The recent increase in wildfires in California calls for studies aimed at understanding the impacts of
climate change on fire risks in this region. In this study we used monthly Terra MODIS 0.05-degree
Climate-Modeling-Grid datasets for NDVI and LST to compute pre-fire-season (June) Fire-Risk Index
maps over the California region for the years 2001, 2021, and 2022. The computation of the Fire-Risk
Index used a formula established in past studies. In order to understand the change in fire risk over the
last two decades we compared the 2021 and 2022 pre-fire-season Fire Risk maps to the corresponding
2001 Fire Risk map. The method involved using 50th, 75th, 90th, and 95th percentiles from the June
2001 Fire Risk data distribution to create Fire Risk category maps for all three years with the following risk
classes: Very-Low, Low, Medium, High, and Very-High. Analysis showed a 74.4% and 27.2% increase in
area under the Very-High Risk category, 96.7% and 44.1% increase in area under the High Risk category,
and 39% and 5.7% increase in area under the Medium Risk category along with corresponding decreases
in area under Low and Very-Low Risk categories for the years 2021 and 2022 respectively when
compared to 2001. Our results suggest a phenomenon of conversion of lower fire risk areas to higher fire
risk areas over the last two decades. However, more data-intensive studies are required to conclusively
establish any such phenomenon and link it to climate change.