Southern California’s 2000-year wildfire history: Long-term trends and decadal variability

Wildfires are one of the most destructive climate-related hazards, causing significant economic and environmental impacts globally. Paleo-wildfire records provide insights into how climate change influences wildfire dynamics, including shifts in severity and frequency. In the western USA, studies using sedimentary charcoal have shown that long-term wildfire variations are closely tied to changes in temperature and aridity. However, short-term variability in paleo-wildfire activity remains understudied.

Here, we investigated high-accumulation varved sediment deposits from the Santa Barbara Basin (SBB) off California. The analyzed cores (SPR0901-05BC and MV0811-14TC) span the last two millennia. By analyzing monosaccharide anhydrides (MAs) as molecular fire-markers at sub-millimeter spatial resolution using mass spectrometry imaging (MSI), we obtained paleo-wildfire records with interannual to decadal temporal resolution. The MAs record shows a similar trend to historical fire data for the last century, demonstrating the feasibility of using MAs as proxies for paleo-wildfire reconstruction.

A mixture model was applied to decompose the raw MAs record into two components: (1) a low-resolution background signal, potentially from fire-derived markers deposited in soil and gradually released into the ocean through long-term erosion and transport, and (2) a high-resolution peak signal, likely reflecting aerosol and/or rapid river deposition following fires. The background signal aligns with wildfire history reconstructed from charcoal records in the western USA, showing high biomass burning during the Medieval Climate Anomaly (MCA) and the settlement era, and low biomass burning during the Little Ice Age (LIA).

By analyzing the density of high-resolution peak signals, we reconstructed a decadal-scale fire frequency record for Southern California. This record provides detailed insights into changes in wildfire frequency during the MCA and the subsequent transition to the LIA. Overall, the fire frequency correlates positively with pre-20th-century variations in temperature and aridity on decadal timescales, indicating that the wildfire frequency was significantly influenced by decadal fluctuations in climate conditions.