This study examines the long-term trends and drivers of change in California's kelp forests over the past century. By curating historical US government kelp canopy inventories and comparing them with contemporary surveys, the authors find that while the overall kelp canopy area declined slightly (-6.9%) between 1910-1912 and 2014-2016, regional changes were dramatic. Central California saw a significant increase (+57.6%, +19.7 km²), while the northern (-63.0%, -8.1 km²) and southern (-52.1%, -18.3 km²) regions experienced losses. The study uses Random Forest models to identify the primary drivers of these changes, with sea otter population density being the most significant factor. Other important factors include benthic substrate, extreme heat, and high annual variation in primary productivity. The findings highlight the importance of sea otter recovery and the need for coastal management strategies to enhance kelp forest resilience to climate change.This study examines the long-term trends and drivers of change in California's kelp forests over the past century. By curating historical US government kelp canopy inventories and comparing them with contemporary surveys, the authors find that while the overall kelp canopy area declined slightly (-6.9%) between 1910-1912 and 2014-2016, regional changes were dramatic. Central California saw a significant increase (+57.6%, +19.7 km²), while the northern (-63.0%, -8.1 km²) and southern (-52.1%, -18.3 km²) regions experienced losses. The study uses Random Forest models to identify the primary drivers of these changes, with sea otter population density being the most significant factor. Other important factors include benthic substrate, extreme heat, and high annual variation in primary productivity. The findings highlight the importance of sea otter recovery and the need for coastal management strategies to enhance kelp forest resilience to climate change.