Climate Science and Policy

Marine heat waves (MHWs), extended periods of unusually high ocean temperatures that significantly impact marine ecosystems, are prevalent in the Southern California Bight (SCB), increased thermal stratification has intensified these events over recent decades, affecting California sea lions by disrupting prey availability, leading to malnutrition and higher mortality rates. Rising atmospheric CO2 levels have exacerbated this issue, contributing to the warming of surface waters. Climate models predict that MHWs will continue to increase in frequency, severity, and duration, posing further risks to marine life. Despite the presence of a significant sea lion colony off the coast of San Diego, little research has been conducted on MHWs in the SCB impact on the local sea lion population. This research examines various aspects of MHWs at San Diego, including defining MHWs, historical occurrences, current trends, and their implications for California sea lions, while exploring potential policy and mitigation strategies to protect Southern California's marine ecosystems. Daily sea Surface Temperature (SST) measurements from La Jolla Scripps Pier (1916-2023) are used to quantify local MHWs relative to a shifting baseline (i.e., one that adjusts for the long-term warming trend), and to a fixed baseline (i.e., the warming trend alone increases MHW occurrences). We investigate critical questions about MHW evolution and duration, considering the continuous warming of the oceans. We consider what the two definitions of MHWs imply for upwelling and nutrient supply to the ocean surface, and the implications for California sea lion prey availability. We identify future research priorities, particularly improved measures of sea lion population counts, and the need for better indicators of MHW impacts beyond SST.