Projected marine heatwaves over the Mediterranean Sea and the network of marine protected areas: a three-dimensional assessment

Marine heatwaves (MHWs) are periods of abnormally warm ocean temperatures that exert significant impacts on marine ecosystems. MHWs are commonly identified as periods when temperature conditions surpass predetermined thermal thresholds. Although they can propagate beneath the ocean's surface, MHWs are typically assessed using sea surface temperatures. In this study, we investigate the future progression and depth penetration of MHWs across the Mediterranean Sea and the existing network of Marine Protected Areas (MPAs). We utilize daily three-dimensional seawater temperature projections from the POLCOMS-ERSEM model for the period 2006-2100 from surface to 2000 m depth under the RCP8.5 forcing scenario, to explore the future spatio-temporal properties of MHWs. We employ two different baseline climatological periods for establishing thresholds based on which MHWs are identified: one remains constant, while the other is adjusted in time to reflect the evolving adaptive capacities of ecosystems in response to long-term warming trends. Our analysis reveals that, regardless of the threshold applied, the Mediterranean MPAs will experience long-lasting and intense MHWs in the future across the ocean's depths. While the utilization of a shifting baseline period results in shorter and milder shallow MHWs within the Mediterranean MPAs (i.e. annual duration: 45 days per year and mean annual intensity: 1.63 degrees C) compared to the fixed baseline approach (i.e. annual duration: 195 days per year and mean annual intensity: 1.89 degrees C), deeper ocean zones exhibit a saturation of MHWs, with year-round events, particularly below 1000 m, under both baseline approaches. Our findings highlight the profound penetration of MHWs into the ocean's depths, posing a serious threat to the protected ecosystems of the Mediterranean, especially in marine subsurface environments. Notably, our findings suggest that MHWs are expected to demonstrate similar spatio-temporal patterns to those observed within the MPAs across the entire Mediterranean basin. MPAs are, thus, anticipated to face equivalent challenges from MHW impacts, affecting their protected ecosystems and biodiversity across various depths.