Marine heatwaves drive recurrent mass mortalities in the Mediterranean Sea

Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015–2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning. Overall, we show that increasing the resolution of empirical observation is critical to enhancing our ability to more effectively understand and manage the consequences of climate change.

French National Research Agency. Grant Number: ANR-17-MPGA-0001; H2020 Environment. Grant Number: SEP-210597628; Interreg Med. Grant Number: 1MED15_3.2_M2_337; Marie Sklodowska-Curie. Grant Number: 801370; Beatriu de Pinós funded by the Secretary of Universities and Research (Government of Catalonia); Ministerio de Ciencia, Innovación y Universidades. Grant Number: RTI2018-095441-B-C21; Spanish Ministries of Economy and Competitiveness. Grant Numbers: CGL2015-71809-P, CTM2012-32603; Greenpeace Italy; National Geographic Society. Grant Number: EC-176R-18; Ente Parco Nazionale del Gargano; Italian Ministry of Education, University and Research. Grant Numbers: AIM 1807508–1, PON 2014–2020; Tropical Signals Program of CIESM; Fondation Albert 2 Monaco (MIMOSA Project); MITERD. Grant Number: CTM2016-77027-R; Departament de Territori i Sostenibilitat of the Generalitat de Catalunya; FCT—Foundation for Science and Technology. Grant Numbers: UIDP/04423/2020, UIDB/04423/2020; FCT/MCTES (PIDDAC); European Regional Development Fund (ERDF); RD Unit. Grant Number: UID/Multi/04423/2019; Regione Autonoma Sardegna. Grant Number: CUP 87G17000070002; Public Investments Programme (PIP) of the Hellenic Republic; EEA GRANTS; MARISCA Project; European Union (European Regional Development Fund, Cohesion Fund); Hellenic Government (Ministry of Development and Investments); CIESM “Tropical Signals,” Stelios Katsanevakis; European Commission; MAVA Fondation; Israel Ministry of Environmental Protection; Stazione Zoologica Anton Dohrn; MCIU/AEI/FEDER. Grant Number: RTI2018-095346-B-I00; Severo Ochoa Centre of Excellence. Grant Number: CEX2019-000928-S; Euromarine.