The vast majority of studies on MMEs in the Mediterranean have focused on foundation species, such as habitat-forming gorgonians and seaweeds (Chimienti et al., 2021 Garrabou et al., 2019 Verdura et al., 2021). These two events affected more than 40 species from various taxa (e.g., Porifera, Cnidaria, Bivalvia, Bryozoa, Ascidiacea) across thousands of kilometers of coastline. The most dramatic reported events in terms of geographic extent and number of affected species occurred in 19 along the Northwestern Mediterranean Sea (Cerrano et al., 2000 Garrabou et al., 2009 Perez et al., 2000). Indeed, Mediterranean MHWs have triggered unprecedented climate-driven MMEs during the last decades and their occurrence is expected to increase in the coming decades (Darmaraki, Somot, Sevault, Nabat, Cabos-Navaez, et al., 2019 Garrabou et al., 2019). While barely recognized by society at large, marine MMEs are one of the most predominant biological impacts of climate change in the Mediterranean Sea (Cramer et al., 2018 Marbà et al., 2015 Rivetti et al., 2014). Although the Mediterranean Sea represents only 0.32% of the total volume of the oceans, its unique geomorphological history led to a markedly high level of biodiversity with 7%–10% of all known marine species and a large proportion of endemic species (Bianchi & Morri, 2000 Coll et al., 2010). The Mediterranean Sea is generally considered a relevant model to assess the ecological effects of climate change on marine biodiversity and to test potential adaptation and mitigation strategies that may be scaled up (Cramer et al., 2018). Understanding the emergent properties and dynamics of these recurrent climatic disturbances is critical for predicting spatial refuges and cumulative ecological responses, and for managing long-term impacts on ecosystems and the provision of services they provide (Hughes et al., 2021 Smith et al., 2021). This increase in the frequency of MHWs has been associated with the rapid increase in mass mortality events (MMEs) worldwide (Garrabou et al., 2009 Hughes et al., 2017 Thomson et al., 2015 Wernberg et al., 2016). In the last 20 years, MHWs have globally doubled in frequency and have become longer-lasting, more intense and more extensive (Collins et al., 2019). MHWs are discrete periods of prolonged anomalously warm water at a particular location (Hobday et al., 2016). Besides gradual ocean warming, acidification, deoxygenation, and sea-level rise (IPCC, 2019), human activities have caused a rapid increase in the frequency and intensity of extreme climatic events, such as marine heatwaves (MHWs Collins et al., 2019 Darmaraki, Somot, Sevault, & Nabat, 2019 Oliver et al., 2018). 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.Īnthropogenic climate change has become a major environmental driver, strongly affecting marine biodiversity and ecosystem functioning, often interacting with other human-induced stressors (Gissi et al., 2021 Smale et al., 2019). 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. Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. 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). 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. 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.

Heat wave killed marine wildlife masse