Marine heatwaves in Northen Sea areas: Occurrence, effects, and expected frequencies

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Marine Heatwaves in Northen Sea areas: Occurrence, effects, and expected frequencies

Marine heatwaves (MHW) are extended periods of anomalously warm ocean temperatures. It is a growing phenomenon globally and in Northern sea areas, but little is known of the extent of the problem and the derived effects on marine organisms, ecosystems, and ecosystem services. This report has been commissioned by the Norwegian Environment Agency with the aim to address this knowledge gap and increase the understanding of MHWs in Northern seas. The long term aim is to use the knowledge acquired through the report as a basis for sustainable marine management in the future.

Through a literature study, a webinar and a gap analysis this report has studied the climatic functions, geographically specific effects, anthropogenic drivers, biological effects, identification and monitoring, and future prognosis of MHWs in Northern sea areas. The main results are that on a global scale, evidence indicates that a majority of all MHWs can be attributed to anthropogenic warming. It seems evident, also in the Northern sea areas, that the frequency, intensity and duration will continue to increase with further temperature increases. According to our results, some areas and ecosystems are more susceptible to MHWs.This includes the Arctic Ocean where the relative temperature changes may have larger effects, and coastal areas where kelp ecosystems are particularly vulnerable. Considering the wider biological effect of MHWs in Northern sea areas, they are not well studied, and there are many uncertainties regarding impacts of MHWs on species and ecosystems. In order to achieve a clearer understanding of these impacts, more extensiv biological monitoring is needed. Although our study found few examples of current monitoring directly aimed at MHWs, there are ongoing monitoring efforts of SST, both satellite derived measurements and in situ measurements. These can be used to provide the information needed to map the extent of MHW in Northern sea areas.

Based on the findings of the literature study, webinar and gap analysis, we propose the following recommendations.

• Historical and current changes in the occurrence of MHWs should be documented
• Adequate MHW monitoring should be in place to inform proper marine  management
• Biological monitoring is recommended for temperature sensitive species and their relative communities
• Temperature data should be gathered to develop appropriate baseline conditions
• The overall resilience of marine systems should be strengthened to  increase their ability to withstand stress induced by MHWs
• Human induced marine pressures should be reduced and protection of  marine areas should be enhanced