New policy brief calls for action to mitigate the impacts of Baltic heatwaves

Global warming has a major impact on the Baltic Sea, both through rising mean water temperatures and through more frequent marine heatwaves. Between 1982 and 2023, 189 marine heatwave events were recorded in the Baltic Sea, and recent years have included some of the warmest sea surface temperatures on record.

This has major consequences for marine life, according to researchers within CoastClim, who have summarised the issue in a new policy brief, written mainly by Norman Göbeler at Tvärminne Zoological Station.

Species that already live close to their upper heat tolerance are usually hit hardest by marine heatwaves and can suffer large biomass losses, while other species may benefit. However, marine heatwaves also affect processes in the sea, which can have consequences for the whole ecosystem.

“Marine heatwaves can intensify eutrophication from within, working against the benefits of measures to reduce nutrient loads. The accelerated metabolic activity across organisms can also increase the production and release of greenhouse gases such as carbon dioxide and methane from coastal waters and sediments,” the researchers write.

Addressing the impacts of marine heatwaves requires action from society, and in the policy brief the researchers give the following recommendations:

• Reduce greenhouse gas emissions to mitigate climate change.
• Increase ecosystem resilience to withstand heatwaves by:
  • speeding up measures to reduce eutrophication,
  • reducing habitat disturbance such as bottom trawling and dredging,
    and
  • restoring coastal habitats and seafloor communities.
    

• Strengthen long-term monitoring of ecosystem change.

Marine heatwaves cannot be stopped locally, but the pressures that make their impacts worse can be reduced, the researchers conclude.

“Reducing local stressors, such as nutrient loads, overfishing and habitat degradation, strengthens the sea’s ability to withstand and recover from marine heatwaves, buying time and resilience while global emissions are brought down. At the same time, more frequent, higher-resolution monitoring is needed to understand how these events unfold and to guide an effective response,” they write.

Read or download the full policy brief

Action is needed to mitigate the impacts of Baltic heatwaves (web text, English)

Action is needed to mitigate the impacts of Baltic heatwaves (layouted pdf, English)

Action is needed to mitigate the impacts of Baltic heatwaves (text version with references, English)

Åtgärder krävs för att möta värmeböljorna i Östersjön (web text, Swedish)

Åtgärder krävs för att möta värmeböljorna i Östersjön (layouted pdf, Swedish)

Itämeren lämpöaaltojen vaikutusten hillitseminen edellyttää toimia (layouted pdf, Finnish)

Baltic Breakfast

Watch the Baltic Breakfast seminar where Norman Göbeler presents more about the research behind the policy brief:

Read a summary of the seminar:

Baltic Breakfast: The ocean is warming – and the Baltic Sea is changing even faster