Climate change is considered one of the top three threats to seabirds. Arctic terns could be particularly vulnerable as they rely on both polar regions and migrate across huge expanses of the planet. In this study, the 21st century trends of three key environmental variables that could impact arctic tern migration and non-breeding seasons were projected under two climate scenarios (‘middle-of-the-road’ and ‘fossil-fuelled development’):
- Primary productivity, which relates to food availability, at key stopover regions and during the non-breeding season,
- the Southern Ocean sea ice extent, which arctic terns follow from November – March due to the high density of prey,
- winds during migratory flights and in the Southern Ocean during the non-breeding season.
Arctic Tern by Joanne Morten
The primary productivity within the North Atlantic is highly likely to decline by 2100 under both climate scenarios. Many arctic terns visit this region during the early stages of their southbound migration, and the region is vitally important for millions of seabirds as well as other taxa including cetaceans and sharks. These projected declines are therefore very concerning. In contrast, there are weak or no notable projected changes to primary productivity in the other regions investigated (Benguela Upwellling region, the subantarctic Indian Ocean and the Southern Ocean).
Figure 3 from Morten et al 2023: Projected changes to primary productivity during the 21st century at four key regions for migrating and non-breeding Arctic Terns
All climate models agreed that under both climate scenarios, the sea ice in the Southern Ocean between October and March is likely to decrease by the end of the 21st century. However, the magnitude of this change is hard to infer as in recent decades there are differences between modelled and observed sea ice levels, with the underlying causes uncertain.
Projected changes to wind conditions are likely to be low (< 7.2 km per hour). However, arctic terns will need to compensate for these wind shifts. During simulated future northbound migrations, virtual arctic terns were pushed towards the African continent to a greater extent than virtual terns migrating in present day wind conditions. This suggests that migrating arctic terns will need to exert more energy to cope with projected changes.
Arctic Tern at breeding colony, Iceland by Joanne Morten
Due to the vast scales in which arctic terns live, they may be able to cope with the projected changes reported in this research. For example, they could mitigate the future declines in the North Atlantic primary productivity by stopping in other regions. Between October and March in the Southern Ocean, arctic terns could follow the sea ice edge as they are not restricted as central place foragers, unlike the breeding seabirds during these months.
In this study, the focus was on the projected environmental changes during the migratory and non-breeding stages only. Arctic terns are likely to be impacted by climate change during the breeding season, and by variables that were not investigated, such as changes to sea surface temperatures. Additionally, the cumulative impacts of the changes to primary productivity, Southern Ocean sea ice and winds are uncertain. Additionally, the consequences of these future changes on other species, with more restricted ranges are very concerning. It is therefore vital that dramatic changes to carbon emissions and policy are implemented to permit species like arctic terns the space to adapt.