Written by Nereus Research Associate Ryan Swanson,

Nereus research fellow Tyler Eddy (Dalhousie University), research associates Miranda Jones (UNEP-WCMC) and Derek Tittensor (Dalhousie University, UNEP-WCMC), Charles Stock (NOAA) and director (science) William Cheung (UBC) are co-authors on an article recently published in the Proceedings of the National Academy of Sciences (PNAS). For the study, the authors combined six global marine ecosystem models (MEMs), two Earth system models (ESMs), four emission scenarios (RCPs) and fishing impact to create a standardized ensemble projection for the effects that warming will have on ocean life.

Four different carbon mitigation scenarios were used: RCP2.6 (low emission, strong mitigation), RCP4.5 and RCP6.0 (intermediate emissions and mitigation) and RCP8.5 (high emissions, “business as usual” scenario). The authors found that, without fishing, global marine biomass would decline by 5% on average under the low emission scenario (RCP2.6) and 17% under the “business as usual” scenario (RCP8.5) by the year 2100, “with an average of 5% decline for every 1 °C of warming.” Additionally, they found that “fishing did not substantially alter the effects of climate change” but did note that this would largely depend on future fishing (e.g., effort, technology, management and conservation), aquaculture and other human impacts.

The decline in global marine biomass will not be even but differ both spatially and by animal size and trophic position in the marine food web. Latitudinally, the authors projected an increase in marine biomass near polar regions and a decrease in temperate and tropical regions.

This is notable, because it counters the IPCC’s Fifth Assessment Report that used single-model results and showed an increase in biomass in temperate and tropical zones and a decrease around Antarctica. The authors partially attribute their projections to warming polar waters resulting in expanded primary production (e.g., increased zooplankton in lower trophic levels) and exceeding a temperature threshold in temperate and tropical waters making them less habitable to current species. In general, larger marine animals (>30cm) in higher trophic levels were shown to decline more than those in lower trophic levels, attributed to trophic amplification. The authors suggest this could be due to changes in “phytoplankton size and composition, lengthening food chains, reduced trophic efficiencies and larger bodies having higher metabolic costs” that would cause a cascading effect toward higher trophic levels that contain larger marine animals that rely on them as a food source.

The ramifications of the study’s projections would be far reaching for marine ecosystems and the animals within them, but also for people who rely on fishing and seafood for food and employment security. Indeed, the authors do suggest serious challenges for meeting certain Sustainable Development Goals by 2030, such as SDG1 (livelihoods), SDG2 (food security), SDG3 (well-being) and SDG14 (life below water). Additionally, they note that nations in latitudes projected to see the largest biomass declines (temperate – tropical) disproportionately rely on seafood and fisheries for their food security and well-being, while increasing biomass in the polar regions raises the potential for conflicts over natural resource access and usage. In conclusion, the authors advise using improved “dynamic and adaptive ecosystem-based management” strategies that could mitigate the impacts of climate change and maintain marine ecosystem health and call on the international community to “[strengthen] the required institutions and management approaches.”