The science of sustainable seafood, explained

Ocean predicted to lose 5% of life per degree of global warming

Climate change is harmful to the ocean and bad for fisheries. We have chronicled many studies linking climate change, fisheries, and conservation and have not shied away from overtly suggesting that climate change is the largest threat to fisheries and biodiversity in the ocean (as opposed to other popular and well-known ocean issues).

A recent paper is the strongest evidence yet. Lotze et al. 2019, a groundbreaking study published open access in June 2019 predicts that life in the ocean will decrease by about 5% per degree of global warming.

Predicted changes in ocean biomass under different emissions scenarios and temperature changes. From Lotze et al. 2019.
Predicted changes in ocean biomass under different emissions scenarios and temperature changes. From Lotze et al. 2019.

This is a powerful figure that clearly shows a linear relationship between biomass and temperature change.

Climate change studies and the gloomy figures they produce can be discouraging, but the figure here can be interpreted positively: Reducing warming by any amount will move up the scale and prevent the deaths of countless marine animals.

Explaining the science: an ensemble of models

Lotze and dozens of scientists from across the world incorporated climate models into ecosystem models to determine how rising global and ocean temperatures (and their associated impacts) will affect animal populations over time.

Nearly every paper measuring something about climate change uses RCP scenarios* to set up a computer model to measure some kind of physical change. For example, a paper from last year used RCPs to measure physical changes inside and outside of marine protected areas (MPAs). What makes Lotze et al. 2019 groundbreaking is that researchers used RCP scenarios to inform a composite of ecosystem models that predict ecosystem changes, not just physical changes. Co-author Marta Coll of the Institute of Marine Science in Barcelona said, “Our study represents a major undertaking, where we used for the first time a comparative and standardized approach of 6 global marine ecosystem models forced with 2 Earth-system models and 4 emission scenarios with and without fishing. The process of incorporating RCP scenarios into an ensemble of various ecosystem models was 6 years in the making, according to another co-author, Tyler Eddy.

There are two major takeaways concerning fisheries:

  1. The composition of fish in the ocean will change dramatically and
  2. Fishing has very little impact on what will happen.

Trophic amplification

Large marine animals higher in the food chain will suffer disproportionately to animals lower in the food chain. Climate change effects will start low in the food chain and amplify through the food web, leaving higher trophic level animals most vulnerable—a phenomenon known as trophic amplification. “Our findings suggest that larger marine animals, many of which are already of conservation concern, could show a particular vulnerability to climate-driven declines, with a domino effect from phytoplankton up the food chain” said co-author Derek Tittensor of the UN Environment World Conservation Monitoring Centre.

According to the models, much of the biomass lost in the ocean will be due to a sharp decrease in organisms larger than 30cm. This has obvious implications for fishermen and women and seafood consumers.

“We know that food production on land is increasingly put at risk by climate change impacts, such as extreme heat and drought. This study adds another disconcerting chapter to the global warming story, by confirming that human-made climate change endangers food resources in the oceans, too. In 2015, all nations agreed upon the Sustainable Development Goals. One of them is to achieve zero hunger by 2030. Our research shows that this will require not only a much more careful management of natural resources, but also rapid reductions in global greenhouse gas emissions” adds co-author Jacob Schewe from the Potsdam Institute for Climate Impact Research.

Fishing impact is virtually non-existent

According to the study’s models, fishing had little to no impact on relative future biomass in the ocean:

The three boxes per color represent, from left to right, total biomass, biomass >10cm and biomass >30cm. You can see slightly higher total relative biomass in the fished models, though the errors bars are much higher in the unfished scenarios. From Lotze et al. 2019.

“The fact that the estimated climate-change impacts are independent of fishing provides an added incentive to develop sustainable and adaptive fishing, responsive to climate change, which we need to feed a world of 9 billion humans” says co-author Manuel Barange, Director of Fisheries and Aquaculture at the Food and Agriculture Organisation (FAO).

However, the fished and unfished scenarios used in the models were basic—the next step in this analysis is to incorporate more realistic fishing scenarios.

Regardless, the conclusion remains that reducing emissions is the most important thing society can do to protect the ocean.

Picture of Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

*Explanation of RCPs: Representative Concentration Pathways (RCPs)— a scientifically backed estimate of radiative forcing (you can think of this as the amount of global warming) based on different emissions scenarios. Basically, an RCP estimates the amount of warming Earth will experience based on the amount of future emissions. RCPs can then be used to set up computer models. There are 4 recognized RCPs: RCP 2.6, 4.5, 6, and 8.5.

RCP 2.6 is what humanity should strive for; governments and policies that severely limit future emissions to keep global warming to within 1.5-2 degrees. RCP 8.5 is the other side of scale, colloquially referred to as the “business as usual” scenario where governments do little to stop emissions and they continue to rise. Thankfully, RCP 8.5 is very unlikely as governments around the world have started acting on emissions. Unfortunately, the emissions-reducing commitments are not enough and RCP 2.6 is also very unlikely (unless a global push by citizens elects progressive, environmental leaders that dismantle the fossil fuel industry,*cough* *cough*). Based on the world today, humanity is on track to end up somewhere in between RCP 2.6 and 8.5.

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