Towards depopulated oceans

Reducing greenhouse gas emissions could (also) protect marine biodiversity. A new study shows that global warming will drastically reduce the abundance of animals in the ocean.

IRD - Ifremer

A warmer, less populated ocean. That's what's on the horizon for 2100, according to the most comprehensive analysis of the effects of climate change on marine ecosystems published to date. According to the study, if greenhouse gas emissions continue at their current rate, 17% of the world's marine animal biomass could disappear by 2100: "This is the total weight of marine animals such as fish, invertebrates and marine mammals in the ocean," explain Yunne Shin and Olivier Maury, co-authors of the study and researchers at the Marbec laboratory (Marine Biodiversity, Exploitation and Conservation).

Large marine animals under threat

Why is global warming affecting ocean populations? Two major processes are at work: increased dissipation and reduced primary production," answers Olivier Maury. The rise in water temperature will accelerate all biological processes, as metabolism is faster in a warmer environment. This in turn increases energy dissipation. A faster metabolism therefore implies a greater need for resources.

The bad news is that the quantity of available resources is actually going to decrease, which is what researchers call the decline in primary production: "This is actually phytoplankton," explains Yunne Shin. These microscopic plant cells form the basis of all ocean food chains, and with global warming and ocean acidification, phytoplankton density isset to decline. Estimates range from -8% to -12% on a global scale.

This reduction in primary production will have a greater impact on large marine animals, many of which are already in a worrying situation. " The decline in phytoplankton has greater repercussions at the top of the food chain, through a domino effect known as trophic amplification," explain Yunne Shin and Olivier Maury.

Major stress

The consequences of global warming will therefore vary from species to species, but also from region to region. And it is the intertropical zones that will pay the heaviest price. " According to some models, water temperatures could reach 35 degrees Celsius in certain regions of the western Pacific, makingthem uninhabitable. Consequences: ocean populations will migrate towards the poles, totally redrawing the world's maps.

While biomass may increase in some polar regions around the Arctic and Antarctic, it is certain to decline drastically in many temperate and tropical ocean regions: "Biomass will decline very sharply, by -40 to -50%, even though these are areas where marine biodiversity is already heavily affected by human activities," explains Yunne Shin.

And the consequences will not be limited to aquatic life: "These are precisely the regions where human populations are directly dependent on ocean resources for food," adds Olivier Maury, " so it's also a question of global food security. In these regions, climate change will represent a major stress not only for marine ecosystems, but also for human societies.

Prevention and adaptation

For the researchers, the key word in the light of these scenarios is anticipation. " We must obviously do our utmost to limit CO2 emissions, but we must also reconsider measures to preserve biodiversity and manage fisheries in the light of climate change", they recommend. This new study is a further incentive to develop a sustainable, adaptive fishery that will help feed the 11 billion people who will inhabit the planet by 2100.

"On the one hand, the number of human beings will increase, while on the other, marine resources, which make an essential contribution to global food security, are likely to decrease, by as much as 30-35% according to some models," warns Olivier Maury. "It is urgent that governments at all levels take note of our results. Our scenarios can be used to anticipate threats and develop prevention and adaptation strategies. The stakes are clear: we need to avoid what we can't manage, and give ourselves the means to manage what we can't avoid.

Unique expertise

To develop these scenarios, 35 researchers from 12 countries and 4 continents have joined forces in the international FishMIP consortium (Fisheries and marine ecosystem model intercomparison project), a group of experts and modelers working on the dynamics of marine ecosystems in the context of climate change. According to their simulations, marine animal biomass will decline regardless of the CO2 emission scenarios considered. " However, this decline would be limited to 5% if global warming were limited to 2 degrees", say the researchers.