Whaling and Climate Change: A Double Blow to Carbon Sequestration
A study conducted at the MARBEC marine biology laboratory, in collaboration with American and Canadian researchers, provides the first-ever estimate of the dynamics of carbon sequestration on the ocean floor via whale carcasses over a two-century period (1890–2100). It demonstrates how whaling and climate change are contributing to a drastic weakening of this natural carbon sink. This sequestration deficit could amount to more than 45 megatons by 2100. This research, led by Anaëlle Durfort, a doctoral student at the University of Montpellier, was published in the scientific journal *Proceedings B* on November 2.
Natural Carbon Pump
Whales, these massive and iconic animals, play a vital role in marine ecosystems: sequestering carbon in the ocean. When a whale dies, its carcass sinks to the ocean floor, taking with it all the carbon contained in its tissues and trapping it there for hundreds or even thousands of years. However, in the Southern Ocean, whales have been decimated by whaling and are now threatened by climate change, thereby jeopardizing this natural carbon sink. This study provides, for the first time, an estimate of the dynamics of carbon sequestration by five species of whales in the Southern Ocean from 1890 to 2100.
Double Punishment
In the 19th century, the natural mortality of these five whale species sequestered 0.4 megatons of carbon each year. In the 20th century, the boom in the whaling industry—made possible by technological advances—led to the overexploitation of these populations, bringing some species to the brink of extinction. As a result, right whales and blue whales have been reduced to less than 1% of their historical abundance. Consequently, by 1972, whales in the Southern Ocean were sequestering only 0.06 megatons of carbon per year—nearly seven times less than in the past. The study estimates that in 2022, overhunting of whales prevented the sequestration of 26.8 megatons of carbon. This sequestration deficit could rise to 45.2 megatons by 2100.
Commercial whaling was officially banned by an international moratorium in 1986. However, these animals remain threatened, particularly by climate change, which has a significant impact on polar waters. The recovery of whale populations—already slow due to their long life cycles—is therefore at risk. The model proposed by the study predicts carbon sequestration of 0.17 megatons per year by 2100 under the current climate change scenario, which represents less than half of its historical value. Thus, past exploitation and current human-induced threats are combining to impair the ocean’s ability to sequester carbon.
Beyond Tons of Carbon
The repercussions of whale loss in the Southern Ocean extend far beyond the weakening of this oceanic carbon sink. Whales interact with many other organisms and, as a result, provide numerous ecological services to marine ecosystems. When whale carcasses sink to the ocean floor, they not only sequester carbon but also serve as a vital food source for organisms living on the seafloor. Throughout their lives, whales also play a central role as nutrient recyclers, thereby fertilizing their entire environment. Indeed, in the Southern Ocean—where iron is an essential but limiting nutrient for primary production—whales produce iron-rich feces that enable phytoplankton, tiny microscopic algae, to thrive. The fertilization of this first link in the food chain then benefits krill… which will in turn be consumed by whales or will also sequester carbon at the bottom of the ocean in a virtuous cycle for nature and the climate.
Protecting whales and their ecological roles?
In addition to the ban on commercial whaling, other measures can be implemented to protect whales and the associated carbon sinks. For example, preserving krill populations by limiting krill fishing or implementing measures to reduce collisions with ships can help whale populations recover. However, the main challenge lies in mitigating climate change, which remains the most serious threat to all living organisms in the oceans, with Antarctica certainly being the most affected.
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Publication date: November 2, 2022
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