Whaling and climate change: a double whammy for carbon sequestration
A study conducted at the MARBEC marine biology laboratory, in collaboration with American and Canadian researchers, provides the first estimate of carbon sequestration dynamics at the bottom of the ocean via whale carcasses over a period of two centuries (1890-2100). It shows how whaling and climate change are contributing to a drastic weakening of this natural carbon pump. The sequestration deficit could reach 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: carbon sequestration in the ocean. When a whale dies, its carcass sinks to the depths, taking with it all the carbon contained in its tissues, trapping it for hundreds or even thousands of years. However, in the Southern Ocean, whales have been decimated by fishing and are now threatened by climate change, putting this natural carbon pump at risk. This study provides the first-ever estimate of the carbon sequestration dynamics of five whale species in the Southern Ocean from 1890 to 2100.
Double punishment
In the 19th century, the natural deaths of these five whale species sequestered 0.4 megatons of carbon each year. In the 20th century, the rise of the whaling industry, made possible by technological advances, led to the overexploitation of these populations, bringing some species to the brink of extinction. Right whales and blue whales were reduced to less than 1% of their historical abundance. As a result, by 1972, whales in the Southern Ocean were sequestering only 0.06 megatons of carbon per year, almost seven times less than in the past. The study estimates that in 2022, overfishing 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 are still threatened, particularly by climate change, which is having a particularly severe impact on polar waters. The recovery of whale populations, already slow due to their long life cycle, is therefore at risk. The model proposed by the study predicts carbon sequestration of 0.17 megatons per year in 2100 under the current climate change scenario, which is less than half its historical value. Thus, past exploitation and current anthropogenic threats are combining to alter the ocean's ability to trap carbon.
Beyond tons of carbon
The repercussions of the loss of whales in the Southern Ocean go far beyond the weakening of this oceanic carbon pump. Whales interact with many other organisms and provide numerous ecological services to marine ecosystems. When they sink to the depths, whale carcasses not only sequester carbon, they also provide an essential food source for organisms living on the seabed. Throughout their lives, whales also play a central role as nutrient recyclers, fertilizing their entire environment. In the Southern Ocean, where iron is an essential nutrient but limits primary production, whales produce iron-rich feces that allow phytoplankton, tiny microscopic algae, to grow. The fertilization of this first link in the food chain then benefits krill, which in turn is consumed by whales or sequesters carbon at the bottom of the ocean in a virtuous circle for nature and the climate.
Preserving whales and their functions?
In addition to banning 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 undoubtedly being the most affected.
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Publication date: November 2, 2022
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