Whales Removed an Abundance of Carbon From Earth. Then Humans Killed 3 Million of Them

Before industrial whaling began, giant baleen whales—including blue, humpback and fin whales—once removed as much carbon from the environment as forest ecosystems spanning entire continents, scientists have found. And restoring their numbers to pre-slaughter levels could help combat climate change, the team says.

Marine ecologist Matthew Savoca, from Stanford University, is the lead author of a study that looks at how much food these giant mammals actually consume. Previous estimates of their food intake was largely based on just a few measurements, so the team looked at data from 321 tagged whales sized between 30 and 100 feet that lived in the Atlantic, Pacific and Southern Oceans.

The tags were able to track the whale movements, allowing them to pinpoint how often each animal was feeding. They also used drone photographs to estimate the volume of water the whales were filtering with each mouthful, based on calculations of their length. Researchers also visited feeding hotspots to establish the density of food, such as krill, that would be ingested during each meal.

Their findings, published in Nature, were a surprise. Giant baleens were shown to eat three times as much as was previously estimated. An adult North Pacific blue whale is thought to eat 16 metric tons of krill every day during foraging season. A bowhead whale would eat about six metric tons of zooplankton per day.

It is an amount of food that is unimaginable," Nicholas Pyenson, from the Smithsonian's National Museum of Natural History, who is an author on the study, told Newsweek. "It's an amount that is roughly twice the annual global fisheries catch or twice all of the krill that are currently alive in the Southern Ocean. Our results also illuminate something that scientists had suspected for the largest whales, but hadn't yet carefully quantified: the scale of their role as ecosystem engineers."

Understanding how much whales eat is key to understanding how much their presence on earth contributes to carbon removal and ocean health.

During industrial whaling expeditions in the 20th century, up to three million whales were killed. This removal of the ocean giants had a profound impact on the ecosystem. Whale feces provided an important source of nutrients to ocean food webs. It provides key nutrients, such as iron, to the surface and allows for blooms of phytoplankton—tiny organisms that act as carbon sponge. Fewer whales meant fewer phytoplankton blooms and reduced carbon removal.

Many baleen whale species are still recovering from the mass slaughter over the last century. Pyenson said that if whale populations are restored to their pre-whaling levels, the function of ocean ecosystems would benefit massively.

"Our results suggest the contribution of whales to global productivity and carbon removal was probably on par with the forest ecosystems of entire continents, in terms of scale," he said in a statement. "That system is still there, and helping whales recover could restore lost ecosystem functioning and provide a natural climate solution."

The team estimates at the start of the 20th century, whales in the Southern Ocean were consuming about 430 million metric tons of krill every year. That is double the amount of krill in the whole of the Southern Ocean today.

The study suggests that if whale populations were restored, and levels of phytoplankton increased accordingly, over 200 million metric tons of carbon could be absorbed and stored in ocean systems. These benefits could also increase year on year, the team says.

"Whales are ecosystem engineers, but until our study, we really didn't have numbers based on real world data to make this comparison," Pyenson told Newsweek. "[It's] not an exaggeration to say that with whaling, we lost an oceanic Amazon, in terms of biomass loss (but not in biodiversity). No species went extinct, but the loss of abundance seems to have had a dramatic effect on ecosystem function.

"If we promote the recovery of these giants, we think that would be a good thing for the health and function of the world's oceans—and good for our own descendants too."