Restoring the populations of a few important groups of animals could help capture huge amounts of carbon from the air and thereby play a role in limiting global warming.
Climate change research has emphasised the importance of vast forests and seagrass meadows as the most efficient way of storing carbon. But bison, elephants, whales, sharks and other massive wild animals also store carbon in their bodies while promoting tree and seagrass growth, preventing carbon-releasing wildfires and packing down ice and soil to keep carbon in the ground, says Oswald Schmitz at Yale University.
“There’s been scepticism in the scientific community that animals matter, because if you just do the accounting, they’d say animals don’t make up much of the carbon on the planet, so they can’t be important,” he says. “What we’re doing is connecting the dots, showing that animals – despite their lack of abundance – have an outsized role, because of the multiplier effects that they create.”
To keep the average global temperature from rising more than 1.5°C above its pre-industrial level, scientists estimate that we need to remove 6.5 gigatonnes of carbon dioxide per year from the atmosphere until 2100. Current models that focus on protecting and restoring forest, wetland, coastal and grassland ecosystems would fall short by an estimated 0.5 to 1.5 gigatonnes per year, says Schmitz.
He and his colleagues reviewed data from previous publications about the environmental effects – including dispersing seeds, trampling, carbon cycling, feeding behaviour, hunting behaviour and methane production – of dozens of kinds of wild animals.
They determined that we could theoretically meet the planet’s carbon reduction goals by protecting six groups of animals and expanding another three. The populations of reef sharks, grey wolves, wildebeest, sea otters, musk oxen and ocean fish need to be maintained at current levels. We would also need populations of at least 500,000 African forest elephants, 2 million American bison and 188,000 baleen whales in the Southern Ocean. Collectively, these populations could help capture approximately 6.41 gigatonnes of carbon dioxide annually, says Schmitz.
Herbivores consume plants that compete with trees for resources, pack down carbon-rich soil as well as ice in the permafrost, maintain grasslands that might otherwise lead to wildfires, and promote new tree growth through seed dispersal, while storing large amounts of carbon in their own bodies for decades.
Whales encourage populations of carbon-capturing phytoplankton at the sea surface through their breath and faeces, and then send massive amounts of carbon deep to the sea floor when they die. Predators, meanwhile, control populations of animals that might otherwise endanger carbon-storing plants on the land and sea if left unchecked.
Schmitz says these animal populations can rebound quickly if the right conditions are in place, but we would need to return vast areas of farmland to nature.
“Instead of being cattle ranchers, let’s think about being carbon ranchers,” he says. “Let’s bring the bison back and actually pay the ranchers for the carbon that they store rather than the meat produced by cattle.”
While the new findings “bring together a broad vision for global rewilding that is admirable”, they don’t provide enough evidence for policy recommendations, says Yadvinder Malhi at the University of Oxford.
“I think there is real potential for synergies between wildlife conservation and carbon storage, [but] I am wary of anything like this being touted as a ‘global warming game changer’,” says Malhi.
“The science is not yet robust enough and the timescales involved in many cases are too slow given the urgency of the climate crisis,” he says. “Trying to get this into international climate frameworks could even be a distraction from the only real global warming game changer, which is keeping fossil fuels in the ground.”