In the Țarcu mountains of Romania, a pioneering experiment is changing the atmosphere around rewilding. Starting in 2014, around 100 European bison were gradually reintroduced to the area, having been wiped out by hunting more than 200 years ago. They now number more than 170 and graze over some 48 square kilometres. That is a success story in itself. But there is more to this project than just bringing back the big beasts. Their domain has also become a carbon hoover, sucking an estimated 200,000 tonnes of carbon dioxide out of the air every year, equivalent to taking 43,000 petrol cars off the road.
The bison themselves aren’t a significant carbon sink. It is their influence on the wider environment – compacting soil, dispersing seeds and creating varied habitats through their browsing – that has turbocharged its ability to absorb carbon. The area in which they roam is now soaking up 10 times the amount that it was before the bison were reintroduced.
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The Țarcu mountains experiment is the first test of a concept that , an ecologist at Yale University, claims has the potential to restore the atmosphere to an earlier state and hence help to arrest climate change. Schmitz and his collaborators argue that if similar projects were rolled out across the globe, both on land and in the sea, a significant amount of carbon would just disappear.
These researchers are now building their evidence base and honing their plans. Meanwhile, though, some climate scientists have raised concerns that attempts to put rewilding at the heart of climate mitigation could backfire. So how do Schmitz’s claims really stack up?
Carbon cycle
A solution to the climate crisis demands two things. First and foremost, that we stop emitting greenhouse gases, or at least get emissions down to net zero by 2050. Second, that we remove the huge amount of CO2 we have pumped into the atmosphere over the past century or so. Fail on either front and we have no chance of keeping the global average temperature rise to below 2°C relative to pre-industrial levels. This would be catastrophic.
The first of these demands is difficult, but doable. How we might achieve the second is less clear. According to Schmitz, to pay back our legacy carbon debt, we need to remove at least 6.5 gigatonnes of CO2 a year, every year, from now until 2100. Existing methods of drawing carbon down from the atmosphere, if expanded to their maximum capacity, would only be able to remove 6 gigatonnes a year, says Schmitz. Less, if we also need to use them to offset ongoing emissions.
A growing pile of evidence supports the idea that animals are powerful drivers of carbon capture
Those existing methods are largely nature-based, such as protecting habitats and planting living carbon sinks – trees, mangroves and seaweeds. There are also technological fixes, such as enhanced rock weathering, where rocks are crushed to dust to increase the rate at which they absorb CO2, or direct air capture, where CO2 is pulled out of the atmosphere by huge machines. But these are emerging technologies that may never actually mature.
The upshot is that, even if we reach net zero by 2050, we appear to still be, at the very least, 500 million tonnes short on CO2 removal per year for the next 75 years.
According to Schmitz, however, this is much less of a problem than it appears, because a powerful but overlooked nature-based solution is already making up much of that shortfall and could even solve the whole problem on its own: wild animals.
Until recently, animals weren’t considered part of the solution at all. Compared with land plants and seaweed, the amount of carbon stored in animal bodies is minuscule, accounting for just 0.3 per cent of all the biomass on Earth, according to a . But around 10 years ago, Schmitz started to think that this might be an oversight.
He was researching an ecological concept called trophic cascades, which holds that ecosystems are primarily shaped from the top down by the feeding behaviour of “apex consumers” – large herbivores and carnivores. Herbivores influence the make-up of vegetation, which fundamentally alters how the ecosystem works. They, in turn, are controlled by their predators, so predators also influence vegetation. Exactly how depends on the context. But, in general, if you take the large animals out of an ecosystem, you radically alter it, usually for the worse.
Biomass
At the same time, research from the Serengeti plains of East Africa and the American prairies was finding that grazing by wildebeest and bison altered nutrient cycles, especially how much carbon could be locked away by plants. “And so I connected the dots,” says Schmitz. “It dawned on me that having animals in the system can transform what the vegetation looks like and should thereby also be able to transform how much carbon is taken up.” In 2014, he and his collaborators published the idea and coined a term for harnessing animals’ underappreciated influence on carbon storage: , or ACC for short.
They point to a growing pile of evidence to support their claims that animals are unrecognised but powerful drivers of carbon capture. On the Serengeti, for example, 1.2 million blue wildebeest range over a vast area, browsing and defecating as they go. By keeping the grass trim, they reduce the frequency and intensity of wildfires, which release stored carbon from the soil. Much of their carbon-rich dung ends up in long-term storage underground, buried by insects. Together, these actions allow the Serengeti ecosystem to store 4.4 million tonnes more CO2 each year than it would in the absence of wildebeest.
In the Pacific Ocean, meanwhile, by feeding on herbivorous urchins that, if left to their own devices, devastate the kelp. The sea otter population of the US and Canada – which has rebounded from near-extinction after being mercilessly hunted for their fur – leads to the capture of 5.2 million tonnes of CO2 a year.
A few million tonnes of CO2 isn’t going to make much of a difference, but many other animals create carbon-sinking ecosystems, and it all adds up. Indeed, Schmitz estimates that existing populations of wildebeest, sea otters and five other species – muskox, grey wolves, tiger sharks, lemon sharks and blacktip reef sharks – already collectively stimulate the storage of 300 million tonnes of CO2 a year. Which is a bit more like it. That sort of figure goes quite a long way towards filling the gap between the conventionally calculated carbon capture potential of nature-based solutions and what we need to remove from the atmosphere to avoid climate catastrophe. And there’s more. If you take into account existing populations of all marine fish, the amount of annual carbon capture and storage attributable to wild animals increases almost 20 times.
Fish have a “tremendous” impact on carbon storage, says Schmitz. “Part of it is in just the sheer biomass of these animals,” he says. But bony fish also fix carbon into insoluble minerals in their intestines as part of their way of dealing with . “It’s a sort of rock-like substance that they poop out and that sinks to the ocean bottom really quickly,” says Schmitz. Collectively, .
Schmitz and his collaborators estimate that, taken together, this group of animals alone – wildebeest, sea otters, muskox, grey wolves, sharks and other marine fish – are already taking 5.8 gigatonnes of CO2 out of the atmosphere each year, not far off the overall 6.5 gigatonnes a year legacy CO2 debt.
That’s a massive contribution, and one that Schmitz says isn’t accounted for in the global carbon budget. If he is right, we have overestimated how much legacy carbon we need to remove because animals are already doing most of it, unnoticed and unsung. “What we need to do is incorporate what the animals currently provide in [carbon] accounting,” says Schmitz. “But because we’re not, we’re maybe letting an important ingredient of ecosystems disappear and that puts us further in the hole. So we need to conserve what we have.”
Apex consumers
As for the rest – 700 million tonnes – that may be achieved by expanding the existing natural methods of CO2 removal. Or else Schmitz calculates that much of it could be taken care of by putting apex consumers back into the ecosystems from which they have been removed, known as trophic rewilding.
A key part of that calculation is a methodology for estimating how much carbon a given rewilding project would remove from the atmosphere, called the Yale/GRA ACC model (GRA stands for the Global Rewilding Alliance, which has funded a lot of Schmitz’s work). “We’re taking the classic principles of carbon cycling – that is, carbon uptake through photosynthesis and carbon recycling because of plants dying and going into the soil – and layering herbivores feeding on the plants on that, and then carnivores feeding on the herbivores,” he says. Using that tool to analyse the , Schmitz and his collaborators were able to estimate the carbon impact – subsequently measured by taking samples in the field – to a fair degree of accuracy.
They have calculated that trophic rewilding of just seven further species around the world would add another 600 million tonnes of carbon drawdown a year. These are forest elephants, American bison and five species of baleen whale: blue, fin, humpback, southern right and Antarctic minke. They wouldn’t need to be brought back to their full, historical populations or ranges, but to something like their status before the industrial revolution, says Schmitz. That means around 500,000 African forest elephants (up from less than 100,000 today), 2 million American bison (up from 30,000 today) and 188,000 baleen whales in the Southern Ocean.
To start making ACC happen at scale, Schmitz and his colleagues are now applying the carbon storage methodology to numerous other existing and potential rewilding projects worldwide, all with a view to figuring out which would have the biggest impact.
Rewilding hotspots
“We’ve got case studies in different habitats and on different continents, deliberately diverse ones,” says at the Global Rewilding Alliance. “The idea is, what are the 50 or 60 or 70 locations around the world that hold the most promise for helping to stabilise the global climate by helping nature’s recovery?” The hope is that this “hotspots” study will lend support to the case for a global rollout of ACC. Some countries are even looking at incorporating rewilding gains into their legally-binding plans for reducing carbon emissions in line with the Paris Agreement, according to Scott.
Such grand plans, however, make some scientists nervous. “Clearly, animals play really important roles in nutrient cycling, including carbon cycling,” says at Cornell University in Ithaca, New York, co-author of a of ACC published in August 2024. “But we shouldn’t overstate their importance in climate change mitigation. We have limited science at this point on select species and select systems.”
What isn’t taken into account, says Duvall, is that some species in some ecosystems – savannah elephants, for example – actually result in an overall release of carbon into the atmosphere. “The reality is that when it comes to carbon, not every animal is doing the same thing and not every system is doing the same thing,” he says. “It’s not an inevitable outcome of functionally intact and biodiverse systems to store carbon.”
His concern is that people will jump to the conclusion that all rewilding projects are carbon negative when they aren’t. He likens it to tree planting, which was once seen as a climate panacea, but turned out to be more complicated and context dependent. Dark foliage can have a warming effect by soaking up sunlight that would otherwise be reflected, for instance.
The guardians of the carbon budget aren’t buying it either. “He is claiming that rewilding upper trophic mammals could induce large carbon storage in vegetation and soil,” says at the University of Exeter’s Global Systems Institute in the UK, which publishes an . “That’s true in theory, and there is some evidence of increased carbon storage locally, for example in South Africa.” However, Friedlingstein disputes the claim that animals are already drawing down vast amounts of unaccounted-for carbon. “Wildlife preservation is, of course, important for biodiversity, but not for climate change.”
Schmitz acknowledges that not all rewilding projects will draw down carbon, but he insists that isn’t a problem because we can just focus on those that do. That is exactly why he and his collaborators are working to identify the specific contexts in which trophic rewilding is worthwhile. “If we’re going to rewild, let’s rewild,” says Schmitz. “But then let’s measure the relevant ecosystem components as we go and learn by doing. This is win-win conservation.”
Conservation and rewilding in the Central Apennines: Italy
Journey into Italy’s Central Apennines region for a fascinating introduction to the concept and practicalities of rewilding.
