The restored forests have the potential to soak up the equivalent of 5.9 gigatonnes (Gt) of carbon dioxide – more than the annual emissions of the US, according to conservation groups.

A team led by WWF used satellite data to build a map of regenerated forests.

Forest regeneration involves restoring natural woodland through little or no intervention.

This ranges from doing nothing at all to planting native trees, fencing off livestock or removing invasive plants.

William Baldwin-Cantello of WWF said natural forest regeneration is often “cheaper, richer in carbon and better for biodiversity than actively planted forests”.

But he said regeneration cannot be taken for granted – “to avoid dangerous climate change we must both halt deforestation and restore natural forests”.

“Deforestation still claims millions of hectares every year, vastly more than is regenerated,” Mr Baldwin-Cantello said.

“To realise the potential of forests as a climate solution, we need support for regeneration in climate delivery plans and must tackle the drivers of deforestation, which in the UK means strong domestic laws to prevent our food causing deforestation overseas.”

The Atlantic Forest in Brazil gives reason for hope, the study said, with an area roughly the size of the Netherlands having regrown since 2000.

In the boreal forests of northern Mongolia, 1.2 million hectares of forest have regenerated in the last 20 years, while other regeneration hotspots include central Africa and the boreal forests of Canada.

But the researchers warned that forests across the world face “significant threats”.

Despite “encouraging signs” with forests along Brazil’s Atlantic coast, deforestation is such that the forested area needs to more than double to reach the minimal threshold for conservation, they said.

The project is a joint venture between WWF, BirdLife International and WCS, who are calling on other experts to help validate and refine their map, which they regard as “an exploratory effort”.

One of the simplest ways to remove carbon dioxide from the air is to plant trees. But scientists say the right trees must be planted in the right place if they are to be effective at reducing carbon emissions.

The vaquita marina is found only in Mexico. It is the most critically endangered sea mammal on the planet, its survival threatened by a deadly clash of interests between fishing and conservation. Scientists estimate there may be fewer than a dozen left in the wild.

Jacques Cousteau, the marine explorer, called the Sea of Cortéz, also known as the Gulf of California, “the world’s aquarium”.

One of its treasures is a silvery-coloured porpoise with wide, panda eyes. But the vaquita’s days may be numbered because of illegal fishing for another protected species: totoaba.

Totoaba, a fish that can grow as large as a vaquita, was a food source before it was placed on Mexico’s endangered list.

“We used to catch it in the 60s and 70s,” remembers Ramón Franco Díaz, president of a fishing federation in the coastal town of San Felipe, on the peninsula of Baja California. “Then the Chinese came with their suitcases full of dollars, and bought our consciences.”

They arrived wanting the totoaba’s swim bladder, an organ that helps the fish stay buoyant. In China it is highly prized for its perceived – though unproven – medicinal properties.

According to the Earth League International NGO, 10-year-old dried swim bladders can sell for $85,000 (£60,000) a kilo in China. The fishermen of San Felipe make only a tiny fraction but in a poor community, business has boomed for the “cocaine of the sea”.

“The illegal fishermen – the criminal elements – are so strong that in the plain light of day you see them with their illegal nets and totoaba,” says Mr Franco Díaz.

Every afternoon during the season, a stream of pick-up trucks towing fishing boats reverses down the concrete slipway of the town’s public beach and into the sea. These craft are mostly unlicensed, and their crew use nets that can kill the vaquita.

“Gillnets might be hundreds of metres long and 10 metres high,” says Valeria Towns, who works with a Mexican NGO, Museo de la Ballena. “They become a wall under water.”

To protect the vaquita, all gillnets are banned in the upper Gulf. However, they are widely used, even by fishermen with permits for halibut or prawns. Mesh size varies with the catch, and the most perilous to the vaquita are gillnets with large mesh used for totoaba.

“It’s not easy for marine mammals to free themselves from those – the vaquita get caught,” says Ms Towns.

Off the coast of San Felipe, all commercial fishing is supposed to be prohibited inside the Vaquita Protection Refuge – an area larger than 1,800 sq km. Inside the refuge is a smaller zone of Zero Tolerance.

Museo de la Ballena supports a handful of fishermen interested in ending reliance on gillnets, and it sponsors alternatives to fishing like oyster cultivation. It is also one of the NGOs that removes gillnets from the protected area.

This is an activity that has escalated tensions between locals and conservationists.

On 31 December 2020, one fisherman was fatally injured and another seriously hurt after their fishing boat collided with a larger ship belonging to the international NGO Sea Shepherd that was out taking up gillnets.

The facts are disputed, but the upshot was a riot in San Felipe where Museo’s ship is moored.

“They were going to burn our boat,” says Ms Towns who was at sea at the time, testing vaquita-friendly nets. “When I arrived back, other fishers that work with this alternative gear, they were protecting our boat, telling them [the rioters], ‘This is not your enemy! Don’t burn this boat’.”

Museo’s ship escaped with broken windows. The Mexican Navy was not so fortunate, one of its patrol boats was set alight in the port.

Now there is an uneasy truce. The Navy says it continues to patrol and remove nets from the Refuge. But there are few NGOs involved – Museo de la Ballena awaits permission to resume work, and the Sea Shepherd never returned to San Felipe after the incident.

Impunity and the absence of law enforcement may account for the dozens of totoaba launches leaving from San Felipe’s beach and heading into the Refuge.

“Not a single authority stops them,” says Ramón Franco Díaz. “If you dared approach them, they’d give you a bullet. Organised crime has stolen the Sea of Cortéz.”

“Before you needed to watch the wind and the sea,” says one former totoaba fisherman. “Now you see a lot of crazy people out there with guns.”

The violent events of 31 December made international headlines, and brought San Felipe into the spotlight.

Now the Mexican government is considering proposals that might gratify the fishermen, but will anger conservationists worried about the vaquita’s precarious fate.

One is to lift the totoaba’s endangered species status. Another is to legalise the other fishing already happening in the Refuge.

“We want to establish different fishing zones for example, for Corvina and shrimp,” says Iván Rico López from the government taskforce exploring sustainability in the upper Gulf.

“The Refuge is huge. If the ban on fishing was respected there, the fishermen just wouldn’t eat. So we have to move towards legalising fishing.”

The Mexican government has also distributed 3,000 “suripera”, vaquita-safe nets. But fishermen complain these reduce their catch by 80%.

“We’ve got to look for ways to increase that,” says Mr Rico López. “We’re looking for alternatives, but we have to convince communities – if they’re not involved in decision-making, we won’t succeed.”

Is it possible to protect this precious mammal and ensure locals still make a living? In San Felipe the illicit trade in totoaba, the menacing involvement of organised crime, and little economic diversity create a toxic mix. There is also an entrenched traditional fishing culture.

Valeria Towns has a warning for San Felipe’s fishing families who ignore the call to make changes to save the vaquita. “I don’t think anyone would buy products from an area where people extinguished a species.”

So after the free-for-all of this totoaba season, would she bet on the vaquita surviving till next year?

“Of course! There’s always hope. If not, I wouldn’t be here.”