As the implications of climate change become starker and the world faces up to a biodiversity crisis that threatens humanity’s existence, a group of campaigners from across the world are saying there is one clear way to get us out of this mess, but that governments are ignoring it.

In an open letter published in the British newspaper The Guardian, the group tells governments that the best and cheapest way to avert a climate catastrophe is to heal nature by restoring and replanting degraded forests and by better conserving the natural world.

“Defending the living world and defending the climate are, in many cases, one and the same. This potential has so far been largely overlooked,” say the 23 signatories to the letter.

“We call on governments to support natural climate solutions with an urgent programme of research, funding and political commitment,” they added.

Vast amounts of carbon can be removed from the air and stored by restoring ecosystems razed by palm oil plantations, cattle ranching and timber and fish production, the letter says. The 23 signatories include the teenage school climate strike activist Greta Thunburg, authors Margaret Atwood, Naomi Klein and Philip Pullman, U.S. climate scientist Michael Mann, and environmental campaigner Bill McKibben.

“The world faces two existential crises, developing with terrifying speed: climate breakdown and ecological breakdown. Neither is being addressed with the urgency needed to prevent our life-support systems from spiralling into collapse,” say the signatories.

They call for the defense, restoration and reestablishment of forests, peatlands, mangroves, salt marshes, natural seabeds and other crucial ecosystems, to remove and store large amounts of carbon from the air. The protection and restoration of these ecosystems can help minimize a sixth great extinction, they say.

The group says that nearly a third of the greenhouse gas reductions needed to hold temperatures to a 1.5 C (2.7 F) rise can be provided by the restoration of natural habitats. But natural solutions are calculated to have attracted just a small fraction of the funding so far committed, according to journalist and author George Monbiot, one of the signatories.

Technology alone cannot solve climate change, Monbiot wrote in The Guardian. Much of the technology proposed to capture carbon is expensive and could pose problems at scale. The cheapest and surest approach, he wrote, is to restore natural forests and allow native trees to repopulate deforested land.