Finland has one of the most ambitious carbon-neutral goals in the world: to reach net zero by 2035. If this feels like a bold pledge, there’s good reason for it: two-thirds of the country is covered in forests, that have for decades absorbed more carbon dioxide than they have put out.

But recently, something has changed: Finland’s carbon sink is no longer working. In fact, in barely over a decade, its forests and peatlands have become a net emitter of carbon dioxide … with devastating consequences for the country’s climate goals.

Biodiversity and environment reporter Patrick Greenfield travels to Finnish Lapland to discover what is happening to its primeval forests and peatlands, one of the last true wildernesses on the continent.

He speaks to representatives of the Sami people and environmental activists, to discover how global heating is changing the forests, and how – in turn – changing forests might be affecting global heating too.

It is a new and still little understood phenomenon, he explains to Helen Pidd, but one with a stark warning to the rest of the world.