Luciana Gatti stares grimly out of the window of the small aircraft as it takes off from the city of Santarém, Brazil, in the heart of the eastern Amazon forest.

The land ecosystems of the world together absorb about 30% of the carbon dioxide released by burning fossil fuels; scientists think that most of this takes place in forests, and the Amazon is by far the world’s largest contiguous forest.

I’ve travelled to Santarém, where the Tapajós River joins the Amazon River, to join Gatti and other scientists trying to determine whether the forest is heading for an irreversible transformation towards a degraded form of savannah.

While passing over one huge, newly razed parcel of Amazon forest, Gatti’s voice crackles over the plane’s intercom.

A 2015 analysis of 321 plots of Amazon primary forest with no overt human impacts reported “a long-term decreasing trend of carbon accumulation”.

That’s a change from previous decades, when censuses indicated that such primary forest in the Amazon was storing more carbon.

The fate of the Amazon is on Gatti’s mind as she climbs a lattice tower in the Tapajós forest - one of the landmarks her pilots fly over as they collect air samples.