At a remote site where the world’s largest rainforest abuts land cleared for big agriculture, Brazilian and American scientists are keeping watch for a critical tipping point – the time when the Amazon ceases to be a carbon sink and turns into a source of carbon emissions.

The contrast is staggering. On one side of a narrow track is cool, moist rainforest, stretching northwest for hundreds of kilometers through the almost intact Xingu indigenous reserve. On the other side is hot, bare ground being prepared to plant soy on a farm the size of 14 Manhattans. This, says my guide, earth systems scientist Michael Coe, is the front line of deforestation in the Amazon – where the rainforest meets agribusiness, but also where a rainforest ecosystem is being degraded into savanna grassland.

It is also “the perfect laboratory” for exploring how forests interact with climate, and how that changes when the forest disappears, says Coe, of the Woods Hole Research Center in Massachusetts. And it is where Brazilian and American scientists are keeping watch for the long-predicted tipping point – the moment when the Amazon, the world’s largest rainforest, begins a process of runaway degradation, when so much forest has been lost that the transition to savanna is irreversible. That will be the moment when the Amazon ceases to be a carbon sink that helps protect the planet from climate change, and turns into a global source for carbon emissions.