Often referred to as the 'lungs of the world', the Amazon doesn't actually provide the grossly over-stated 20 per cent of the planet's oxygen that's been alleged, but it's still one of the world's finely tuned biological systems, and could tip over into ecological chaos - with widespread implications for the region and the planet.

Its vast stores of atmospheric moisture - airborne rivers that thread away from the forest across Latin America - buffer the region from the ravaging effects of drought. It also regulates the climate by storing vast quantities of CO2 in its vegetation and carbon-rich soil - though last year, a study found it was now emitting more CO2 than it stored, mostly thanks to deliberate fires set to clear the land for soy and beef production.

The researchers used various data sources, including satellite data on vegetation optical depth (VOD), a measure of the total biomass of trees and plants in a given area. With this data, they were able to build a picture of changes over time in the forest's resilience, meaning its ability to recover from a disturbance.

What they found was stark: since the early 2000s, three-quarters of the Amazon has been losing resilience. That doesn't mean those areas have lost vegetation cover, but their vegetation's ability to weather disturbances is diminishing over time.

"The rainforest can look more or less the same, yet it can be losing resilience - making it slower to recover from a major event like a drought," explains Tim Lenton, director of the Global Systems Institute (GSI) at Exeter University, UK.

The study also found the warning signs of a vastly approaching tipping point, in the form of 'critical slowing down'. Critical slowing down is a behaviour in systems research: it suggests a system that can occur in two different states is swiftly approaching a shift from one state to the other - in this case, an ecosystem shifting from tropical rainforest to savannah grassland.

"The Amazon rainforest is a highly complex system, so it's very difficult to predict if and when a tipping point could be reached," notes study co-author Chris Boulton, also of Exeter's GSI.

But the smoking gun for these changes is easily identifiable.

"Deforestation and climate change are likely to be the main drivers of this decline," explains co-author Niklas Boers, of the Potsdam Institute for Climate Impact Research (PIK), and the Technical University of Munich, Germany.

"Resilience is being lost faster in parts of the rainforest that are closer to human activity, as well as those with less rainfall.

"Many researchers have theorised that a tipping point could be reached, but our study provides vital empirical evidence that we are approaching that threshold."

Depressingly, we may not know when that tipping point is reached until it's too late.

"If too much resilience is lost, dieback may become inevitable - but that won't become obvious until the major event that tips the system over," says Boers.

The research is part of the Tipping Points in the Earth System (TiPES) project funded by the EU's Horizon 2020 programme.