In her the fourth post of the Amazon on the Brink series, LAB author and editor Sue Branford looks at the mounting evidence that the extent of deforestation matters: where it is greater it disrupts the natural water cycle, reducing rainfall and increasing the resulting dieback of trees.

It has long been known by scientists that the Amazon forest would suffer dieback, that is, the widespread decline in the vitality of its trees and thus its eventual death, if global warming reached 3.5 °C. But a new study (Nature, doi.org/q5td) by Nico Wunderling, from the Potsdam Institute for Climate Impact Research in Germany, argues that the size of the forest also plays a key role and, if deforestation increases to 22 per cent, this dieback is likely to happen with an increase of just 1.5 °C in the global temperature.

Wunderling explains his reasoning. Prevailing winds transport moisture from the Atlantic Ocean, and it falls as rain over the Amazon. Evaporation and transpiration (that is, the process by which water is released from the plants, largely through small openings called stomata located on the leaves) mean that plants return three-quarters of this water to the atmosphere.

This water falls as rain and most returns to the atmosphere, where the process begins again. The rains form what have been called ‘flying rivers’ that carry moisture across the entire rainforest. The bigger the forest, the more the same rain falls, lowering the risk of dieback. The rain also benefits much of the rest of Brazil, as the rivers are eventually forced south, when they hit the Andes.

Deforestation, even in a few areas, disrupts this process. When an area of forest is razed, more than half of its rainwater runs off into streams and eventually flows into the sea. That starves the flying rivers of moisture and reduces rainfall.

Because the level of deforestation varies across the Amazon basin, the amount of rainfall reflects this variation. Satellite observations and rain gauge measurements show that the amount of rain falling in the southern Amazon basin declined by 8 to 11 per cent between 1980 and 2019.

That is because tree cover in that part of the Amazon shrank by 16 per cent in roughly the same period, mainly because the forest was cut down and burned for beef cattle ranching. The northern Amazon basin has suffered far less deforestation and saw only a slight decrease in precipitation, which was not statistically significant.

No one knows what level of deforestation will lead to total dieback. ‘The Amazon is more sensitive than we used to think, which is bad news,’ Wunderling says. ‘Maybe we’re closer to a deforestation threshold than we thought. But I think there’s lots of uncertainty.’

What is clear is that a prolonged period of drought, stemming from a global event, makes the situation worse. The most common is a natural climate event, popularly known as El Niño, through which the warming of the eastern equatorial Pacific Ocean results in the development of unusually warm waters along the coast of South America. It was El Niño, the fifth-most powerful in recorded history. that led to a historic drought in the Amazon in 2023–2024. The Rio Negro fell to its lowest level in 122 years.

Characterized by temperatures at least 0.5 °C above the norm, El Niño hugely affects the weather and often pushes global temperatures to new heights. El Niño events are occurring more frequently and lasting longer than in the past.

A new El Niño event, which could be ‘one of the largest El Niño events in the historical record going back to 1950’, according to the National Oceanic and Atmospheric Administration (NOAA), is already occurring and is expected to peak next year, putting 2027 in the running to break global heat records. It could produce a series of devastating effects, ranging from supercharged rainstorms to drought.

This will undoubtedly have a big impact in the Amazon, though no one knows precisely what it will be. Its consequences could be permanent. According to Luiz Eduardo Aragão, a researcher at Brazil’s National Institute for Space Research (INPE), ‘the forest is likely to lose biomass over time, reducing its productivity and its ability to recycle water and maintain its original microclimatic characteristics.’