Scientists reveal what would happen if the Amazon rainforest dried out

Independent

29-05-2025

Beneath the dense canopy of the Amazon, a clearing reveals the stark reality of prolonged drought. Fallen logs decay, trees thin out, and the temperature rises where sunlight penetrates.
This scene is the result of 24 years of severe drought in the world's largest rainforest. But it is human-made – this degraded patch of forest, roughly the size of a soccer field, is the site of a scientific experiment.
Launched in 2000 by Brazilian and British scientists, the Esecaflor project, short for "Forest Drought Study Project" in Portuguese, simulates a future where climate change reduces rainfall in the Amazon.
As the longest-running project of its kind, it has contributed to numerous academic articles across meteorology, ecology, and physiology.
The implications of drought on the Amazon, an area twice the size of India spanning several South American nations, extend globally. The rainforest stores a vast amount of carbon dioxide, a key greenhouse gas driving climate change.
According to one study, the Amazon holds the equivalent of two years of global carbon emissions. When trees are cut or die from drought, they release stored carbon into the atmosphere, accelerating global warming.
Creating drought conditions and observing the results
To mimic stress from drought, the project, located in the Caxiuana National Forest, assembled about 6,000 transparent plastic rectangular panels across one hectare (2.5 acres), diverting around 50 per cent of the rainfall from the forest floor.
They were set one metre above ground (3.3 ft) on the sides to four metres (13.1 ft) above ground in the center. The water was funnelled into gutters and channelled through trenches dug around the plot's perimeter.
Next to it, an identical plot was left untouched to serve as a control. In both areas, instruments were attached to trees, placed on the ground and buried to measure soil moisture, air temperature, tree growth, sap flow and root development, among other data. Two metal towers sit above each plot.
In each tower, Nasa radars measure how much water is in the plants, which helps researchers understand overall forest stress. The data is sent to the space agency's Jet Propulsion Laboratory in California, where it is processed.
'The forest initially appeared to be resistant to the drought," said Lucy Rowland, an ecology professor at the University of Exeter.
That began to change about eight years in, however. "We saw a really big decline in biomass, big losses and mortality of the largest trees,' said Professor Rowland.
This resulted in the loss of approximately 40 per cent of the total weight of the vegetation and the carbon stored within it from the plot. The main findings were detailed in a study published in May in the journal Nature Ecology & Evolution. It shows that during the years of vegetation loss, the rainforest shifted from a carbon sink, that is, a storer of carbon dioxide, to a carbon emitter, before eventually stabilising.
There was one piece of good news: the decades-long drought didn't turn the rainforest into a savanna, or large grassy plain, as earlier model-based studies had predicted.
Next steps include measuring forest recovery
In November, most of the 6,000 transparent plastic covers were removed, and now scientists are observing how the forest changes. There is currently no end date for the project.
'The forest has already adapted. Now we want to understand what happens next,' said meteorologist João de Athaydes, vice coordinator of Esecaflor, a professor at the Federal University of Para and coauthor of the Nature study. 'The idea is to see whether the forest can regenerate and return to the baseline from when we started the project.'
During a visit in April, Dr Athaydes guided journalists through the site, which had many researchers. The area was so remote that most researchers had endured a full-day boat trip from the city of Belem, which will host the next annual U.N. climate talks later this year. During the days in the field, the scientists stayed at the Ferreira Penna Scientific Base of the Emilio Goeldi Museum, a few hundred yards from the plots.
Four teams were at work. One collected soil samples to measure root growth in the top layer. Another gathered weather data and tracking soil temperature and moisture. A third was measured vegetation moisture and sap flow. The fourt focused on plant physiology.
"We know very little about how drought influences soil processes,' said ecologist Rachel Selman, researcher at the University of Edinburgh and one of the co-authors of the Nature study, during a break.
Esecaflor's drought simulation draws some parallels with the past two years, when much of the Amazon rainforest, under the influence of El Nino and the impact of climate change, endured its most severe dry spells on record. The devastating consequences ranged from the death of dozens of river dolphins due to warming and receding waters to vast wildfires in old-growth areas.
Professor Rowland explained that the recent El Nino brought short-term, intense impacts to the Amazon, not just through reduced rainfall but also with spikes in temperature and vapour pressure deficit, a measure of how dry the air is. In contrast, the Esecaflor experiment focused only on manipulating soil moisture to study the effects of long-term shifts in rainfall.
'But in both cases, we're seeing a loss of the forest's ability to absorb carbon,' she said. 'Instead, carbon is being released back into the atmosphere, along with the loss of forest cover.'