How the Amazon's 'Boiling River' foreshadows a warmer world

When you drive towards Peru's Boiling River, along a bumpy, four-hour track through the rainforest, it's only after you transcend a crest in the landscape that you see it ahead of you, says Alyssa Kullberg, a postdoctoral researcher in plant ecology at the Swiss Federal Technology Institute of Lausanne (EPFL). Huge billows of steam rise from a clutch of trees in the wide, saucer-like depression below.

"It was so magical," says Kullberg, remembering the first time she saw it with her own eyes. The Boiling River, also known as the Shanay-timpishka or La Bomba, is part of a tributary in east-central Peru that connects to the mighty Amazon River.

The hills in this area were scoured by fossil fuel companies looking for oil reserves in the 1930s but the secrets of the legendary Boiling River itself are only now being told in depth by Western scientists. Researchers have, for instance, determined that the river is heated by geothermal sources deep in the ground below. Kullberg first visited this mysterious place in 2022 along with a team from the US and Peru, including Riley Fortier, currently a PhD candidate at the University of Miami. As the researchers trekked through the jungle, they noticed something unusual about the plant life around them.

"It was just really evident to all of us that there was a clear and noticeable change along the river," says Fortier. "The forest felt maybe scrubbier. There weren't as [many] big trees and it felt a little bit drier as well, the leaf litter was crunchier."

Fortier recalls marvelling at how hot this stretch of the jungle was, even for the balmy Amazon. He and other members of the team realised that this place represented a possible snapshot of how climate change might alter the Amazon, as global warming pushes average air temperatures higher than they are today. In that sense, the Boiling River could be seen as a kind of natural experiment – a possible glimpse of the future.

But studying it wasn't going to be easy: "It's like doing fieldwork in a sauna," says Fortier.

In a paper published in October, Fortier, Kullberg and colleagues from the US and Peru describe how they tracked a year's worth of air temperature readings near the Boiling River using 13 temperature logging devices. The researchers positioned these along a stretch of the river that included cooler areas more typical of the wider forest. The average annual temperature ranged from 24-25oC (75-77F) in those cooler places to 28-29oC (82-84F) in the warmest parts. The maximum air temperatures logged, in a handful of the hottest locations along the Boiling River, approached 45oC (113F). A past analysis – that has not yet been published in a peer-reviewed journal – by the geothermal scientist Andrés Ruzo found that the average water temperature is 86C (187F).

The team also battled stifling conditions to make a detailed analysis of which plant species were present. They carefully studied vegetation in a series of plots sampled along the river and detected an important correlation: where the river was hotter, plant life was less dense and some species were absent entirely. "There was way less vegetation in the understorey," says Kullberg. "Even though it's very steamy, the vegetation looked a lot drier."

Some large trees such as Guarea grandifolia, an evergreen that can grow up to 50m (164ft) tall, seemed to struggle near the hottest parts of the river, for example. Overall, the heat seemed to have a negative impact on biodiversity. The sheer amount of steam in the air might even deter flying insects or other animals from the area, suggests Fortier, though the team's study did not examine this specifically.

Plant species that are known to tolerate high temperatures were more common in the hotter areas, which in itself may not be unexpected – but the team was surprised to see this effect even over very small distances. The entire length of their study area was not more than around 2km (1.24 miles). Plus, the hottest parts of the Boiling River are intermittent – a handful of particularly steamy patches here and there. The study results suggest that, as soon as temperatures reach a certain point, plant life responds almost immediately.

"I thought it was great," says Chris Boulton of the University of Exeter, who was not involved in the study, referring to the team's interpretation of the Boiling River as a natural experiment. "It's a clever thing to do." The Boiling River is an example of how the Amazon might change in the future, says Diego Oliveira Brandão, a member of the technical-scientific secretariat of the Science Panel for the Amazon, a scientific research organisation. He adds that he is concerned about the impact that such consequences of climate change could have on indigenous peoples. "These populations depend on biological resources," he says.

Boulton agrees, pointing out that Indigenous groups in the Amazon have already faced significant threats such as flooding and drought, which in some cases has been exacerbated by climate change.

Higher temperatures in the Amazon could threaten the very functioning of many plants there, says Rodolfo Nóbrega at the University of Bristol – and the Boiling River illustrates this perfectly. "As you increase [the region's] temperature, even though you have water availability [nearby], you might reduce the photosynthetic capability of the plants," he says. "What I believe is happening is the plants are being stressed by temperature even though there is water around." Though he notes that the study authors did not measure the temperature or abundance of groundwater.

Kullberg says that, while the Boiling River does hint at how increased temperatures could affect biodiversity and plant growth, it is important to remember that this part of the Amazon might not exactly mirror the rainforest's future in a broad sense. One would not expect so much steam anywhere else, for example. And large weather effects, such as changes in storms or rainfall, will also influence how the forest as a whole evolves in the coming years.

There is another reason why the Boiling River might not fully represent the conditions of the wider Amazon basin under the influence of further climate change. Nóbrega points out that the Amazon is a massive place. It spans portions of nine different countries, including Brazil, Peru, Bolivia, Colombia and the French Guiana, an oversea French territory. In total, it covers an area of more than 6.7 million sq km (2.6 million square miles). "What you find in one area might not be scientifically relevant for another area that has another rainfall pattern or another plant distribution," he says.

Previously, Boulton and colleagues have studied the possibility that the Amazon is reaching a "tipping point", a moment at which climate change and deforestation would force the forest into rapid decline.

"You might see sudden dieback of trees, maybe over a decade or so," says Boulton. He notes that the Amazon isn't just getting hotter and drier because of climate change, though. One particularly insidious problem is deforestation, which can cut off atmospheric rivers that flow in the air above the forest. These would otherwise bring moisture into the forest in the form of rainfall. "If you cut down trees, you ruin that link, basically, you make it drier," he explains.

A major report on various global tipping points published in 2023, authored by more than 200 researchers including Boulton, explored the risk that the Amazon rainforest could soon turn into a much drier place – something resembling savannah rather than jungle.

And yet by studying the Boiling River, we can get an idea of which species are most likely to survive in these harsh new conditions, suggests Fortier. He notes the example of the giant Ceiba tree (Ceiba lupuna), which can grow to be up to 50m (164ft) tall. This tree seemed resilient to higher temperatures near the Boiling River, according to Kullberg, an observation backed up by previous research.

Kullberg notes that the Ceiba tree can store water in its trunk, which helps it to survive drought conditions. Confirming that specific plants can cope with the extreme environment of the Boiling River could help conservationists decide which parts of the wider rainforest require the greatest protection, says Fortier. Perhaps it will even be possible to maintain more forgiving microclimates beneath a forest canopy made up of resilient species, adds Kullberg.

Boulton views protecting the Amazon as a way of protecting humanity far beyond the forest itself. The risk is that, if the rainforest does reach a catastrophic tipping point beyond which it begins to die back rapidly, then the whole world will likely suffer. "If the forest goes, a lot of that carbon is going to go up into the atmosphere and that's going to affect the climate," he says. "It's not just local, it's global."

The Boiling River, then, isn't just a glimpse of the future. It is also a warning.