Can a new way to measure tropical rainforest vulnerability help save them?

A team of top scientists, brought together by the National Geographic Society, built an index to detect which forests need urgent help.

It’s more than just the Amazon.

Rainforests across the tropics, from Indonesia to Central America and from Madagascar to the jungles around the Mekong River, are being cut or burned—to make way for ranches, farms, and palm oil plantations, to be logged for wood or cleared for roads and other human development. Across the globe, up to 20 percent of tropical rainforests has been cleared just since the 1990s, while another 10 percent has been damaged as climate change brings higher temperatures, longer dry seasons, and more frequent droughts.

Now, an analysis by a team of more than 50 top scientists and conservationists suggests that all this change is hitting the damp, rich forests hard. In most corners of the tropics, forests are losing their capacity to store carbon and recycle water and are more vulnerable to collapse than previously thought. At the projected rate of climate change and escalating land-use activities, the forests may even become a source of carbon to the atmosphere.

Some areas are changing faster than others. This team of experts, brought together by the National Geographic Society, with the support of Rolex, combined 40 years of satellite data with other forest observations to create a “vulnerability index,” which scientists plan to use in coming years to track which stretches of tropical rainforest need help the most urgently. The research was published today in the journal One Earth.

If pushed too far, vast stretches of tropical rainforest could see widespread tree deaths, or could transition to a new state, becoming drier, savanna-like woodlands. That would devastate some of the most wildlife-rich regions on earth and potentially worsen climate change because intact rainforests slurp large volumes of carbon dioxide from the atmosphere. While such a shift, in general, would likely happen gradually, scientists worry that some forests, particularly the Amazon, could transform rapidly into something new.

“I think we all have this view that things are bad out there,” says Kristofer Covey, an ecologist and biochemist at Skidmore College and co-author of the study. “The goal here is to understand. Which things are how bad? And where and to what extent, and can we then use that information to make better decisions going forward?”

They hope this approach can provide an early warning system to direct conservation resources, which are limited, to the forests most at risk.

“The public should understand that it’s not just deforestation,” says lead author Sassan Saatchi, a forest carbon expert at NASA’s Jet Propulsion Laboratory. “The way forests function is changing. Since about 2000, we're seeing a new phenomenon. The impact from climate change has accelerated.”

Physicals for tropical forests 

The state of tropical rainforests differs from one continent to the next. African forests are seeing more fire than other places, while the Amazon is seeing more water loss than Asian forests. Forest productivity is declining significantly across the Amazon, while productivity remains stable in the Congo and is even increasing in the tropical forests of China, thanks in part to recovery from past abuses and significant efforts to replant trees.

Vulnerability of rainforests can be measured in many different ways, and previous research often focused on small areas. That, Saatchi says, often confused researchers and conservationists trying to prioritize forest restoration.

These researchers used satellites and other models and measurements to track land temperature, above-ground photosynthesis and production, and shifts in the overall abundance and diversity of wild species. They also examined loss of tree cover from deforestation and fire, as well as changes in how much carbon and water are transferred between plants and the atmosphere.

That massive collection of information allowed them to establish a finely detailed, uniform system for evaluating forest health, much as a doctor might check weight, heart rate, blood pressure, and cholesterol during an annual physical.

And much as some people struggle with heart issues while others have lung troubles, all these forests “are all going through different stressors at different time scales,” says co-author Katia Fernandes, a University of Arkansas expert on fire and drought in the Amazon.

For instance, in Asia, land use changes currently cause more damage than climate change. In central Africa, meanwhile, forests are seeing greater water loss and higher temperature increases than Asia. But the Congo as a whole, for the moment, remains mostly intact. While it is seeing some consequences from climate change—many trees in Gabon, for example, are producing less fruit, which means less food for some wildlife—it has avoided widespread tree deaths and its vegetation, in many other ways, is increasing. Scientists suspect Africa’s long history of water stress may actually have left forests better adapted to droughts.

So far, it seems, “the Congo likely looks OK because humans have cleared less intensively, and the drying of the atmosphere isn’t enough to hurt the trees and may even make them grow faster because it clears the clouds, letting more sun through,” Covey says.

It came as little surprise to the team that the region facing the most stress in the most ways was the Amazon.

The Amazon remains at the greatest risk

“The Amazon stands out as at particular risk even when considering it alongside other global rainforest challenges,” Covey says. “Widespread deforestation paired with a rapidly changing climate are notably impacting ecosystem function across a whole suite of metrics.”

With its golden lion tamarins, colorful birds, and giant stinging wasps, the Amazon’s richness and biodiversity is unparalleled. It’s home to 10 percent of the world’s species and more than two million types of insects. Its trees and soils store the equivalent of four or five years of human carbon emissions, and the forest creates much of its own water as moisture moves in off the Atlantic Ocean, gets sucked up by plant roots, and then is returned to the atmosphere through leaves. A single water molecule can cycle through the forest four or five times.

But deforestation, which has been rising under Brazilian President Jair Bolsonaro, hit a 12-year high last year. Fast-growing, drought-tolerant trees are outcompeting short species that do well in wet conditions. Rains come in pounding bursts, causing floods. Droughts last longer and occur more often—three major droughts have hit in 16 years. Fires are burning more explosively. The rate of tree deaths is increasing.

All this prompted two researchers in 2017 to conclude that if deforestation weren’t stopped and fossil-fuel burning weren’t curbed, a change in the moisture cycle in parts of the Amazon could ignite a self-reinforcing spiral that kills millions of trees or turns the forest into a dry woodland. They believed such a tipping point could come if as little as 20 percent of the Amazon were cleared, which is roughly how much already has been lost.

Both of those authors—Thomas Lovejoy, a George Mason University professor and senior fellow at the United Nations Foundation, and Carlos Nobre, a senior researcher at the University of São Paulo—are co-authors on the new study.

Saatchi says it’s imperative that deforestation stop—a challenge that comes with its own set of complications. But even with that, it’s not enough to stop the damage. Active reforestation is desperately needed. “We still don’t know as much as we want to know about how the system is going to react,” Saatchi says. Or how quickly. “We need to restore these systems.”

By simply putting all these measurements together for the first time, the scientists have been able to paint a much clearer, if more troubling, picture in the Amazon and elsewhere. And while it largely backs what other scientists have predicted, “it’s more concerning because it’s even more believable,” says Nate McDowell, a forest expert and earth scientist with the Pacific Northwest National Laboratory, who was not part of the research team.

“These people, especially the lead authors, are known for being very, very careful,” McDowell says. “And they’re suggesting something very alarming: As we have warmed the planet, some areas of forest are approaching threshold-like behavior.

“The system is slowing down,” he says.

It is not, however, too late to change course. Saatchi’s team hopes this rigorous analysis will convince people that we are significantly altering these important landscapes. But they also hope their new tool will be used to help monitor further changes—and help direct resources toward recovery.

The National Geographic Society, committed to illuminating and protecting the wonder of our world, funded this work. Learn more about the Society’s support of Explorers researching and documenting our planet’s critical and diverse landscapes.

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