We’ve known for decades that tropical rainforests are special. They’re nearly unrivaled in biodiversity, and research has shown that they absorb more carbon dioxide than any other ecosystem. A recent study showed that the tropics sequester four times as much carbon dioxide as temperate and boreal ecosystems combined—and several studies have estimated that all terrestrial ecosystems combined sequester as much as 30 percent of the total carbon dioxide in the atmosphere each year.

We’ve also known for decades that these ecosystems are at risk of vanishing. As much as 20 percent of tropical rainforests have been cleared in the last 30 years, with an additional 10 percent lost to degradation. Beyond these direct threats, forests worldwide, and especially rainforests, are experiencing severe losses due to climate change—notably higher temperatures and drought.

Until now, there haven’t been means to systematically keep tabs on the health of these critical ecosystems. But a collaboration of nearly 50 institutions has recently developed a comprehensive index to measure the health and vulnerability of all tropical rainforests around the world. The result is a potential warning system that allows scientists and policymakers to monitor and prioritize which forests are at the highest risk of irreversible damage and loss.

“Rainforests regulate the Earth’s climate; if they cannot function well—the patterns of climate will change almost everywhere on Earth,” says lead author Sassan Saatchi of the Jet Propulsion Laboratory at the California Institute of Technology and the Institute of Environment and Sustainability at the University of California, Los Angeles. “If we lose the ability of the tropical forests to function normally, for example absorb the carbon from the atmosphere, it may mean that all of the efforts we’re doing in terms of climate change mitigation may become moot.”

Preventing tipping points

This new tropical forest vulnerability index (TFVI) combines close to a dozen data sets spanning up to 37 years of measurements (1982–2018). Past efforts have focused on limited regions or have largely relied on fieldwork, but the TFVI brings together a wide range of available measurements and models of rainforest stresses and responses.

Stress measurements included climate data about temperature, the dryness of the air (vapor pressure deficit), and the amount of water entering and leaving each system (the water balance). Responses included tree cover, carbon storage, above-ground biomass, productivity, and evapotranspiration—the amount of water that each system exchanges with the atmosphere during photosynthesis. Available measurements also included an existing biodiversity index and the temperature of the land surface or, in forests, the temperature of the tree and leaf surfaces. In addition, models and satellite data were calibrated with ground readings as much as possible.

When combined, these measurements give a historical record of the overall health and functioning of rainforests around the world over the past several decades. From this, it’s possible to see how much stress different forests have survived in the past and to flag current and future stress readings that are far outside of past variations. The researchers suggest that too many stresses may create feedback loops that lead to tipping points, beyond which a forest will no longer recover, and the damage will become irreversible. Such tipping points can be rapid and cause mass tree deaths, or they can trigger relatively gradual transitions to a different ecosystem type, like a savanna.

Global rainforest health check

Based on current measurements, the index shows that forests in the Amazon are particularly vulnerable, while African forests in the Congo Basin are more resilient. The authors suspect this may be due to less development in the Congo Basin, as well as a history of more frequent droughts there. Forests in Asia are particularly threatened by land use and forest fragmentation.

The intention of the index is to help the early identification of forests that are in the most need of additional protections, as well as to provide specific guidance on exactly which stress factors the forests are experiencing. While some interventions, like slowing climate change, may require longer-term solutions, others—like forest fragmentation—could be managed with restoration projects.

The authors were also careful to include measures of uncertainty throughout the index, and the code for the index is freely accessible so that anyone can use it and, hopefully, improve upon it as well. Every month, the index is updated with the latest data, and the team expects to launch an online version in the next year.

“This index is not the ultimate answer. One of the biggest caveats is that, as much as we know about the ecosystem, we are always surprised how the ecosystem works, and it can still become vulnerable to something that we don’t know about, or it may become resilient to something that we thought it has been vulnerable to,” says Saatchi. “But this is a work in progress, and the more knowledge we gain in terms of the function of these ecosystems, the better we can predict which direction they’ll go. The index will help us to continuously take the pulse of global rainforests as their health is changing.”

Science Advances, 2021. DOI: 10.1126/sciadv.abe9829

K.E.D. Coan is a freelance journalist covering climate and environment stories at Ars Technica. She has a PhD in chemistry and chemical biology.