Tropical forests are the most biodiverse terrestrial ecosystems on the planet, as well as some of the biggest climate regulators. Now, a new study by an international team of researchers finds that the planet’s tropical forest canopies could be closer to crucial high-temperature thresholds than was previously believed.

For the study, the researchers used data from warming experiments conducted across Earth’s tropical forests, along with high-resolution thermal imaging data from an instrument located on the International Space Station, a press release from the UK’s University of Plymouth said.

The researchers discovered that some tropical leaves are beginning to reach, and sometimes even exceed, temperatures that stop them from functioning, and that as the climate crisis wears on, whole tree canopies could die.

“If we adopt a do-nothing response to climate change and tropical forest air temperatures increase by greater than 4C, there could be massive leaf death, possible tree mortality and species turnover across all tropical forests,” said Chris Doughty, a Northern Arizona University associate professor of ecoinformatics and the lead author of the study, as reported by The Guardian.

The research team said the findings of their study could have dire consequences since tropical forests are one of the main climate regulators and house almost all of the biodiversity on Earth, the press release said.

Data from the work of Dr. Sophie Fauset — an associate professor in terrestrial ecology at University of Plymouth and one of the authors of the study — on leaf temperatures in West Africa, Brazil, China and the UK showed that the temperatures of individual leaves could get as warm as 18 degrees Celsius above the air temperature in a montane forest in Brazil.

“Trees are a critical part of our planet’s response to climate change, and tropical forests play a key role in housing species diversity and regulating the planet’s climate. If they are damaged by increases in temperatures, we are losing a key line of defence and limiting nature’s ability to mitigate the impacts of human activity,” Fauset said. “Other research I have been involved in has looked at the sensitivity of tropical forests to climate, and shown that tree carbon stock is reduced at maximum temperatures above 32°C during the hottest part of the year. If we don’t do more to address climate change, the consequences could be severe.”

The study, “Tropical forests are approaching critical temperature thresholds,” was published in the journal Nature.

From their understanding of current leaf temperatures in tropical forests around the globe, the research team was able to model how the leaves might respond to ongoing human-caused global heating.

In order to estimate what proportion of leaves might come close to critical temperatures if air temperatures increase by two, three and four degrees Celsius, the researchers combined data on canopy top leaf warming from experiments in Puerto Rico, Brazil and Australia.

The results revealed that greater than one percent of the canopy’s leaves exceeded critical threshold temperatures — estimated to be 116.06 degrees Fahrenheit — a minimum of once each year.

“We should do all we can to avoid high-emissions scenarios. Under low-emissions scenarios, almost all tropical forest tree leaves can avoid death from overheating and the trees will survive,” said Simon Lewis, a global change science professor at University College London, as The Guardian reported.

The researchers also simulated how tropical forests would fare under future planetary warming scenarios and explored the impact of hotter temperatures and more frequent droughts.

The results showed that the plants’ ability to cool themselves with evaporation would be reduced, and that the hotter, drier temperatures would speed up leaf temperature increases and potential tree mortality.

“Given that tropical forests’ key role in housing species diversity and regulating the planet’s climate, insights into their future can build an understanding of the trajectory of the planet. But our model is not fate. It suggests that with some basic climate mitigation, we can address this issue, and helps pinpoint a few key areas that need further research,” Doughty said in the press release. “It also shows that by avoiding high-emissions pathways and deforestation, we can protect the fate of these critical realms of carbon, water, and biodiversity.”

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