Extreme heat could make photosynthesis impossible for tropical trees and plants that may all die as a result, a new study found.

Visitors walk through the world heritage listed daintree rainforest on November 14, 2012 in Mossman Gorge, Australia.

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• Trees stop making food for themselves when they get too hot, a new study shows.
• If tropical air temperatures reach 116 degrees Fahrenheit, a lot of the rainforest could die. 
• This is the first study to narrow in on a threshold that we need to avoid. 

When trees get hot, their leaves begin to sweat. If they stay hot for too long, they deplete their water supply, exhausting themselves.

It's then that photosynthesis, the backbone of plant life, breaks down. 

The plant stops being able to care for itself, and begins to die, scientist Gregory Goldsmith, an assistant professor of biology at Chapman University, said in a press briefing.

So what happens in a world that by all signs, will continue to get warmer?

For a new Nature study, scientists across the country found that photosynthesis begins to fail in tropical trees at 116 degrees Fahrenheit (46.7 degrees Celsius). 

The researchers also found that a small percentage of leaves — .01% — have already surpassed this limit at least once per season.

If the world continues to grow warmer, massive amounts of the tropical canopy could die off. However, in the paper, researchers state that "it is still within our power to decide the fate of these critical realms of carbon, water, and biodiversity.

What we didn't know

Chloroplasts carry chlorophyll which makes them green. These chloroplasts actually circulate around within each cell.

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Scientists already knew extreme heat makes leaves unable to photosynthesize, said Goldsmith, a co-author of the study. But "this study is really the first study to establish how close tropical forest canopies may be to these limits," he said. 

To determine the threshold for what too hot really means, the researchers used data from climate monitoring satellites, temperature towers in tropical forests, and countless sensors they taped onto individual leaves in the canopy.

They studied five forests in Brazil, Puerto Rico, Panama, and Australia, ScienceAlert reported.

These three perspectives combined to give them an idea of when the leaves, which are harbingers of health for the rest of the tree, begin to malfunction.

A Cruise self-driving car in the middle of traffic in San Francisco.

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Without change, we're headed toward disaster

The nutrients a tree needs to sustain itself are created when it photosynthesizes. So if photosynthesis is interrupted, a plant will essentially starve to death. 

Since plants take in carbon dioxide when they photosynthesize, they can help moderate the amount of greenhouse gases in the air.

But if a plant dies, all the carbon it's stored in its body gets released into the atmosphere. 

Tropical forests make up about 12% of the Earth's land surface. So if this heat threshold is reached, that whole chunk of Earth's surface might begin to die off, releasing greenhouse gases into the air with it, said Christopher Doughty, a professor of ecoinformatics at Northern Arizona University. 

If all the trees in the tropical rainforests died off, that would release an estimated 228.7 petagrams of carbon into the atmosphere, according to a 2012 study.

One petagram of carbon is equal to 1 Gigaton (or a billion metric tons), so 228 petagrams are equivalent to six times the amount of carbon emissions emitted worldwide in 2022, alone.

"If that all went into the atmosphere, that would accelerate climate change," Doughty said. He was the one up in the trees, taping each sensor delicately to the tropical canopy.

Though the researchers found that .01% of the leaves in the tropical rainforest have reached their heat-induced limit, there is room for error in the measurements, Doughty cautioned.

They arrived at their measurements based on the data they were able to gather from key areas globally, but they weren't able to actually measure all the tropical leaf temperatures across the globe.

A photo of the canopy of the Amazon rainforest.

Ignacio Palacios/Getty Images

They estimated them by combining their ground data with satellite data, but there might be slight deviations from their estimation. 

Even so, if no action is taken to prevent further climate change, Doughty said it's possible their predictions will come true.

But making even moderate changes, like enforcing our current climate agreements and reducing tropical deforestation, would help. 

These sorts of changes could give us a good chance of avoiding what would be a tragic loss of some of Earth's most biologically-rich ecosystem. This makes Doughty hopeful.

"I feel optimistic," he said.

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