Some leaves in tropical forests from South America to Southeast Asia have been so affected by the heat that they may no longer be able to photosynthesize, with major consequences for the world’s forests, according to a new study in Nature, published by CNN.

The ability of leaves to photosynthesize, the process by which they produce energy from carbon dioxide, sunlight and water, is not achieved when their temperature reach about 46.7 degrees Celsius.

Although the temperature limit seems high, the leaves can be much warmer than the air temperature, according to the report published on Wednesday in Nature by a group of scientists from countries such as the USA, Australia and Brazil.

The scientists used temperature data transmitted by thermal satellite sensors in the International Space Station, 400 kilometers (almost 250 miles) above Earth. They combined these with ground observations from leaf heating experiments.

Instead of looking at average temperatures, scientists looked at extremes, said Christopher Doughty, an associate professor of ecological informatics at Northern Arizona University and an author of the report. They found that average forest canopy temperatures peaked at 34 degrees Celsius, but some exceeded 40 degrees Celsius.

Currently, 0.01% of leaves pass the critical temperature threshold beyond which their ability to photosynthesize breaks down, according to the report, potentially killing the leaf and the tree.

That percentage, though small, is set to increase as the world warms, the report said, threatening the world’s rainforests, which cover about 12 percent of the planet and are home to more than half the world’s species. They also provide a vital role in absorbing and storing carbon and help regulate the global climate.

Rainforests can withstand about 4 degrees Celsius of global warming before they reach a tipping point in their ability to photosynthesize, according to the report.

If warming exceeds that level, the amount of leaves that exceed critical temperature thresholds could increase to 1.4 percent, potentially causing large-scale leaf loss and the death of the entire tree, according to the report.

This level of warming is not expected under current climate policies, which are estimated to reach 2.7 degrees Celsius above pre-industrial levels. But it is within the range of the most pessimistic climate scenarios if the world continues to burn fossil fuels.

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While these figures may seem small, the risk is significant given how critical tropical trees are to life, the climate system and the planet.

“Almost all life, including humans, depends on photosynthesis for food either directly or indirectly,” said Kevin Collins, senior lecturer in environmental systems at the Open University, who was not involved in the research.

The findings show that global warming is jeopardizing this vital process, he told the Science Media Centre. However, he added, there are more immediate concerns for rainforests, including deforestation, fires and drought.

Christopher Steele, a forest ecosystem professor at Oregon State University who was also not involved in the report, said the research provided some new insights. While much emphasis has been placed on the impact of drought on tree loss, he told CNN, “this paper says it’s not just drought — we also need to be really concerned about the temperatures that the leaves are reaching.”

Others were more cautious. Chloe Brimicombe, a climatologist at the University of Graz in Austria, said that given how few leaves reach the critical temperature threshold and how high their heating needs to become before the tipping point, “this suggests in theory that rainforests are quite resilient to climate change”.

The report’s authors said that despite the uncertainties, the research gives important insights into how tropical forests will respond to climate change.