A tiny proportion of leaves in tropical rainforests are getting so hot they are breaching the threshold at which they can photosynthesise, according to a new study.
Around 0.01% of all leaves currently surpass this critical temperature at least once a year, the study – published today in peer-reviewed journal Nature – found.
Photosynthesis is the process by which plants convert carbon dioxide to energy and is essential to the maintenance of life on Earth.
As carbon dioxide is a climate-heating gas, and oxygen is released in exchange, photosynthesis also helps to slow climate change.
Although the numbers involved are small, one of the report’s authors Dr Joshua Fisher from Chapman University in the US, said the findings are the “canary in the coal mine”.
“And you want to be able to detect something happening before it’s widespread,” he said.
Tropical forests serve as critical carbon stores, host most of the world’s biodiversity and may be particularly sensitive to increasing temperatures. The critical temperature beyond which the photosynthetic process in tropical trees begins to fail is about 46.7C on average.
The scientists analysed satellite readings and verified them with some measurements on the ground in some locations, by sending up equipment to the tree canopy.
But they said the increase would be non-linear, and therefore “our ability to see that first bit of information is really important”.
The paper projects the number of leaves affected would increase to 1.4% if the planet warmed by around 4C, though current policies put Earth on track for around 2.4C to 2.7C of warming by 2100.
Dr Leslie Mabon, environmental systems lecturer from the Open University, who was not involved with the study, said although the proportion of leaves affected seems small, the study is “another indication that as climate change intensifies, we risk upsetting the natural environments on which we rely.
“This is why it is so important that we do everything we can to avoid the uncertainties associated with higher levels of global heating, by reducing the burning of fossil fuels and preventing deforestation.”
Dr Chloe Brimicombe, climate scientist and extreme heat researcher at the University of Graz, who was not involved with the study, said the model forest dynamics are “much more complex” than just the temperature.
“Other studies show that heat and drought negatively affects whole ecosystems including a forest and the health of trees,” she said.
“This is another nature study in as many weeks that is making claims about temperature related tipping points.
“But, you no longer need to read a scientific paper to see climate change is happening, it’s frequently visible outside your window.”