Trees are struggling to trap carbon dioxide in warmer, drier climates, meaning that they may no longer serve as a solution for offsetting humanity's carbon footprint as global warming accelerates, a study by Penn State university found.

"We found that trees in warmer, drier climates are essentially coughing instead of breathing," said Max Lloyd, assistant research professor of geosciences at Penn State and lead author of the study recently published in Proceedings of the National Academy of Sciences. "They are sending CO2 right back into the atmosphere far more than trees in cooler, wetter conditions."

Trees typically remove CO2 from the atmosphere through photosynthesis, yet under stressful conditions, they release CO2 back into the atmosphere via a process called “photorespiration.”

The researchers demonstrated that photorespiration rates are up to twice as high in warmer climates, particularly when water is scarce. This pattern starts to emerge when average daytime temperatures exceed 20C and gets worse as temperatures rise further.

Last year was the hottest year on record and global warming is accelerating as the climate models are wrong, underestimating the rate at which temperatures are rising.

"Plants and climate are inextricably linked," Lloyd said as cited by Science Daily. "The biggest drawdown of CO2 from our atmosphere is photosynthesizing organisms. It's a big knob on the composition of the atmosphere, so that means small changes have a large impact."

Plants currently absorb an estimated 25% of the CO2 emitted by human activities each year, according to the US Department of Energy. However, as the climate warms and water becomes scarcer, this proportion is expected to decline. "The world will be getting warmer, which means plants will be less able to draw down that CO2," Lloyd added.

As part of the changes caused by the climate crisis, last year the Amazon basin suffered from a severe drought while the whole of South America suffered from record heat waves. The Amazon rainforest absorbs a quarter of all the CO2 absorbed by all the land on Earth. The amount absorbed today, however, is already 30% less than it was in the 1990s because of deforestation and now the forest is becoming stressed that share is likely to fall further.

Previously, studying photorespiration was nearly impossible at scale due to the reliance on real-time measurements in living plants or well-preserved specimens. The Penn study came up with a new method using isotopes that offers researchers a tool for predicting how well trees might "breathe" in the future and how they coped with different climates in the past.