Yet another drawback caused by global warming has come to light the other day as a new study led by The Pennsylvania State University (Penn State) researchers revealed that trees are struggling to sequester heat-trapping carbon dioxide (CO2) in warmer, drier climates. This means trees may no longer serve as a solution for offsetting humanity’s carbon footprint as the planet continues to warm. “We found that trees in warmer, drier climates are essentially coughing instead of breathing,” said Max Lloyd, assistant professor of geosciences at Penn State and lead author on 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.”
Through the process of photosynthesis, trees remove CO2 from the atmosphere to produce new growth. Yet, under stressful conditions, trees release CO2 back to the atmosphere, a process called photorespiration. With an analysis of a global dataset of tree tissue, the research team demonstrated that the rate of photorespiration is up to two times higher in warmer climates, especially when water is limited. They found the threshold for this response in subtropical climates begins to be crossed when average daytime temperatures exceed roughly 20 degree Celsius and worsens as temperatures rise further.
“We have knocked this essential cycle off balance,” Lloyd said. “Plants and climate are inextricably linked. The biggest draw down of CO2 from our atmosphere is photosynthesising 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, but this percentage is likely to decrease in the future as the climate warms, Lloyd explained, especially if water is scarcer.
“When we think about climate futures, we predict that CO2 will go up, which in theory is good for plants because those are the molecules they breathe in,” Lloyd said. “But we’ve shown there will be a tradeoff that some prevailing models don’t account for. The world will be getting warmer, which means plants will be less able to draw down that CO2.”
The amount of carbon dioxide in the atmosphere is rapidly rising; it is already greater than at any time in the last 3.6mn years, according to the US National Oceanic and Atmospheric Administration’s 2020 data. But that period is relatively recent in geologic time, Lloyd explained. As the climate continues to change, this study underscores the need for a more nuanced approach to understanding the complex relationship between plants and the atmosphere, guiding future efforts towards more effective climate action. Other authors on the paper are Rebekah A Stein, Daniel A Stolper, Daniel E Ibarra and Todd E Dawson of the University of California, Berkeley; Richard S Barclay and Scott L Wing of the Smithsonian National Museum of Natural History and David W Stahle of the University of Arkansas. The work was funded in part by the Agouron Institute, the Heising-Simons Foundation, and the US National Science Foundation.