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Earth’s ability to absorb nearly one-third of man-made carbon emissions from plants could be halved over the next two decades at the current rate of warming, new study found Science Advances by researchers at Northern Arizona University, Woodwell Climate Research Center and University of Waikato, New Zealand. Using more than two decades of data from measuring towers in all of the world’s major biomes, the team identified a critical temperature tipping point beyond which the ability of plants to capture and store atmospheric carbon – a cumulative effect called a “terrestrial carbon sink” —Decreasing as temperatures continue to rise.
The terrestrial biosphere – the activity of terrestrial plants and soil microbes – does a large part of the “respiration” of the Earth, exchanging carbon dioxide and oxygen. Ecosystems around the world take up carbon dioxide through photosynthesis and release it into the atmosphere through the respiration of microbes and plants. In recent decades, the biosphere has generally absorbed more carbon than it released, which mitigates climate change.
But as record high temperatures continue to spread across the world, this may not continue; researchers from the NAU, Woodwell Climate and Waikato have detected a temperature threshold beyond which the uptake of carbon by plants slows down and the release of carbon accelerates.
Principal author Katharyn duffy, a postdoctoral researcher at NAU, noticed sharp drops in photosynthesis above this temperature threshold in almost every biome around the world, even after removing other effects such as water and sunlight .
“The Earth has an ever-increasing fever, and just like the human body, we know that every biological process has a range of temperatures at which it functions optimally, and above which function deteriorates,” said Duffy. “So, we wanted to ask, how much can plants resist?”
This study is the first to detect a temperature threshold for photosynthesis from observational data on a global scale. While temperature thresholds for photosynthesis and respiration have been studied in the laboratory, Fluxnet data provides a window into what ecosystems on Earth actually experience and how they react.
“We know the temperature optima for humans are around 37 degrees Celsius (98 degrees Fahrenheit), but we in the scientific community didn’t know what those optima were for the terrestrial biosphere,” said Duffy.
She teamed up with researchers from Woodwell Climate and the University of Waikato who recently developed a new approach to answer this question: the theory of macro-molecular rates (MMRT). Based on the principles of thermodynamics, the MMRT allowed researchers to generate temperature curves for each major biome and the globe.
The results were alarming.
The researchers found that the temperature “peaks” for carbon uptake – 18 degrees C for the most common C3 plants and 28 degrees C for C4 plants – are already exceeded in nature, but found no temperature control on breathing. This means that in many biomes, continued warming will cause photosynthesis to decline as respiration rates increase exponentially, shifting the balance of ecosystems from carbon sink to carbon source and accelerating climate change.
“Different types of plants vary in the details of their responses to temperature, but all show a drop in photosynthesis when it’s too hot,” said NAU co-author George koch.
Currently, less than 10 percent of the Earth’s biosphere experiences temperatures above this photosynthetic maximum. But at the current rate of emissions, up to half of the terrestrial biosphere could experience temperatures above this productivity threshold by mid-century – and some of the most carbon-rich biomes in the world, including the tropical rainforests of Amazonia and Southeast Asia and the taiga in Russia and Canada will be among the first to reach this tipping point.
“The most striking thing our analysis showed is that the temperature optima for photosynthesis in all ecosystems were so low,” said Vic Arcus, a biologist at the University of Waikato and co-author of the study. “Combined with the increased rate of ecosystem respiration through the temperatures we observed, our results suggest that any increase in temperature above 18 degrees C is potentially detrimental to the terrestrial carbon sink. Without curbing warming to stay at or below the levels set in the Paris Climate Agreement, the terrestrial carbon sink will not continue to offset our emissions and save us time.
Graphic by Victor O. Leshyk, Center for Ecosystem Science and Society
Kate Petersen | Center for Ecosystem Science and Society
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