The Earth’s ability to absorb nearly a third of man – made carbon emissions through plants could be halved within the next two decades at the current warming rate, according to a new study in Science Progress 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 every major biome around the globe, the team identified a critical temperature point, beyond which the plants’ ability to collect and store atmospheric carbon – a cumulative effect called the “land carbon wash” – Falling , when the temperature continues to rise.
The terrestrial biosphere – the activity of terrestrial plants and soil microbes ̵
But as record temperatures continue to spread across the globe, this may not continue; NAU, Woodwell Climate and Waikato researchers have discovered a temperature limit beyond which plant uptake of carbon is lowered and carbon release accelerates.
Main author Katharyn Duffy, a postdoctoral researcher at NAU, noted sharp declines in photosynthesis above this temperature limit in almost all biomes across the globe, even after removing other effects such as water and sunlight.
“The earth has a constantly growing fever, and like the human body, we know that every biological process has a range of temperatures at which it functions optimally and those above which function deteriorates,” Duffy said. “So we would ask, how much can plants withstand?”
This study is the first to record a temperature limit for photosynthesis from global-scale observational data. While temperature limits for photosynthesis and respiration have been studied in the laboratory, Fluxnet data provides a window into what ecosystems around the Earth are actually experiencing and how they respond.
“We know that the temperature optima for humans is around 37 degrees Celsius (98 degrees Fahrenheit), but we in the scientific community did not know what these optima were for the terrestrial biosphere,” Duffy said.
She collaborated with researchers at Woodwell Climate and the University of Waikato who recently developed a new approach to answering this question: MacroMolecular Rate Theory (MMRT). Based on the principles of thermodynamics, MMRT allowed scientists 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 more widespread C3 plants and 28 degrees C for C4 plants – are already exceeded in nature, but then no temperature control of respiration. This means that in many biomes, continued warming will cause photosynthesis to decrease, while respiration rates increase exponentially, overturning the balance between ecosystems from carbon decline to carbon source and accelerating climate change.
“Different types of plants vary in the details of their temperature response, but all show decreases in photosynthesis when it gets too hot,” said NAU co-author. George Koch.
Right now, less than 10 percent of the terrestrial biosphere is experiencing temperatures beyond this photosynthetic maximum. But at current emission rates, up to half of the terrestrial biosphere could experience temperatures above this mid-century productivity limit – and some of the most carbon-rich biomes in the world, including tropical rainforests in the Amazon and Southeast Asia and Taiga in Russia and Canada will be among the first to hit this turning point.
“The most striking thing our analysis showed is that the temperature optimum for photosynthesis in all ecosystems was 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 across the temperatures we observed, our findings suggest that any temperature rise above 18 degrees C is potentially detrimental to the terrestrial carbon sink. Without curbing warming to remain at or below the levels set out in the Paris Climate Agreement, carbon washing will not continue to compensate for our emissions and buy us time. ”
Graphics by Victor O. Leshyk, Center for Ecosystem Science and Society
Kate Petersen | Center for Ecosystem Science and Society