The Earth’s ability to absorb nearly a third of human carbon emissions through plants could be halved within the next two decades at the current warming rate, according to a new study in Science Advances by researchers at Northern Arizona University, the Woodwell Climate Research Center and the University of Waikato, New Zealand. Using more than two decades of data from measurement towers in every major biome around the world, the team identified a critical tipping point for temperature within which plants’ ability to capture and store atmospheric carbon, a cumulative effect called the ‘land coal water basin’ – Decreases as temperatures continue to rise.
The terrestrial biosphere – the activity of terrestrial plants and soil microbes – carries out much of the earth’s “respiration”, exchanging carbon dioxide and oxygen. Ecosystems around the world absorb carbon dioxide through photosynthesis and release it back into the atmosphere by breathing microbes and plants. Over the past few decades, the biosphere has generally ingested more carbon than it released, mitigating climate change.
But as record temperatures continue to spread around the world, it cannot continue; the researchers from the NAU, Woodwell Climate and Waikato observed a temperature threshold within which the uptake of carbon into the plant slows down and the release of carbon accelerates.
Lead author Katharyn Duffy, a postdoctoral researcher at the NAU, has noticed sharp declines in photosynthesis above this temperature threshold in almost every biome around the world, even after other effects such as water and sunlight have been removed.
“The earth has a constantly growing fever, and just like the human body, we know that every biological process has a range of temperatures at which it performs optimally, and those over which function deteriorates,” Duffy said. “So we wanted to ask how much can plants withstand?”
This study is the first to detect a temperature threshold for photosynthesis from observation data on a global scale. While studying temperature thresholds for photosynthesis and respiration in the laboratory, the Fluxnet data provide a window into what ecosystems around the world experience and how they respond.
“We know that the temperature optima for humans is about 37 degrees Celsius (98 degrees Fahrenheit), but we in the scientific community did not know what the optima was for the terrestrial biosphere,” Duffy said.
She is collaborating with researchers from Woodwell Climate and the University of Waikato who recently developed a new approach to answering the question: MacroMolecular Rate Theory (MMRT). Using the principles of thermodynamics, MMRT enabled the researchers to generate temperature curves for each major biome and the entire world.
The results were worrying.
The researchers found that the temperature ‘peak’ for carbon uptake – 18 degrees C for the more widespread C3 plants and 28 degrees C for C4 plants – was already exceeded in nature, but did not see any temperature control on respiration. This means that continued warming in many biomes will slow down photosynthesis while increasing the rate of respiration exponentially, tilting the balance of ecosystems from carbon zinc to carbon source and accelerating climate change.
“The different types of plants differ in the details of their temperature responses, but all show decreases in photosynthesis when it gets too hot,” said NAU co-author. George Koch.
Currently, less than ten percent of terrestrial biosphere experiences temperatures outside this photosynthetic maximum. But at the current emission rate, up to half of the Earth’s biosphere could experience temperatures beyond that productivity threshold by the middle of the century – and some of the most carbon-rich biomes in the world, including tropical rainforests in the Amazon and Southeast Asia and the Taiga in Russia and Canada, will be one of the first to hit that tipping point.
“The most striking thing our analysis showed is that the temperature optima for photosynthesis in all ecosystems was so low,” said Vic Arcus, a biologist at the University of Waikato and co-author of the study. ‘In combination with the increased rate of respiration of the ecosystem over the temperatures we observed, our findings suggest that any temperature rise above 18 degrees C is potentially harmful to the terrestrial carbon basin. Without limiting warming to stay at or below the levels set in the Paris climate agreement, the country’s carbon footprint will not continue to compensate for our emissions and buy time for us. ”
Graph by Victor O. Leshyk, Center for Ecosystem Science and Society
Kate Petersen | Center for Ecosystem Science and Society