Earth’s ability to absorb nearly a third of man-made carbon emissions from plants could be halved over the next two decades at the current rate of warming, according to a new study in Scientists progress by researchers from Northern Arizona University, the Woodwell Climate Research Center and the 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 “terrestrial carbon sink” – decreases 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 attract 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, thereby mitigating climate change.
But as record-breaking temperatures continue to spread across the world, this may not continue; researchers from 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.
Lead author Katharyn Duffy, postdoctoral researcher at NAU, noticed a sharp drop in photosynthesis above this temperature threshold in almost every biome in 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 said Duffy. “So, we wanted to ask, how much can plants take?”
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 Earth’s ecosystems are actually experiencing and how they react.
“We know that the temperature optima for humans are around 37 degrees Celsius (98 degrees Fahrenheit), but we in the scientific community did not 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: Macromolecular Rate Theory (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 have seen no temperature control on breathing. This means that in many biomes, continued warming will cause photosynthesis to decline while respiration rates increase exponentially, shifting the balance of ecosystems from carbon sink to carbon source and accelerating climate change.
“The different types of plants vary in the details of their responses to temperature, but all show a drop in photosynthesis when it is 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 Earth’s biosphere could experience temperatures exceeding this productivity threshold by mid-century – and some of the most carbon-rich biomes in the world, notably tropical rainforests. Amazon 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 optimum temperatures 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 increase in the ecosystem’s respiration rate 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 dampening warming for remain at or below the levels established in the Parisian climate. Granted, the terrestrial carbon sink will not continue to offset our emissions and save us time. “
Funding for this research was provided by the National Aeronautics and Space Administration (grant NNX12AK12G), the National Science Foundation (NSF) East-Asia Pacific Summer Institute Fellowship (1614404), the Royal Society of New Zealand Foreign Partnership Program (EAP- UOW1601) and the New Zealand Marsden Fund (grant 16-UOW-027). This work used eddy covariance data acquired and shared by the FLUXNET community, including AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NEC -TERN, TCOS -Siberia and USCCC networks.