At the current rate of warming, the Earth's ability to absorb nearly a third of man-made carbon emissions from plants could be halved within the next two decades, according to a new study in Advances in science by researchers from Northern Arizona University, Woodwell Climate Research Center, and the University of Waikato, New Zealand. Using more than two decades of data from measuring towers in every major biomass in the world, the team identified a critical temperature tipping point at which plants are able to capture and store atmospheric carbon – a cumulative effect known as the "land carbon sink" "- decreases with increasing temperatures.
The terrestrial biosphere – the activity of land plants and soil microbes – makes up a large part of the earth's "respiration" and exchanges carbon dioxide and oxygen. Ecosystems around the world attract carbon dioxide through photosynthesis and return it to the atmosphere through the respiration of microbes and plants. Over the past few decades, the biosphere has generally absorbed more carbon than it released, which is mitigating climate change.
However, as record temperatures continue to spread around the world, this may not continue. The researchers at NAU, Woodwell Climate and Waikato have identified a temperature threshold above which the carbon uptake of plants slows down and the carbon release accelerates.
Lead author Katharyn Duffy, a postdoctoral fellow at NAU, noted a sharp drop in photosynthesis above this temperature threshold in almost every biome around the world, even after other effects such as water and sunlight were removed.
"The earth has a growing fever, and just like the human body, we know that every biological process has a range of temperatures at which it functions optimally and over which it degrades," said Duffy. "So we wanted to ask how much plants can withstand?"
This study is the first to determine a temperature threshold for photosynthesis from observational data at a global level. While temperature thresholds for photosynthesis and respiration were examined in the laboratory, the Fluxnet data offer an insight into the actual experiences of the ecosystems on earth and their reactions.
"We know the temperature optimals for humans are 37 degrees Celsius, but we in the scientific community did not know what those optimals 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 answering this question: MacroMolecular Rate Theory (MMRT). Using MMRT, which is based on the principles of thermodynamics, the researchers were able to create temperature curves for every major biome and globe.
The results were alarming.
The researchers found that the temperature peaks for carbon uptake – 18 ° C for the more common C3 plants and 28 ° C for C4 plants – are already exceeded in nature, but saw no temperature control in breathing. This means that in many biomes, continued warming causes photosynthesis to decrease while breathing rate increases exponentially, disrupting the balance of ecosystems from the carbon sink to the carbon source and accelerating climate change.
"Different species of plants differ in the details of their temperature responses, but all show a decrease in photosynthesis when it gets too warm," said George Koch, co-author of the NAU.
Currently, less than 10 percent of the terrestrial biosphere is exposed to temperatures above this photosynthetic maximum. However, at the current rate of emissions, temperatures above this productivity threshold could be up to half of the terrestrial biosphere by mid-century – and some of the world's most carbon-rich biomes, including tropical rainforests in the Amazon and Southeast Asia, will be among the first to see the taiga in Russia and Canada who reach this turning point.
"The most striking thing our analysis showed is that the temperature optima for photosynthesis in all ecosystems were so low," said Vic Arcus, biologist at the University of Waikato and co-author of the study. "Combined with the increased respiration rate of the ecosystem at the observed temperatures, our results suggest that a temperature rise above 18 ° C may affect the terrestrial carbon sink. Without curbing warming to stay at or below the levels set in the Parisian climate the rural carbon sink will no longer offset our emissions and buy us time. "
Funding for this research was provided by the National Aeronautics and Space Agency (grant NNX12AK12G), the National Science Foundation's East Asian Summer Institute (NSF) grant (1614404), the Royal Society of New Zealand's Foreign Partnership Program (EAP-UOW1601), and the New Zealand Marsden Fund (Grant 16-UOW-027). This work used vortex covariance data collected and shared by the FLUXNET community including AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NECC , OzFlux Siberia and USCCC networks.