Plants are better at taking up carbon than we thought—but they may not be as good at holding on to it.
“If we want to limit climate change and make policies that are going to keep us under a certain level of global warming, then we need to understand how the natural system is responding and how it’s serving as a carbon sink.”
Current climate models underestimate how much carbon plants sequester but overestimate how long that carbon stays locked away, researchers report in Science. The findings could inform future strategies to store and manage atmospheric carbon dioxide (CO2).
“If we want to limit climate change and make policies that are going to keep us under a certain level of global warming, then we need to understand how the natural system is responding and how it’s serving as a carbon sink,” said study lead author Heather Graven, an atmospheric scientist at Imperial College London.
Following Carbon-14
The team developed a global carbon budget using carbon isotopes from nuclear weapons detonated in the 1950s and 1960s. The bombs introduced elevated levels of radioactive carbon-14 into the atmosphere. That spike allowed researchers to track the movement of carbon-14 through Earth and its atmosphere, including into plants when they convert CO2 into sugars.
A carbon-14 atom has eight neutrons, rather than the usual six, inside its nucleus, alongside six protons. Carbon-14 forms when neutrons produced by cosmic rays—or nuclear weapons—interact with nitrogen in the atmosphere.
Graven and her colleagues analyzed carbon-14 uptake in plants around the world between 1963 and 1967. During that period, Earth’s biosphere contained more carbon-14 than usual because of nuclear weapons testing. But because there were no significant detonations in the mid-1960s, the concentration of carbon-14 in the global biosphere remained relatively constant.
The team first pored through old data to determine how carbon-14 moved through the biosphere. Carbon-14 initially formed in the stratosphere, and data from the mid-1960s revealed how much of it left the stratosphere in the following years. That carbon-14 shifted into other parts of the biosphere—land, oceans, and the troposphere.
The researchers calculated how much carbon-14 passed into the troposphere and used computer simulations to estimate how much was taken up by the oceans. The rest, they figured, must have gone into the terrestrial biosphere. On land, plants usually snap up carbon first through photosynthesis. That carbon later gets transported to soil or back into the atmosphere when the plant dies and microorganisms start to break it down.
Between 1963 and 1967, most of the terrestrial carbon-14 was tied up in vegetation, before it had time to move into soil or decaying plant litter.
Carbon Budget Overhaul
After crunching the numbers, the team determined that the amount of carbon taken up by plants during the study period was higher than current models estimate. The same likely holds true today: Graven and her coauthors estimated that plants today should be taking up 80 billion metric tons of carbon each year, whereas current models predict between only 43 billion and 76 billion metric tons per year.
The findings hint at disparities in outgoing carbon, too. Satellite data and computer simulations give researchers a sense of how much carbon is currently stored in vegetation. Knowing how much carbon plants are taking in per year and how much is stored in existing plants, the team determined that plants are releasing carbon at higher rates than expected. “If there’s more going in, then there has to be more coming out,” said Graven. “So the amount of time that the carbon is in the vegetation is shorter.”
“We can’t rely on the biosphere to mop up our CO2. We need to stop emitting it if we want to reduce or limit climate change.”
High carbon turnover could potentially affect the role of land in future carbon storage, said César Terrer, an Earth systems scientist at the Massachusetts Institute of Technology who was not involved in the new research. Rising temperatures and natural disasters could affect plants’ ability to take up carbon.
Low uptake and high turnover might one day tip the scales, turning vegetation into a net source of atmospheric carbon rather than a sink, Terrer said.
The results suggest that locking away atmospheric carbon in plants might not pack as much of a punch as previously thought. “This idea of carbon dioxide removal into terrestrial vegetation is limited. So, really, just trying to return the focus to reducing fossil fuel emissions as soon as possible is really the key to limiting climate change,” Graven said.
“We can’t rely on the biosphere to mop up our CO2,” Graven said. “We need to stop emitting it if we want to reduce or limit climate change.”
—Skyler Ware (@skylerdware), Science Writer
Citation: Ware, S. (2024), Carbon cycles through plants more quickly than expected, Eos, 105, https://doi.org/10.1029/2024EO240306. Published on 22 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
Related