Forests are hitting their limit for carbon intake

January 21, 2021

In the near future, forests may not be able to sequester as much carbon as previously. (Unsplash/Sebastian Unrau)

Plants’ ability to keep absorbing close to one-third of human-caused carbon emissions could be slashed in half by 2040, as forests and other land ecosystems start releasing more carbon than they store, according to the first study to identify a photosynthesis “temperature tipping point” based on on-site data from around the world.

Beyond that point, plants will be less able to capture and store atmospheric carbon as the world warms, according to the study published last week in Science Advances, effectively transforming the “land carbon sink” that’s important to controlling climate change.

Among the first ecosystems to hit the tipping point will be carbon-rich tropical rainforests in the Amazon and Southeast Asia, as well as cold Taiga forests in Russia and Canada, according to the study.

“This is not fully baked-in yet,” lead author Katharyn Duffy, a postdoctoral researcher at Northern Arizona University’s School of Informatics, Computing and Cyber Systems, said in an interview with The Academic Times. “Even though the tipping point is in the near future, we can act now and avoid drastic altering of the global carbon cycle from the biosphere perspective.”

Using a new “MacroMolecular Rate Theory” based in thermodynamic principles to create temperature curves for each of the planet’s major biomes, Duffy’s team determined that less than 10% of the terrestrial biosphere, or land plants and soil microbes, have already reached the photosynthesis tipping point — but there is no such threshold for carbon release, which takes place through respiration. 

The global photosynthesis peaks are 64.4 degrees Fahrenheit for many common plants, known as C3 plants because the first carbon compounds they produce contain three carbon atoms, and 82.4 degrees Fahrenheit for C4 plants, which have adapted to reduce photorespiration.

“At higher temperatures, respiration rates continue to rise, in contrast to sharply declining rates of photosynthesis,” reads the study by authors from NAU, Woods Hole Research Center in Massachusetts and the University of Waikato and Manaaki Whenua − Landcare Research in New Zealand. “Under business-as-usual emissions, this divergence elicits a near-halving of the land sink strength by as early as 2040.”

The researchers examined temperatures from the global FLUXNET meteorological sensor network from the last two decades, over which the biosphere has generally helped to mitigate climate change by absorbing more carbon than it put back out.

“While biomes will eventually shift spatially in response to warming, this process is unlikely to be a smooth migration, but rather a rapid disturbance-driven loss of present biomes (with additional emissions of carbon to the atmosphere), followed by a slower establishment of biomes more suited to the emerging climate,” they wrote. 

“This further suggests that we are rapidly entering temperature regimes where biosphere productivity will precipitously decline,” they continued, “and calls into question the future viability of the land sink.”

It also could spell trouble for the Paris Agreement, in which countries’ Intended Nationally Determined Contributions lean heavily on land carbon uptake. Nearly all nations are party to the accord, which President Joe Biden moved to rejoin during his first hours in office Wednesday, after the U.S. formally exited under his predecessor Donald Trump in November. The treaty seeks to limit global temperature rise to no more than 34.7 degrees Fahrenheit and well below 35.6 degrees Fahrenheit, as compared with pre-industrial levels.

If emissions going forward are significantly lowered through proactive global action, most land carbon uptake could be preserved, the study notes.

But in a surprising finding, the researchers also saw that the mean temperature of the warmest quarter annually has already warmed 35.24 degrees Fahrenheit since the FLUXNET network began about a quarter-century ago, exceeding “thermal maxima” in the last decade and surpassing total warming, including oceans. 

“What that means is that any additional warming is going to push us into more and more time past that peak for photosynthesis,” Duffy explained.

So far, plants don’t appear to be acclimating to the change, and they’re unlikely to do so to a significant extent, the researchers said.

Duffy now aspires to figure out what the temperature curve means for these areas’ long-term health.

“For how many months of the year can an ecosystem be in photosynthetic decline, or at temperatures where we believe photosynthesis has quote unquote stopped?” she said. “Here we're not talking about photosynthesis but we’re talking about ecosystem collapse, potentially. And that is work that we absolutely need to pursue in order to protect and be proactive about ecosystem management and things like that.”

In the meantime, Duffy said, understanding temperature dependence could help highlight areas such as the Southwest that may be in more urgent need of restoration. There, 100 years of wildfire suppression has turned forests into densely packed “doghair thickets,” a stressful, structure-altering condition that could result in losses from fires and climate change alike. 

“We need to be considering future temperatures that we are likely to experience,” Duffy said, “and how those temperatures may affect growth and yield of, say, stands for timber.”

The study “How close are we to the temperature tipping point of the terrestrial biosphere?,” published Jan. 13 in Science Advances, was authored by Katharyn A. Duffy, Northern Arizona University; Christopher R. Schwalm, Northern Arizona University and Woods Hole Research Center; Vickery L. Arcus, University of Waikato; George W. Koch, Northern Arizona University; Liyin L. Liang, University of Waikato and Manaaki Whenua − Landcare Research; and Louis A. Schipper, University of Waikato.

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