Photograph by Keith Ladzinski, Nat Geo Image Collection
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Forests like the Hall of Mosses in Washington's Olympic National Park are critical carbon sinks, regions that soak up more carbon than they emit.

Photograph by Keith Ladzinski, Nat Geo Image Collection

Plants help absorb our carbon, but for how much longer?

Since the industrial era, plants have been binging on carbon, but scientists worry they might soon have their fill.

All the shrubs, vines, and trees that surround you play a critical part in pulling excess carbon out of the atmosphere, and a new study argues that plants are, to date, helping absorb excess carbon emissions.

But at some point plants will get their fill of carbon, and the climate change helping hand they've extended will begin to recede. Exactly when that will happen is a question that scientists are racing to answer.

Since the Industrial Revolution began in the early 20th century, the amount of carbon in the atmosphere caused by human activity has rapidly increased. Using computer models, the study's authors concluded that photosynthesis has increased by 30 percent.

“It's kind of a silver lining in an otherwise stormy sky,” says Lucas Cernusak, a study author and ecoyphysiologist from James Cook University in Australia.

The study was published in the journal Trends in Plant Science.

How can they tell?

Cernusak and his colleagues used data from a 2017 Nature study that measured carbonyl sulfide found in ice cores and air samples. In addition to carbon dioxide, plants take in carbonyl sulfide during their natural carbon cycle, and that is frequently used to measure photosynthesis on a global scale.

“Terrestrial plants are removing about 29 percent of our emissions that would otherwise contribute to growth of the atmospheric CO2 concentration. What our model analysis showed is that the role of terrestrial photosynthesis in driving this land carbon sink is larger than estimated in most other models,” says Cernusak.

The carbon sink refers to the amount of carbon taken in by plants versus the amount they might naturally emit through deforestation or respiration.

Some scientists are less confident about using carbonyl sulfide as a method to measure photosynthesis.

Kerrie Sendall is a biologist from Georgia Southern University who studies how plants grow under various climate change scenarios.

Because plant absorption of carbonyl sulfide can vary by the amount of light they get, Sendall says the study's estimates “could be overestimated,” but she notes that most methods of measuring global photosynthesis have a degree of uncertainty.

Greener and leafier

Regardless of the rate at which photosynthesis has increased, scientists agree that excess carbon is acting like a fertilizer for plants, boosting their growth.

“There's evidence that trees are leafier, and that there's more wood,” says Cernusak. “The wood is really where more most of the carbon is absorbed in the mass of the plant.”

Scientists at the Oak Ride National Laboratory have observed that when plants are exposed to increasing levels of CO2, the size of pores on a leaf increase.

In Sendall's own experimental research, she exposed plants to double the amount of carbon dioxide they were used to.

Under those drastically increased CO2 conditions, “The makeup of their leaf tissues is a little different,” she says. “It makes it tougher for herbivores to eat and harder for larvae to grow on.”

Tipping point

The levels of atmospheric CO2 are rising and it’s assumed that eventually, plants won’t be able to keep up.

“The response of the land carbon sink to increasing atmospheric CO2 remains the largest uncertainty in global carbon cycle modeling to date, and this is a huge contributor to uncertainty in climate change projections,” the Oak Ride National Laboratory notes on their website.

Clearing land for ranching or agriculture and fossil fuel emissions are the biggest influences on the carbon cycle. Without dialing those two things way back, scientists say a tipping point is inevitable.

“More of the CO2 we emit will stay in the air, CO2 concentrations will rise quickly, and climate change will occur more rapidly,” says Danielle Way, an ecophysiologist from Western University.

What can we do?

Scientists from the University of Illinois and the Department of Agriculture have been experimenting with ways to genetically modify plants to store up even more carbon. An enzyme called rubisco is responsible for capturing CO2 for photosynthesis, and scientists want to make it more efficient.

Recent tests of modified crops have shown that beefing up rubisco increases yields by about 40 percent, but using the modified plant enzyme on a large commercial scale could take more than a decade to implement. So far tests have only been done on common crops like tobacco, and it's unclear how rubisco would alter trees, which capture the most carbon.
In September of 2018 environmental groups met in San Francisco to devise a plan to save forests, a natural asset they say is the “forgotten climate solution.”

“I think policy makers should respond to our findings by acknowledging that the terrestrial biosphere is functioning for the moment as an efficient carbon sink,” says James Cook University’s Cernusak. “Take immediate measures to protect forests so that they can continue functioning in this way, and get to work immediately to de-carbonize our energy production.”