Will plants always absorb carbon?

Studies show that absolutely all the shrubs, vines and trees that surround us play an important role in absorbing excess carbon from the atmosphere. But at some point, plants can take on so much carbon that their helping hand in combating climate change begins to dwindle. When exactly will this happen? Scientists are trying to find an answer to this question.

Ever since the Industrial Revolution began in the early 20th century, the amount of carbon in the atmosphere caused by human activities has skyrocketed. Using computer models, the authors, published in Trends in Plant Science, found that at the same time, photosynthesis increased by 30%.

“It’s like a ray of light in a dark sky,” says Lukas Chernusak, study author and ecophysiologist at James Cook University in Australia.

How was it determined?

Chernusak and colleagues used data from environmental studies from 2017, which measured carbonyl sulfide found in ice cores and air samples. In addition to carbon dioxide, plants take up carbonyl sulfide during their natural carbon cycle and this is often used to measure photosynthesis on a global scale.

“Land plants absorb about 29% of our emissions, which would otherwise contribute to atmospheric CO2 concentrations. Analysis of our model showed that the role of terrestrial photosynthesis in driving this process of carbon sequestration is greater than most other models have suggested,” Chernusak says.

But some scientists aren’t so sure about using carbonyl sulfide as a method of measuring photosynthesis.

Kerry Sendall is a biologist at Georgia Southern University who studies how plants grow under different climate change scenarios.

Because carbonyl sulfide uptake by plants can vary depending on the amount of light they receive, Sendall says the results of the study “may be overestimated,” but she also notes that most methods for measuring global photosynthesis have some degree of uncertainty.

Greener and thicker

Regardless of how much photosynthesis has increased, scientists agree that the excess carbon acts as a fertilizer for plants, accelerating their growth.

“There is evidence that the foliage of the trees has become denser and the wood is denser,” Cernusak says.

Scientists from the Oak Ride National Laboratory also noted that when plants are exposed to increased levels of CO2, the pore size on the leaves increases.

Sendall, in her own experimental studies, exposed plants to twice the amount of carbon dioxide they normally receive. Under these conditions, according to Sendall’s observations, the composition of leaf tissues changed in such a way that it became more difficult for herbivores to eat them.

The tipping point

The level of CO2 in the atmosphere is rising, and it is expected that eventually the plants will not be able to cope with it.

“The response of a carbon sink to an increase in atmospheric CO2 remains the largest uncertainty in global carbon cycle modeling to date, and it is a major driver of uncertainty in climate change projections,” the Oak Ride National Laboratory notes on its website.

Land clearing for cultivation or agriculture and fossil fuel emissions have the biggest impact on the carbon cycle. Scientists are sure that if humanity does not stop doing this, a tipping point is inevitable.

“More carbon emissions will be trapped in the atmosphere, the concentration will increase rapidly, and at the same time, climate change will occur faster,” says Daniel Way, an ecophysiologist at Western University.

Что мы можем сделать?

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

Recent trials of modified crops have shown that upgrading the quality of rubisco increases yields by about 40%, but using the modified plant enzyme on a large commercial scale can take more than a decade. So far, tests have only been done on common crops like tobacco, and it’s not clear how rubisco will change the trees that sequester the most carbon.

In September 2018, environmental groups met in San Francisco to develop a plan to conserve forests, which they say is “the forgotten solution to climate change.”

“I think policy makers should respond to our findings by recognizing that the terrestrial biosphere currently functions as an efficient carbon sink,” Cernusak says. “The first thing to do is to take immediate action to protect forests so they can continue to sequester carbon and start working immediately to decarbonize the energy sector.”

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