Tech by VICE

Climate Models Overestimate How Much CO2 Plants Can Remove From the Atmosphere

Carbon dioxide levels might increase even more rapidly than expected based on new research on the molecular mechanisms of photosynthesis.

by Daniel Oberhaus
Oct 3 2018, 1:00pm

Image: Shutterstock

Scientists have been overestimating plant life’s ability to suck up carbon dioxide from the atmosphere, new research suggests, indicating that the threat posed by climate change is even worse than we thought.

The Earth is unfathomably complex system and creating an accurate model of its climate requires scientists to model hundreds of subsystems and the ways these systems interact—everything from macro-level atmospheric and oceanic changes to the rates at which plants recycle carbon dioxide.

According to recent research on photosynthesis conducted by scientists from seven different universities, however, climate models until now may have been overestimating how much CO2 plants take in during the process of producing energy.. This suggests that atmospheric carbon dioxide levels may increase even faster than expected and accelerate global warming.

“The takeaway is that plants’ ability to help us control atmospheric carbon dioxide levels is weaker than we thought,” Tom Sharkey, a biochemist at Michigan State University’s Plant Research Laboratory and one of the study’s authors, said in a statement.

There are many different climate models, each with its own strengths and weaknesses depending on the granularity of the data. Since the mid-90s, the World Climate Research Program has hosted the Coupled Model Intercomparison Project (CMIP), a framework that allows different climate models to be compared. Despite the differences between the climate models used for CMIP, all of them have a large range of uncertainty about the exchange of carbon dioxide between plants and the atmosphere. This makes it difficult to create accurate climate models and understand how things like deforestation affect climate change.

Photosynthesis is able to sequester about one-third of all human CO2 emissions, but the way this process is modeled has led to “considerable uncertainty” about the CO2 uptake by plants, according to the new research. One of the main sources of this uncertainty is that many climate models do not accurately account for triose phosphate utilization (TPU), which dictates limitations on a plant’s CO2 intake. In other words, its uncertain whether this represents the upper limit on the amount of carbon dioxide plants on Earth can process.

During photosynthesis plants turn CO2 into triose phosphate, a molecule that is used to make sucrose, which is in turn used by plants for energy. By studying this process at the molecular level in leaves, the researchers were able to quantify the limits of how much CO2 a plant can process.

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“When photosynthesis gets too much carbon dioxide, it can’t process it into sugars fast enough,” Tom Sharkey, a biochemist at Michigan State University’s Plant Research Laboratory said in a statement. “Photosynthesis cannot indefinitely increase its productivity levels. It reaches a ceiling and more carbon dioxide won’t help.”

According to the researchers, TPU limitation in plants is affected by a number of factors. For example, the limitation is more stringent at higher light levels, colder temperatures and high CO2 levels.

When TPU parameters were incorporated into existing climate models, the researchers found that it resulted in atmospheric carbon dioxide levels increasing faster than expected.

“The prognosis is more alarming than we previously thought,” Sharkey said. “We need to better understand TPU limitation because it is affected by many factors.”