– By Priyanka Runwal
Because plants absorb carbon dioxide from the atmosphere and convert it into food, forests and other similar ecosystems are considered to be some of the planet’s most important carbon sinks. In fact, the United States and many other countries that participated in last month’s UN Climate Change Conference have made nature-based solutions a critical feature of their carbon dioxide mitigation framework under the Paris Agreement.
As human activities cause more carbon dioxide to be emitted into the atmosphere, scientists have debated whether plants are responding by photosynthesizing more and sucking up even more carbon dioxide than they already do – and if so, is it a little or a lot more. An international team of researchers led jointly by UC Berkeley and Lawrence Berkeley National Laboratory (Berkeley Lab), has found that plants are photosynthesizing at a higher rate, with a 12% increase global photosynthesis from 1982 through 2020. The global carbon dioxide concentrations in atmosphere increased by about 17% over the same time period, from 360 parts/million (ppm) up to 420ppm.
The 12% increase of photosynthesis results in 14 petagrams more carbon being removed from the atmosphere each year by plants. This is roughly equivalent to the carbon emitted globally from fossil fuels. in 2020 alone. While not all carbon removed from the atmosphere through photosynthesis is stored within ecosystems, some is later released to the atmosphere through respiration. The study however reports a direct connection between increased photosynthesis, and global carbon storage. The studyNature published the article.
“This is a very large increase in photosynthesis, but it’s nowhere close to removing the amount of carbon dioxide we’re putting into the atmosphere,” said Berkeley Lab scientist Trevor Keenan, lead author of the study. “It’s not stopping climate change by any means, but it is helping us slow it down.”
It is crucial to reduce carbon dioxide, as it remains in the atmosphere for longer periods of time than other greenhouse gases that are driving global warming. Photosynthesis and soils capture around a third the carbon dioxide that is released into the atmosphere every decade from fossil fuel burning.
Plants open tiny pores on the leaf surfaces during photosynthesis to absorb carbon dioxide from the atmosphere and make their own food. Scientists can measure photosynthetic activity by placing a leaf in an enclosed chamber. They can then quantify the drop in carbon dioxide levels inside the chamber. But it’s far more difficult to measure how much carbon dioxide an entire forest takes up.
Initiatives such as AmeriFlux, a network of measurement sites coordinated by the Department of Energy’s AmeriFlux Management Project at Berkeley Lab, scientists from across the world have built over 500 micrometeorological towers in forests and other ecosystems to measure the exchange of greenhouse gases between the atmosphere and the vegetation and soil. While these flux towers can help estimate photosynthesis rates, they’re expensive and thus limited in their geographic coverage, and few have been deployed long-term.
This explains why scientists rely upon satellite images to map how green the Earth is and what plants are covering it. This allows them infer global photosynthetic activity. But, with increasing carbon dioxide emissions, these estimates that are solely based on greenness have become problematic.
The history of the picture
Satellite images can capture extra green to account the increased number of leaves plants produce due to their rapid growth. But they often don’t account for each leaf’s increased efficiency to photosynthesize. Also, this efficiency doesn’t increase at the same rate at which carbon dioxide builds up in the atmosphere.
Previous attempts to determine how increased carbon dioxide concentrations affect photosynthesis rates have produced wildly varying results. These ranged from very little to no effects at the low end to large effects at the high end.
“That magnitude is really important to understand,” said Keenan, who is also an assistant professor in UC Berkeley’s Department of Environmental Science, Policy and Management. “If the increase [in photosynthesis] is small, then we may not have the carbon sink we expect.”
Keenan and his research team took a different approach. They looked at almost three decades of Global Carbon Project carbon sink estimates. These were compared to predictions from satellite images taken of the Earth between 1982 and 2012. They also used models that use carbon exchange between land and atmosphere to estimate carbon sinks.
“Our estimate of a 12% increase comes right in the middle of the other estimates,” he said. “And in the process of generating our estimate, it allowed us to re-examine the other estimates and understand why they were overly large or small. That gave us confidence in our results.”
While this study highlights the importance of protecting ecosystems that are currently helping slow down the rate of climate change, Keenan notes that it’s unclear how long forests will continue to perform this service.
“We don’t know what the future will hold as far as how plants will continue to respond to increasing carbon dioxide,” he said. “We expect it will saturate at some point, but we don’t know when or to what degree. The capacity of land sinks to offset our emissions will be much lower at that point. And land sinks are currently the only nature-based solution that we have in our toolkit to combat climate change.”
NASA and the DOE Office of Science supported part of the study. Among the coauthors were Nicholas Smith, Yao Zhang, Xiangzhong Louo, and Shazhou, all of whom are now at other institutions.
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