Because plants take up 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. Now an international
team of researchers led by Lawrence Berkeley National Laboratory (Berkeley
Lab) and UC Berkeley have used a novel methodology combining remote sensing,
machine learning, and terrestrial biosphere models to find that plants are
indeed photosynthesizing more, to the tune of 12% higher global
photosynthesis from 1982 to 2020. In that same time period, global carbon
dioxide concentrations in the atmosphere grew about 17%, from 360 parts per
million (ppm) to 420 ppm.
The 12% increase in photosynthesis translates to 14 petagrams of additional
carbon taken out of the atmosphere by plants each year, roughly the
equivalent of the carbon emitted worldwide from burning fossil fuels in 2020
alone. Not all of the carbon taken out of the atmosphere through
photosynthesis is stored in ecosystems, as much is later released back to
the atmosphere through respiration, but the study reports a direct link
between the increased photosynthesis and increased global carbon storage.
The study was published in Nature.
“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.”
Measuring photosynthesis
Because carbon dioxide stays in the atmosphere decades longer than other
greenhouse gases driving global warming, efforts to reduce it are critical
to mitigating climate change. Plants, through photosynthesis, and soils
sequester roughly a third of carbon dioxide emissions released into the
atmosphere each decade from the burning of fossil fuels.
During photosynthesis, plants open tiny pores on their leaf surfaces to suck
carbon dioxide from the air and produce their own food. To measure this
photosynthetic activity, scientists can put a leaf in a closed chamber and
quantify the dropping carbon dioxide levels in the air inside. But it’s far
more difficult to measure how much carbon dioxide an entire forest takes up.
Through 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 on satellite images to map how much of the
Earth is green and thus covered by plants, which allows them to infer global
photosynthetic activity. But with rising carbon dioxide emissions, those
estimates based solely on greenness become problematic.
Bringing history in the picture
Satellite images can capture the extra green to account for additional
leaves plants put out due to accelerated 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 efforts to estimate how photosynthesis rates respond to increased
carbon dioxide concentrations found widely varying results, from little to
no effects on the low end, to very large effects on 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.”
So Keenan and his team of researchers took a new approach: they looked back
at nearly three decades of carbon sink estimates made by the Global Carbon
Project. They compared these with predictions from satellite images of the
Earth taken between 1982 and 2012 and models using carbon exchange between
the atmosphere and land to make carbon sink estimates.
“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. At that
point land sinks will have a much lower capacity to offset our emissions.
And land sinks are currently the only nature-based solution that we have in
our toolkit to combat climate change.”
Reference:
Keenan, T.F., Luo, X., De Kauwe, M.G. et al. A constraint on historic growth
in global photosynthesis due to increasing CO2. Nature 600, 253-258 (2021).
DOI: 10.1038/s41586-021-04096-9