Slowing climate change, even if possible, requires more than earlier estimates suggested. Green plants that release oxygen by turning carbon dioxide and water into sugars are among the planet’s essential carbon sinks. The basic idea was that as atmospheric CO2 rose, photosynthesis would speed up globally, helping to curb warming. That held true for much of the last century, but since 2000, photosynthesis has lagged because the atmosphere has grown drier. When plants have less water, photosynthesis slows and more CO2 remains in the air.
A team of Earth scientists from the Grassland Research Institute of the Chinese Academy of Agricultural Sciences, working with colleagues across several U.S. institutions, has found evidence that the worldwide boost in photosynthesis driven by higher CO2 has slowed significantly.
In their study, published in Science, researchers tracked changes in global photosynthesis rates in recent years. They did not anticipate the weaker results.
forest in Romania. Unsplash
Earlier research showed that as CO2 increased, plants accelerated photosynthesis and drew down more carbon from the atmosphere; the net effect was thought to help curb global warming. That trend held early on, until around 2000, when newer data show that rising CO2 alone no longer produces the same boost because the atmosphere is drier.
Everything changed in 2000
To gauge global photosynthesis, researchers analyzed data from ground monitors around the world spanning 1982 to 2016. These monitors record environmental statistics such as CO2 and water vapor in the air.
They also examined satellite images of vegetation-covered regions and combined those images with multiple machine learning tools to detect subtle changes, including shifts in leaf color and photosynthesis levels.
Using these data, the team built models to illustrate how global photosynthesis has evolved from 2000 to today and to forecast future trends.
Energy production facility. Pixabay
The study reaffirmed an earlier finding: global photosynthesis rose as atmospheric CO2 rose over the past century.
Yet from 2000 onward, the rate of increase in photosynthesis slowed, and some scientists fear it could stall entirely as the climate becomes hotter and drier.
The main takeaway is that unchecked warming will accelerate unless greenhouse gas emissions fall dramatically, threatening the continuity of life on the planet. There is more: nature-based solutions to meet Paris Agreement goals are essential.
17% more CO2 in the atmosphere
A Nature study published two years earlier reported a 12% rise in global photosynthesis between 1982 and 2020, while CO2 concentrations climbed by 17% in the same period.
This uptick in photosynthesis removed about 14 petagrams of carbon from the atmosphere each year, roughly equivalent to the carbon released globally from fossil fuel burning in 2020 alone.
Monkfish seed field. Pixabay
Not all of the carbon captured through photosynthesis remains stored in ecosystems; some returns to the atmosphere through respiration. The new study cautioned that the dramatic jump in photosynthesis does not erase the massive CO2 emitted by human activity.
Still, the scientists remained optimistic: the increase in photosynthesis helps counter climate change, even if it does not stop it entirely. That optimism cooled as the latest findings showed the rise in photosynthesis is far smaller than the rise in emissions and is shrinking as the atmosphere dries out.
Because CO2 stays in the atmosphere for decades longer than other greenhouse gases, reducing it remains critical to mitigating climate change.
Plants and soils sequester about a third of the CO2 emitted from fossil fuels each decade, and that sequestration is increasingly under pressure.
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