Everything changed in 2000
Climate science has long taught that plants, by turning carbon dioxide and water into sugars, serve as major carbon sinks. For over a century, higher CO2 levels were believed to boost photosynthesis worldwide, which would curb warming. Early results did show faster photosynthesis as CO2 rose, but since 2000 those gains slowed. A drier atmosphere means less water in plant tissues, and when water is scarce, photosynthesis slows and CO2 remains in the air longer.
A team of earth scientists from the Grassland Research Institute of the Chinese Academy of Agricultural Sciences, alongside colleagues from several U.S. institutions, uncovered telling evidence. It shows that the global rise in photosynthesis driven by CO2 has slowed significantly. This finding challenges the earlier expectation that rising CO2 would continually bolster plant carbon uptake across the planet. The study appears in Science, where the researchers report the measured deceleration of photosynthesis in recent years and discuss what this means for climate outcomes. (Science, 2023)
A key question: how well does the world really absorb CO2 through photosynthesis? Ground-based monitors worldwide, tracking environmental variables such as CO2 and water vapor from 1982 through 2016, provided a foundation. Supplemental satellite imagery offered a broader view of vegetation health and cover. By merging these data streams with advanced machine learning techniques, the team identified subtle shifts in leaf color and other indicators that are hard to spot with the naked eye. These tools helped map global photosynthesis trends from 2000 onward, creating models that project future changes in uptake. (Science, 2023)
Early work showed that CO2 rises were accompanied by greater plant photosynthesis and more carbon drawn from the atmosphere. But since 2000, the pace of this increase has slowed, and the most worrying implication is that rising temperatures and drying conditions may eventually halt the growth in photosynthesis. If this happens, the trajectory of global warming could accelerate, unless greenhouse gas emissions fall sharply. Nature-based solutions could help close the gap and contribute to emissions reductions under the Paris Agreement. (Science, 2023)
17% more CO2 in the atmosphere
A Nature study published two years earlier noted that global photosynthesis rose by about 12 percent between 1982 and 2020, even as atmospheric CO2 concentrations climbed by roughly 17 percent. The boost in photosynthesis translated into plants pulling approximately 14 petagrams of carbon from the atmosphere each year—roughly equivalent to the yearly carbon emissions from fossil fuel burning in 2020. (Nature, 2022)
Yet not all of that carbon captured by photosynthesis stays locked away. A large portion is later released again into the air through plant respiration and decomposition. The new evidence warned that such a massive uptick in photosynthesis did not erase the scale of human-emitted CO2. In fact, it underscored the persistent gap between increased carbon uptake and the total emissions driving climate change. (Nature, 2022)
Despite that cautious note, scientists remained hopeful. The rise in photosynthesis was seen as a partial ally in slowing climate change, rather than a cure. The lead author of the study, Trevor Keenan, described the effect as a partial offset rather than a solution. The newer data, however, suggested that the momentum behind photosynthesis is fading as global temperatures rise and droughts become more common. (Science, 2023)
The bottom line is clear: CO2 persists in the atmosphere for decades longer than other greenhouse gases, so reducing emissions remains essential for mitigating climate change. Plants and soils currently trap roughly a third of annual CO2 emissions from fossil fuel burning, but that proportion is shrinking as conditions become harsher for photosynthetic activity. (Science, 2023)
References and further reading point to the same conclusion: ongoing monitoring and modeling are crucial to understanding how much carbon the biosphere can continue to sequester. The study emphasizes the need for sustained emissions reductions alongside nature-based strategies to maximize climate benefits. (Science, 2023)
Across different datasets and modeling approaches, the consensus remains that natural processes help, but they do not replace the imperative to cut greenhouse gases. The interplay between atmospheric chemistry, plant physiology, and water availability is complex, and predictions will depend on climate trajectories, land-use decisions, and ecosystem management. (Science, 2023)
Ultimately, the scientific community agrees on a straightforward path: keep CO2 in check, protect and restore ecosystems, and accelerate policies that reduce emissions. Only then can nature-based assets contribute meaningfully to an overall climate strategy. (Science, 2023)
—End of study overview. (Nature, 2022; Science, 2023)