Rising atmospheric CO2 can stimulate photosynthesis in many C3 plants, but estimates of the historical global effect have differed widely across satellite products, long-term proxies and terrestrial biosphere models.
A 2023 Nature Climate Change study combined several independent approaches to constrain that uncertainty.
Global GPP cannot be measured directly
The authors combined terrestrial biosphere models, ecological optimality theory, remote-sensing approaches and an emergent constraint based on global carbon-budget estimates.
After accounting for the direct effect of rising CO2 on canopy photosynthetic efficiency, satellite-based sensitivities became more consistent with model ensembles.
Estimated CO2 effect: 13.5 ± 3.5%
The analysis suggests that CO2 fertilization increased annual global terrestrial photosynthesis by 13.5 ± 3.5% between 1981 and 2020, equivalent to 15.9 ± 2.9 PgC.
This is an estimate of the CO2 contribution, not the total observed change from every environmental driver. Temperature, rainfall, nutrient limitation, land use and disturbance also affect GPP.
CO2 fertilization does not cancel climate change
The fertilization effect contributes to the land carbon sink but does not offset fossil-fuel emissions. Its future strength may also be constrained by water, nitrogen, phosphorus and increasing heat or drought stress.
The major contribution of the paper is therefore not a claim that “more CO2 is good for plants,” but a tighter global constraint on one of the largest uncertain feedbacks in the carbon cycle.
For related context, see Large Trees Are Increasing in the United States and Amazonia—but That Does Not Prove Warming Made Trees Bigger.
For related context, see When Does Tree Planting Stop Cooling the Climate? Albedo Accounting Across 172 Carbon-Market Projects.
For related context, see How Much Crop Choice Could Low Latitudes Lose? Climatic Niches of 30 Food Crops Under 1.5–4°C Warming.
Reference
- Keenan TF et al. A constraint on historic growth in global photosynthesis due to rising CO2. Nature Climate Change. 2023;13:1376–1381. https://doi.org/10.1038/s41558-023-01867-2
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