Recent research reveals that climate models have overestimated the CO2 fertilization effect on plant growth by about 11% due to inaccurate representation of nitrogen fixation processes. This means the land carbon sink is weaker than previously thought, leading to more CO2 remaining in the atmosphere. The study highlights the need for improved modeling of nitrogen cycles and emphasizes rapid emission reductions to mitigate climate change.
If you have been following climate science, you might be familiar with the reassuring idea that as we emit more carbon dioxide (CO2) into the atmosphere, plants will grow faster and absorb more CO2. This phenomenon is known as the CO2 fertilization effect and has been a key assumption in many climate projections for decades. However, a new paper published in late 2025 in the Proceedings of the National Academy of Sciences challenges this comforting narrative.
Earth system models, which are used to generate the Intergovernmental Panel on Climate Change (IPCC) climate projections, have been overestimating a crucial natural process that enables the CO2 fertilization effect. Once this error is corrected, it appears that plants will not grow as much under elevated CO2 levels as previously assumed. Consequently, the land carbon sink—which absorbs roughly a quarter of our emissions annually—is weaker than expected.
Plants require carbon, water, sunlight, and various nutrients to perform photosynthesis. One of the most important nutrients is nitrogen. However, atmospheric nitrogen (N2) is unusable by plants directly. Only certain bacteria and archaea can fix atmospheric nitrogen into a usable form through a process called biological nitrogen fixation (BNF).
Plants obtain nitrogen either from soil nitrogen (from decomposition or fertilizers) or from nitrogen fixed by microbes. Some plants, like legumes, form symbiotic relationships with nitrogen-fixing bacteria, while others rely on free-living microbes in soils, mosses, lichens, and biological soil crusts.
Biological nitrogen fixation is energy-intensive, requiring significant carbon investment. Therefore, when plants or microbes increase nitrogen fixation, they must divert carbon to this process, representing a tradeoff rather than free growth.
Earth system models must simulate nitrogen availability to accurately simulate plant growth. Since BNF is the main source of new nitrogen in natural ecosystems, models estimate BNF globally. However, the new research by Ku Gizbreck and colleagues reveals that these models use a crude approach that does not distinguish between nitrogen fixation in agricultural systems, natural ecosystems, symbiotic fixation inside plant tissues, and free-living fixation in soils.
The researchers compiled the most comprehensive observational dataset ever assembled, including thousands of measurements from forests, grasslands, croplands, and deserts. When comparing this data to model outputs, the mismatch was too significant to ignore.
Natural ecosystems like forests and grasslands absorb the most CO2 and act as the lungs of the biosphere. Models allocating too much nitrogen to these ecosystems overestimate their growth and carbon uptake potential. Conversely, underestimating nitrogen fixation in agricultural areas misrepresents nitrogen hotspots geographically.
Models typically estimate BNF using relationships with net primary production (NPP) and actual evapotranspiration (AET). Some newer models incorporate the carbon cost of nitrogen fixation. Despite differences, all models fail to replicate observed geographical patterns of BNF.
The new data shows that BNF increases slowly with AET and NPP in natural ecosystems but increases more steeply in agricultural ecosystems. Models ignore this difference, leading to underestimation of agricultural BNF and overestimation of natural BNF.
This misrepresentation also affects simulations of nitrogen-related emissions such as nitrous oxide, nitrogen oxides, ammonia, and nitrous acid, most of which originate from agriculture.
The research distinguishes between symbiotic BNF (microbes inside plant tissues) fixing about 28 Tg N/year and free-living BNF (microbes in soils, mosses, lichens, dead wood) fixing about 36 Tg N/year. Models do not make this distinction, leading to overestimation of BNF in productive biomes and underestimation in barren regions.
Since forests and grasslands drive most global CO2 uptake (net ecosystem production, NEP), models artificially inflate the potential carbon sink.
The researchers analyzed 39 existing earth system models and found that when real-world nitrogen fixation data is incorporated, the CO2 fertilization effect is overstated by roughly 11%. While this may seem small, in the context of the global carbon cycle, it is significant.
This overestimation translates into a smaller land carbon sink and more CO2 remaining in the atmosphere than previously thought.
Nitrogen limitation in natural processes has been known for decades. Industrial processes like the Haber-Bosch method produce ammonia for fertilizers, which farmers apply extensively. However, this leads to nitrogen pollution and emissions of nitrous oxide, a greenhouse gas 273 times more potent than CO2 over 100 years.
Genetic engineering efforts to improve nitrogen use efficiency in crops have faced challenges due to the tightly integrated nature of plant metabolism. Recent research suggests that designing microbial ecosystems to improve nitrogen cycling is possible but only under specific conditions and soils, not as a global solution.
Therefore, assuming forests can magically increase nitrogen availability to sustain growth under high CO2 is unrealistic.
The new analysis does not predict a collapse of the land carbon sink. Forests and grasslands will continue to absorb carbon and photosynthesis will still be enhanced by elevated CO2 to some extent. However, the land sink is weaker and the CO2 fertilization effect smaller than many models projected.
This widens the gap between emissions and natural absorption, emphasizing the urgent need for rapid reductions in human-induced greenhouse gas emissions.
The researchers suggest:
These improvements will lead to more accurate climate projections.
There is no natural loophole or hidden mechanism that will save us from the consequences of continued greenhouse gas emissions. The responsibility lies with humanity to reduce emissions rapidly to maintain habitable conditions on Earth.
If you are a researcher or interested in this field, sharing your views on these findings is valuable. For everyone else, reflecting on this information is crucial as we collectively face the challenges of climate change.
Remember to stay informed and engaged, and just have a think.
