The 'Plant Food' Argument
At its core, the CO2 fertilisation effect is a real phenomenon. Plants use carbon dioxide for photosynthesis, the process that fuels their growth. In theory, providing more of a key ingredient—in this case, atmospheric CO2—should boost this process, making
plants grow faster and larger. For decades, this has been presented in parts of the climate debate as a potential silver lining, an ecological bonus that could help offset some of the negative impacts of emissions by turning the atmosphere's excess carbon into lush forests and bountiful harvests.
Evidence of a Greener Planet
This isn't just a theory; we have evidence for it. Satellite data has shown a significant 'greening' of the Earth over the past few decades, with an increase in leaf cover equivalent to twice the area of the continental United States. Studies have attributed a large portion of this greening—as much as 70%—directly to the rising levels of CO2 in the atmosphere. This has led some to conclude that the Earth's vegetation will act as an ever-expanding sink, soaking up a significant portion of our emissions and helping to regulate the climate. But this is where the story gets more complicated.
The Catch in the Canopy
A plant can’t live on CO2 alone. Just as a person needs a balanced diet, a plant requires a host of other resources to thrive, most notably water and soil nutrients like nitrogen and phosphorus. The CO2 fertilisation effect can only work as long as these other ingredients are available in sufficient quantities. Once a plant runs out of available nitrogen or phosphorus in the soil, it doesn't matter how much carbon dioxide is in the air; its growth will be limited. This is the crucial nuance that recent scientific work has been exploring in great detail.
What the Latest Science Reveals
Recent studies, some published as recently as late 2025, have put this nutrient limitation under the microscope. One major finding is that many climate models have been overly optimistic, assuming that nature would find a way to make more nitrogen available to plants to keep up with the rising CO2. New research suggests that natural nitrogen availability has been overestimated by as much as 50% in some models. When these models are corrected with more realistic data, the projected CO2 fertilisation effect is reduced significantly. Furthermore, long-term experiments show that any initial boost in plant growth from extra CO2 often diminishes over time as soil nutrients are depleted. For tropical forests, which grow on ancient, nutrient-poor soils, the effect is even more constrained.
More Growth, Less Nutrition
There's another critical layer of nuance. Even when plants do grow faster and larger, the quality of that growth can decline. Several studies have found that crops like wheat, rice, and potatoes grown under elevated CO2 levels contain less protein and essential minerals like zinc and iron. One study on rice found decreases in several B vitamins. This means that while yields might increase in some cases, the nutritional value of the food we eat could be diluted. The accelerated growth essentially forces plants to spread the same amount of nutrients over a larger biomass, creating a hidden cost to the CO2 fertilisation effect.
