CO₂ Fertilization Effect

The CO₂ fertilization effect is the increase in plant growth and photosynthesis that can occur when the concentration of carbon dioxide in the air rises. Because CO₂ is a raw material for photosynthesis, more of it can boost growth, especially in plants that use the C3 pathway.

The enzyme Rubisco is not saturated at today's CO₂ levels, and extra CO₂ also competes with oxygen, reducing photorespiration. Plants may also lose less water because they can partly close their stomata while still taking in enough carbon. These effects are strongest in C3 plants such as wheat, rice and trees, and weaker in C4 plants, which already concentrate CO₂.

Free-air CO₂ enrichment (FACE) experiments, which raise CO₂ around whole stands of plants outdoors, typically find growth and yield gains on the order of 10 to 20 percent for C3 crops at elevated CO₂. Satellite records show that the Earth has become greener since the 1980s, and studies attribute a large share of that to CO₂ fertilization. The extra uptake is part of the carbon sink that removes a fraction of human emissions.

Growth can be held back by shortages of nitrogen, phosphorus or water, so the benefit often fades over time. Crops grown at high CO₂ can also contain less protein, zinc and iron. Warming, drought and fire may outweigh the gains, as examined in effects of climate change and the carbon cycle, so it should not be counted on to offset emissions. Its role is accounted for in an Earth system model.