The CO fertilization effect or carbon fertilization effect causes an increased rate of photosynthesis while limiting leaf transpiration in plants. Both processes result from increased levels of atmospheric carbon dioxide (CO).  The carbon fertilization effect varies depending on plant species, air and soil temperature, and availability of water and nutrients. Net primary productivity (NPP) might positively respond to the carbon fertilization effect. Although, evidence shows that enhanced rates of photosynthesis in plants due to CO fertilization do not directly enhance all plant growth, and thus carbon storage. The carbon fertilization effect has been reported to be the cause of 44% of gross primary productivity (GPP) increase since the 2000s. Earth System Models, Land System Models and Dynamic Global Vegetation Models are used to investigate and interpret vegetation trends related to increasing levels of atmospheric CO. However, the ecosystem processes associated with the CO fertilization effect remain uncertain and therefore are challenging to model.
Terrestrial ecosystems have reduced atmospheric CO concentrations and have partially mitigated climate change effects. The response by plants to the carbon fertilization effect is unlikely to significantly reduce atmospheric CO concentration over the next century due to the increasing anthropogenic influences on atmospheric CO. Earth's vegetated lands have shown significant greening since the early 1980s largely due to rising levels of atmospheric CO. 
Theory predicts the tropics to have the largest uptake due to the carbon fertilization effect, but this has not been observed. The amount of  uptake from  fertilization also depends on how forests respond to climate change, and if they are protected from deforestation. 
Changes in atmospheric carbon dioxide may reduce the nutritional quality of some crops, with for instance wheat having less protein and less of some minerals.  Food crops could see a reduction of protein, iron and zinc content in common food crops of 3 to 17%.