It is difficult for climate modelers to quantify the complex responses of biodiversity to increased carbon dioxide in the atmosphere. A recent study from the Carnegie Institution for Science suggests a feedback loop where increased carbon dioxide causes vegetation to directly contribute to global warming.
Plants absorb carbon dioxide for photosynthesis through the stomata, or pores, in their leaves. Afterwards, they emit water through these same pores in a process called Evapotranspiration: a natural cooling process like perspiration for a plant.
Evapotranspiration doesn’t just cool off the plant; the enormous quantity of water emitted by vegetation cools the surrounding air significantly.
The problem is, increased carbon dioxide in the atmosphere, causes a plant’s stomata to shrink. This means less water can be emitted and the natural cooling effect is diminished.
In their study, Long Cao and Ken Caldeira modeled a climate with twice the amount of CO2 in the atmosphere. They found that 16% of the heat increase they observed was directly attributable to the hindrance CO2 had on evapotranspiration. Carbon dioxide, therefore, not only contributes to global warming as a heat-trapping greenhouse gas, but it also has a direct impact on plant cooling function.
Other complicating factors must be considered, such as the potential for increased leaf area or for shifting albedo in an higher CO2 atmosphere. Overall, however, the study demonstrates the importance of understanding how plant communities respond to increased carbon dioxide and of being able to quantify this response in future climate models.