How Plants Could Impact Global Warming

Biodiversity influences climate at local, regional and global levels, yet most climate models do not take biodiversity into consideration because its variables and effects are too diverse and complex to compute. Two recent studies, however, demonstrate the importance of being able to consider the response of vegetation to elevated levels of carbon dioxide in climate models as we try to predict our climate future.

Scientists at the Carnegie Institution for Science found that carbon dioxide’s direct effects on vegetation contribute to global warming. Through the pores called stomata in their leaves, plants take in carbon dioxide from the atmosphere that they use for photosynthesis. They then give off water through the stomata in a process called evapotranspiration which cools the plant just as perspiration cools human beings. Evapotranspiration also cools the surrounding air—a tree can transpire up to ten gallons of water on a hot day. But when carbon dioxide levels increase, plants’ stomata shrink, releasing less water into the air and reducing the cooling effect.

Carnegie scientists Long Cao and Ken Caldeira doubled the level of carbon dioxide in their model and found that globally the reduced evapotranspiration was responsible for 16 percent of the land warming; the rest was due to CO2’s heat-trapping effects. In North America and Asia, more than 25 percent of the warming was due to the impact of increased CO2 on vegetation. “There is no longer any doubt that carbon dioxide decreases evaporative cooling by plants and that this decreased cooling adds to global warming,” said Cao. “This effect would cause significant warming even if carbon dioxide were not a greenhouse gas.”

Another effect of the doubled CO2 is increased runoff from the land as more precipitation bypasses the plant’s evapotranspiration system and makes its way directly into streams and rivers.

The Carnegie study did not take into consideration other effects of increased carbon dioxide such as an increase in leaf area, variations in vegetative distribution and resulting changes in albedo (the reflectivity of Earth’s surfaces which affects how much solar radiation is absorbed).  These aspects were fixed in their model. But they cited earlier research on increased CO2 that showed that cooling due to increased leaf area produced an overall cooling effect over land, and that a decrease in albedo due to the expansion of coniferous forests resulted in land warming.

“These results really show that how plants respond to carbon dioxide is very important for making good climate predictions,” said Caldeira. “So if we want to improve climate predictions, we need to improve the representation of land plants in the climate models.

A new NASA study that did take plant growth into consideration found that doubling the level of CO2 resulted in a cooling effect. The model used by Lahouari Bounoua of the Goddard Space Flight Center in Greenbelt, Md., was innovative in its consideration of a reaction that plants have to increased CO2 called “down-regulation.” Down-regulation is the process that enables plants to use water and nutrients more efficiently when there is increased CO2, so that they are able to maintain previous levels of photosynthesis, which can ultimately boost leaf growth.

The increased leaf area resulted in more evapotranspiration globally, and thus created a cooling effect.  The amount of cooling in the study measured -0.6 degrees C (-1.1 F) over land, compared to models that didn’t include down-regulation.

Bounoua stressed, however, that the cooling was not enough to offset the warming trends that are predicted.

Climate models usually factor in a doubling of CO2 to simulate global warming, and scientists generally agree that under this scenario, temperatures would increase from 2 to 4.5 degrees C (3.5 to 8.0 F). Bounoua’s model found a warming of 1.94 degrees C globally without the inclusion of down-regulation. The range in temperature results from unknowns about various “feedbacks,” i.e. how the various systems on Earth such as clouds, plant growth, methane release, the water cycle, albedo, etc. might respond to warming and interact with each other.

Bounoua and her colleagues also looked at how plant growth is stimulated by warmer temperatures, increased precipitation in some areas, and the plants’ more efficient use of water and nutrients when CO2 is doubled. The results suggest that in the long term, increases in vegetation due to elevated CO2 might reduce temperatures after CO2 levels stabilize.

“As we learn more about how these systems react, we can learn more about how the climate will change,” said the study’s co-author Forrest Hall. “Each year we get better and better.”

New climate models are being designed to consider dynamic global vegetation that allows plant types to shift interactively with climate, and ecosystem demography that accounts for how communities of diverse plants might respond to climate change over time.