Increased carbon dioxide's effect on plants

Trees and other plants help keep the planet cool, but rising levels of carbon dioxide in the atmosphere are turning down this global air conditioner. According to a new study by researchers at the Carnegie Institution for Science, in some regions more than a quarter of the warming from increased carbon dioxide is due to its direct impact on vegetation. For scientists trying to predict global climate change in the coming century, the study underscores the importance of including plants in their climate models.

"Plants have a very complex and diverse influence on the climate system," says study co-author Ken Caldeira of Carnegie's Department of Global Ecology. "Plants take carbon dioxide out of the atmosphere, but they also have other effects, such as changing the amount of evaporation from the land surface. It's impossible to make good climate predictions without taking all of these factors into account."

Plants absorb carbon dioxide for photosynthesis through pores (called stomata). When carbon dioxide levels are high, the leaf pores shrink. This causes less water to be released, diminishing the tree's cooling power. "There is no longer any doubt that carbon dioxide decreases evaporative cooling by plants and that this decreased cooling adds to global warming," says researchers. "This effect would cause significant warming even if carbon dioxide were not a greenhouse gas."

The researchers also found that their model predicted that high carbon dioxide will increase the runoff from the land surface in most areas, because more water from precipitation bypasses the plant cooling system and flows directly to rivers and streams.

Steve - as this effect further stresses plants, they emit more pollen to compete for survival. This is why pollen counts get progressively worse every year.