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Too Much CO2 Causes Plants Not to Cool The Earth As They Should, Says Study

The percentage of predicted warming due to the effect of high carbon dioxide concentrations on plants.

Trees and green plants, generally, are used by the planet as a way to keep itself cool. A regular tree can evaporate as much as ten gallons of water a day, acting as a natural air conditioner for its surroundings. So trees are important for their CO2-sequestration capabilities and keeping things cool(er).

Still, when carbon dioxide levels are too high, the greenhouse gas causes the pores (stomata) that the leaf transpires through to shrink, and thus not to release its normal water amounts. A study performed by scientists from the Carnegie Institute for Science shows that more than a quarter of the warming from increased CO2 levels in some areas of the world is due to this effect.

The warming effects of carbon dioxide as a greenhouse gas have been known for a long time, says study co-author Ken Caldeira of Carnegie’s Department of Global Ecology. But he and fellow Carnegie scientist Long Cao were concerned that it is not as widely recognized that carbon dioxide also warms our planet by its direct effects on plants. Previous work by Carnegie’s Chris Field and Joe Berry had indicated that the effects were important. “There is no longer any doubt that carbon dioxide decreases evaporative cooling by plants and that this decreased cooling adds to global warming,” says Cao. “This effect would cause significant warming even if carbon dioxide were not a greenhouse gas.”

In their model, the researchers doubled the concentration of atmospheric carbon dioxide and recorded the magnitude and geographic pattern of warming from different factors. They found that, averaged over the entire globe, the evapotranspiration effects of plants account for 16% of warming of the land surface, with greenhouse effects accounting for the rest. But in some regions, such as parts of North America and eastern Asia, it can be more than 25% of the total warming. “If we think of a doubling of carbon dioxide as causing about four degrees of warming, in many places three of those degrees are coming from the effect of carbon dioxide in the atmosphere, and one is coming from the direct effect of carbon dioxide on plants.”

The model that the Carnegie researchers presented shows that the excess CO2 will even increase the runoff from the land surface in most of the areas. Water from precipitations will bypass the plants’ natural evaporation system and flow directly into rivers and streams. No earlier model predicted runoffs having as a cause the changes in evapotranspiration due to high CO2 levels.

Seeing how plants respond to various levels of carbon dioxide could help climate predictions and improve the representation of climate models worldwide.

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