"Changes in canopy transpiration, canopy evaporation, and soil evaporation in response to the effect of CO2-radiative forcing, CO2-physiological forcing, and the combined CO2-radiative and physiological forcing. All results shown here are annual mean changes in response to a doubling of atmospheric CO2 calculated from the last 70-yr results of 100-yr simulations. Hatched areas are regions where changes are not statistically significant at the 5% level using the Student t-test." Image credit & caption: Excerpted from image in supplemental data for paper, PNAS.
As reported in Importance of carbon dioxide physiological forcing to future climate change, there is an "emerging consensus that the physiological effects of increasing atmospheric CO2 on land plants will increase global warming beyond that caused by the radiative effects of CO2." Excess CO2 in the air results in plant leaves closing the stomata or 'pores.' This results in reduced evapo-transpiration. What water does not go up (by evapo-transpiration) must either be discharged to ground water or run off the land. The authors conclude that the "physiological effect [of doubling atmospheric CO2] increases runoff by 8.4 ± 0.6%, primarily by diminishing evapo-transpiration from the continents. This is in addition to the runoff increases caused by radiative forcing increasing the atmospheric precipitation, reported as an increase of 5.2 ± 0.6%. Of course, increased runnoff associated with evapo-transpiration loss will be seen in landscapes with high plant cover - not in deserts.
The Caldeira Lab at Stanford provides the following, very nice graphic depicting the cumulative effect on climate forcing (aside from the runoff increases mentioned above) of evapo-transpiration reductions caused by the doubling of CO2. We are talking then about an average 25% increase.
"Percentage of warming due to the effect of carbon dioxide on plants. Carbon dioxide warms the Earth because it is a greenhouse gas in the atmosphere. However, carbon dioxide causes plants to provide less evaporative cooling. Our study finds that, in some places, over 25% of warming from a doubling of atmospheric CO2 content is a result of decreased evaporative cooling by plants."
The full article may be purchased and downloaded from Proceedings of National Academies here.
EurekaAlert has a news item covering the research.
Now Glenn Beck will be able to claim that the way to save the world from Climate Change is to cut down trees.
Next on the research docket: realizing that the carbon bomb is acidifying plankton to the point where the feedback loop may become stuck in full open position.