Submitted to: Journal of Crop Production
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 1999
Publication Date: N/A
Interpretive Summary: Atmospheric carbon dioxide (CO2) concentration has risen from about 270 ppm before 1700 to about 370 ppm today. Global temperatures might rise 2.8 to 5.2 degrees Celsius (5 to 9 degrees Fahrenheit) if CO2 increases to twice its present concentration. USDA-ARS scientists at Gainesville, Florida measured evapotranspiration responses (water use) of plants to rising CO2 and temperature. Stomata, the small pores on leaves that regulate plant water loss, partially close as CO2 increases. Doubling of CO2 decreases leaf stomatal conductance of water vapor about 40 percent. However, water use by crop plants usually decreases only 12 percent (or less) for two reasons. Firstly, as stomatal conductance decreases, leaf temperature will rise which forces more water vapor out of the leaf. Secondly, doubled CO2 will cause more leaf area to grow which gives a larger surface area for evapotranspiration. Increasing temperature, however, causes about 4 to 5 percent increase in evapotranspiration for each 1 degree Celsius rise in air temperature. Thus, rising CO2 may decrease slightly the amount of water used by crops in the future, but global warming of about 3 degrees Celsius would completely offset the water-savings effect of doubled CO2.
Technical Abstract: Atmospheric carbon dioxide (CO2) concentration has risen from about 270 ppm before 1700 to about 370 ppm currently. General Circulation Models (GCM) predict a global temperature rise of 2.8 to 5.2 degrees Celsius for a doubling of CO2. This paper reports evapotranspiration responses of plants to rising CO2 and temperature. Doubling of CO2 will decrease leaf stomatal conductance to water vapor about 40 percent. However, water use by crops decreases only 12 percent or less for two reasons. Firstly, the energy balance causes leaf temperatures to rise as stomatal conductance is decreased, thereby increasing transpiration which offsets part of the decreased leaf conductance. Secondly, CO2 enrichment causes increased leaf area which provides more surface for transpiration and also offsets part of the effect of decreased stomatal conductance. Experiments in both ambient and elevated CO2 showed that transpiration rates increased about 4 to 5 percent per 1 degree Celsius rise in temperature. Thus, rising CO2 may decrease slightly the amount of water used by crops in the future, but global warming of only 3 degrees Celsius would offset the water-savings effect of doubled CO2.