Rising CO2 and potential global warming will cause changes in evapotranspiration (ET). Our research objective was to determine the impact of CO2 and air temperature on canopy ET, water use efficiency (WUE), foliage temperature, and canopy resistance (Rc) of soybean [Glycine max (L.) Merr.]. Plants were grown in sunlit, controlled‐environment chambers at cyclic maximum/minimum air temperatures from 28/18°C to 44/34°C and CO2 of 350 or 700 μmol mol−1 Maximum ET rate in the early afternoon at 35 d after planting ranged from 7.5 mol m−2 s−1 at 28/18°C to 19.0 mol m−2 s−1 at 44/34°C. Daily ET during the middle of the season ranged from 260 mol H2O m−2 d−1 (4.7 mm d−1) at 28/18°C to 660 mol H2O m−2 d−1 (11.9 mm d−1) at 44/34°C. Mean daily ET was linearly related to mean air temperature (Tair) as: [Mean daily ET = 21.4 × Tair − 306, r2 = 0.99 (mol H2O m−2 d−1), or mean daily ET = 0.385 × Tair − 5.5 (mm d−1)]. Doubled CO2 caused a 9% decrease in ET at 28/18°C, but CO2 had little effect at 40/30°C or 44/34°C. Whole‐day WUE declined linearly with air temperature, with a slope of −0.150 [(μmol CO2 mmol−1 H2O) °C−1]. Changes in ET and WUE were governed by changes in foliage temperature and Rc. In conclusion, increases in temperature anticipated by climate change could more than offset decreases of ET that would be caused by increases in CO2

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Methane (CH4) effluxes by paddy-culture rice (Oryza sativa L.) contribute about 16% of the total anthropogenic emissions. Since radiative forcing of CH4 at current atmospheric concentrations is 21 times greater on a per mole basis than that of carbon dioxide (CO2), it is imperative that the impact of global change on rice CH4 emissions be evaluated. Rice (cv. IR72) was planted in sunlit, closed-circulation, controlled-environment chambers in which CH4 efflux densities were measured daily. The CO2 concentration was maintained at either 330 or 660 micromol mol(-1). Air temperatures were controlled to daily maxima and minima of 32/23, 35/26, and 38/29 degrees C at each CO2 treatment. Emissions of CH4 each day were determined during a 4-h period after venting and resealing the chambers at 0800 h. Diurnal CH4 effluxes on 77, 98, and 119 d after planting (DAP) were obtained similarly at 4-h intervals. Emissions over four-plant hills and over flooded bare soil were measured at 53, 63, and 100 DAP. Emissions were negligible before 40 DAP. Thereafter, emissions were observed first in high-CO2, high-temperature treatments and reached a sustained maximum efflux density of about 7 mg m(-2) h(-1) (0.17 g m(-2) d(-1)) near the end of the growing season. Total seasonal CH4 emission was fourfold greater for high-CO2, high-temperature treatments than for the low-CO2, low-temperature treatment, probably due to more root sloughing or exudates, since about sixfold more acetate was found in the soil at 71 DAP. Both rising CO2 and increasing temperatures could lead to a positive feedback on global warming by increasing the emissions of CH4 from rice.

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