What was done
The authors studied well watered (flooded) and fertilized rice (Oryza sativa L.) plants that they fumigated with air containing either 380 or 680 ppm CO2 from the panicle formation stage at 59 days after transplanting (DAT, from seedling trays into pots) within controlled-environment chambers maintained at either high (32°C) or low (22°C) night temperatures, with day temperature held constant at 32°C, until either 107 or 114 DAT. During this latter period, they measured the flux of methane (CH4) between the pots and the atmosphere each day at 10:00 and 22:00 hours; and at the conclusion of the experiment they determined the dry weight of each organ of all of the plants employed in the study.

What was learned
Cheng et al. report that the extra 300 ppm of CO2 increased CH4 emissions by 32.2% in the low night temperature treatment, but by only 3.5% in the high night temperature treatment. Likewise, they found that the elevated CO2 increased the dry weight gained by the plants in the low night temperature treatment by 38.4%, but by a smaller 12.7% in the high night temperature treatment.

What it means
An interesting metric that can be derived from these data is the ratio of the percent increase in CO2-induced biomass production (a positive effect) to the percent increase in CO2-induced CH4 emissions (a negative effect) as the air's CO2 concentration rose from 380 to 680 ppm. This benefit/cost ratio was 1.19 in Cheng et al.'s low-night-temperature treatment and 3.63 in their high-night-temperature treatment, which indicates that in transiting from the low-night-temperature to the high-night-temperature environment in their experiment as the air's CO2 concentration rose by 300 ppm, the benefit/cost ratio rose by a little over 200%. Consequently, because night temperatures rose significantly faster than day temperatures throughout most of the world over the last several decades, this phenomenon may well have had a net two-pronged positive effect on the biosphere, as appears to be the case from the many materials we have archived under the headings of Greening of the Earth and Methane (Atmospheric Concentrations) in our Subject Index; and it could well have a similar positive effect in the future, increasing the magnitude of the aerial fertilization effect of atmospheric CO2 enrichment at a considerably faster relative rate than it increases the relative rate of CO2-induced methane emissions to the atmosphere.