What was done
The authors grew rice (Oryza sativa L. cv. Akita Komachi) in 1999 and 2000 in paddy culture at Shizukuishi, Iwate, Japan in a FACE study where the air's CO2 concentration was increased 200 ppm above ambient.

What was learned
In the words of the authors, the extra CO2 "significantly increased the CH4 emissions by 38% in 1999 and 51% in 2000," which phenomenon they attributed to "accelerated CH4 production as a result of increased root exudates and root autolysis products and to the increased plant-mediated CH4 emission because of the higher rice tiller numbers under FACE conditions."

What it means
Since methane is many times more effective than CO2 as a greenhouse gas on an equal mass basis, this result does not bode well for the future radiative forcing of climate as portrayed by state-of-the-art climate models.  Other CO2 enrichment studies of rice have produced just the opposite result, however, suggesting we do not have a full and complete understanding of this phenomenon.  [See, for example, Schrope et al. (1999) and Kruger and Frenzel (2003).]  More research will be needed to better define this important issue.

References
Kruger, M. and Frenzel, P.  2003.  Effects of N-fertilisation on CH4 oxidation and production, and consequences for CH4 emissions from microcosms and rice fields.  Global Change Biology 9: 773-784.

Schrope, M.K., Chanton, J.P., Allen, L.H.. and Baker, J.T.  1999.  Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa.  Global Change Biology 5: 587-599.