Also, you're presenting an argument that has actually been looked at for decades in the literature, which would be evident from a leisurely google scholar search of terms such as: climate change, CO2, respiration, net ecosystem production, terrestrial vegetation, food supply, etc.
I've attempted to grab some links to papers that seem to be publicly available. [5] is a review from the Royal Society, and [3] is straight out of Science if you care about credentials.
2001. [1] http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2486.2001....
1994. [2] http://ecoethics.net/cyprus-institute.us/PDF/Rosensweig-Food...
2006. [3] http://naldc.nal.usda.gov/download/26790/PDF
2004. [4] http://www.preventionweb.net/files/1090_foodproduction.pdf
2005. [5] http://rstb.royalsocietypublishing.org/content/royptb/360/14...