Has anybody studied deminshing returns of co2 in the atmosphere?

I know from my study of agriculture and economics (my degree is in agricultural economics) that any variable input results in diminishing returns when input is added at a higher and higher level. Has anyone studied atmospheric co2 in this manner? In agriculture, which is a natural science, if any variable (such as water or nitrogen) results in diminishing returns when added at higher and higher levels. I have received the same results in econometric models when variables are added when building econometric models. My hypothesis is that increasing levels of atmospheric co2 will result in global cooling not global warming. Any comments? Any research in this area?