Does CO2 have a positive fertilization effect?

Higher CO2 levels are beneficial for many crops. In plant species with a 'C3' photosynthetic pathway, like rice and wheat, higher CO2 directly stimulates photosynthetic rates. However, this mechanism does not affect C4 crops like maize.

Higher CO2 allows leaf pores, called stomata, to shrink, which results in reduced water stress for all crops.

BUT atmpospheric CO2 is the highest in  probably 20 million years (NOAA 2009)
If CO2 fertilization is effective in the fields surely it would be obvious by now.

The theoretical benefits of CO2 increase that are cited above are the limits, that are catastrophically dangerous to the planet and so of less relevance than being assumed in the big picture of global climate change.

The net effect on yields for C3 crops has been measured as an average increase of 14% for 580 ppm (Ainsworth et al., 2008). For C4 species such as maize and sorghum, very few experiments have been conducted, but the observed effect is much smaller and often statistically insignificant (Leakey, 2009).

Results of experimental studies indicate that many crop plants, including wheat and soy, respond to elevated CO2 with increased growth, although not uniformly so. On the other hand, newer field studies carried out in ““free air CO2 enrichment”” (FACE) environments showed that the growth response of some crop types was significantly (50%) smaller than expected (e.g., Long et al., 2006). The response of crop plants to carbon fertilization in the field remains an issue of some debate.

For C3 crops, the negative effects of warming are often balanced by positive CO2 effects up to 2-3 °C local warming in temperate regions, after which negative warming effects dominate. 

Because temperate land areas will warm faster than the global average, this corresponds to roughly 1.25-2 °C in global average temperature. 

For C4 crops, even modest amounts of warming are detrimental in major growing regions given the small response to CO2. C4 crops are essential to world food security as they include maize, millet, sorghum, sugar cane and many pasture grasses.

The IPCC assessment says that any CO2 fertilization effect for the lower latitudes will have a negligible impact and so crop yields will decline with a very small temperature increase.

For the mid and the high latitudes the IPCC chooses its language carefully saying that the CO2 fertilization effect may result in a small increase in crop yields for a while initially as temperature increases, after which crop yields will decline.
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