Look at the detail

The fact that some crops seem to do a bit better under increased CO2 is getting quite an airing in the anti carbon tax/climate change skeptic blogs at the moment. Just this morning, for example, Bolt has a US economist saying:

The experimental evidence suggests that at least 10 percentage points of the increase in wheat and rice yields since 1750 is the result of the roughly 35% increase in CO2 in the atmosphere that has occurred over the same period.

Well, I'm not sure about the claimed yield increases, but you have to keep this recent finding in mind:

The study covered 24 cultivars studied in 112 experimental treatments from 11 countries. A significant growth dilution effect on grain protein was found: a change in grain yield of 10% by O₃ was associated with a change in grain protein yield of 8.1% (R² = 0.96), whereas a change in yield effect of 10% by CO₂ was linked to a change in grain protein yield effect of 7.5% (R² = 0.74). ...

An important and novel finding was that elevated CO₂ has a direct negative effect on grain protein accumulation independent of the yield effect, supporting recent evidence of CO₂-induced impairment of nitrate uptake/assimilation.

So, you might get more wheat, but have to eat all that extra to get the same amount of protein.

And the other point is, of course, that you don't get any wheat to eat at all if you have more severe droughts and baking summer heat.

Speaking of heat, John Nielsen-Gammon has been looking at the Texas drought. Texas certainly seems a dry place: in terms of length, this current one is not exceptional. However, combined with the degree of heat, even the cautious Nielsen-Gammon is saying this:

I don’t consider it to be the worst drought on record, because the 1950s drought lasted for seven years, and 1956 alone gives 2011 a run for its money. But, combine it with July being the warmest month on record for Texas, and it probably becomes the most unbearable. It may well be the worst drought on record for agriculture.

More generally (by which I mean, including outside of Texas) the mid West heat wave is breaking lots of records, but seems to be attracting little attention here.