Scientists Expect Decreased Yields With Global Warming

Center on Food Security and the Environment (via Sowing Agricultural Seeds Daily):

If global temperatures continue to rise, the amount of crops farmers can harvest will sharply decline during the next 100 years.
Stanford professor David Lobell and an international team of climate scientists modeled future crop yields under several global climate scenarios throughout the 21st century. They found that if average global temperatures rise by more than two degrees Celsius, farmers are likely to get less wheat, rice and maize out of each plot of land. Yields are expected to fall by an average of 4.9 percent for every one degree Celsius rise in average temperature. Year-to-year variability of harvests is also expected to rise, as drought and flooding become more frequent. Crop yield losses will speed up throughout the century, with declines in yield beginning around 2030 and with the fastest drop happening in the second half of the century.
Lobell, an associate professor of Environmental Earth System Science and the associate director of the Center on Food Security and the Environment at Stanford, reviewed over 1,700 published studies with a team of climate scientists from the United States, United Kingdom and Australia. The team found that if farmers adapt to climate change within the next few years, they have a better chance of avoiding or even reversing the predicted decline of wheat and rice yields in some regions. Agricultural adaptation strategies like irrigating fields and developing new crop breeds could increase projected yields between 7 percent and 15 percent.
The new study also highlights the need for better data on the potential future impacts of other factors that affect crop yields, like the prevalence of pests and plant diseases, and the availability of water supply. A full version of the study can be found online at Nature Climate Change.

That makes some sense in relation to corn and wheat, neither of which react well to high temperatures.  I'm not really familiar with rice.  I personally would anticipate yields to remain relatively flat overall, as yield drags due to temperature rise cancels out yield improvements due to genetic improvement. However it works out, I anticipate that population increase combined with temperature rise leads to food scarcity.