A recent (Feb 2007) report from Lawrence Livermore National Laboratory and the Dept. of Global Ecology at the Carnegie Institution investigated the impact that climate change has had on harvest yields for the world's main crops: wheat, maize, rice, soy, barley, sorghum ("Production of these crops accounts for over 40% of global cropland area, 55% of non-meat calories and over 70% of animal feed"(1)).
The short paper considered not only temperature and precipitation changes but also technological advances. While not all crops were impacted to the same amount (rice and soybeans less), they did come to the conclusion that "At the global scale, warming from 1981 to 2002 very likely offset some of the yield gains from technological advances, rising CO2 and other non-climatic factors." (2)
In other words, technology increased yields but climate change appears to have taken those gains away for some crops.
The question becomes: can agriculture technology continue to advance crop yields at the same rate it did during this period, or will the temperature and precipitation change faster than technology gains thereby decreasing overall yields at a time of increasing population?
Sources for (1) and (2): David B Lobell and Christopher B Field, "Global scale climate–crop yield relationships and the impacts of recent warming," Environmental Research Letters, Volume 2, Number 1, January-March 2007, http://www.iop.org/EJ/article/1748-9326/2/1/014002/erl7_1_014002.html