Background
The authors write that "climate change under various emission scenarios is highly uncertain but is expected to affect agricultural crop production in the 21st century." Yet in spite of the great uncertainty that they associate with climate change projections to the year 2094, they proceed to use this approach as the primary driver of their projected changes in crop production.

What was done
First of all, based on the A2 (medium-high) and B1 (low) emission scenarios from the IPCC's Fourth Assessment Report, Lee et al. developed 18 climate change projections for the two emission scenarios "by applying different climate models and downscaling methods," which they then used to derive a suite of projected yield changes for a number of different crops using the biogeochemical model DAYCENT (Del Grosso et al., 2002), after which they considered the experimentally-determined impacts of projected increases in the air's CO2 content on the yields of the crops, obtaining their final results by combining the two sets of projections.

What was learned
With respect to only the climate-change-derived results, the three University of California, Davis researchers report that "all crop yields except for alfalfa significantly declined by 2094 under A2, but less under B1," noting that "under A2, yields decreased in the following order: cotton (25%) > sunflower (24%) > wheat (14%) > rice (10%) > tomato and maize (9%)." But then they state that "the CO2 fertilization effects were predicted to potentially offset these yield declines (>30%)."

What it means
To most rational people, these findings would imply that California Central Valley yields in 2094 for all of the studied crops would likely be greater than they are currently (actually as of 2009 in Lee et al.'s study). And, in fact, they say that the combined effects of climate change and CO2 fertilization "increased crop yields by 2-16% more than the yields under climate change only under A2 by 2094." However, they then indicate they have "less confidence" in predicting the response of crop yields to projected increases in atmospheric CO2 concentration than to projected changes in climate. And so they say that they subsequently only "focused on the temperature and precipitation effects on yield in a future changed environment," thereby omitting the well-known and experimentally-derived projected effects of atmospheric CO2 enrichment on the yields of the crops.

What makes this development even more distressing is that they conclude that California needs to implement statewide adaptation scenarios and management strategies that "maintain or increase yields while mitigating emissions of greenhouse gases," which include the one sure thing (CO2) that can actually counteract the deleterious effects of what they originally described as the "highly uncertain" projected climate change between now and the end of the century. Is this science? ... or is it politics?

Reference
Del Grosso, S., Ojima, D., Parton, W., Mosier, A., Peterson, G. and Schimel, D. 2002. Simulated effects of dryland cropping intensification on soil organic matter and greenhouse gas exchanges using the DAYCENT ecosystem model. Environmental Pollution 116: S75-S83.