What was done
Using yield data obtained from the United Nations Food and Agriculture Organization, growing season temperature and precipitation data obtained from the Climate Research Unit (Mitchell and Jones, 2005), and the spatial distribution of crop area obtained from Leff et al. (2004) for the six most widely grown crops in the world (wheat, rice, maize, soybeans, barley and sorghum), the authors plotted year-to-year changes in global yields against similar changes in growing season temperature and precipitation for the period 1961-2002.

What was learned
Lobell and Field found that roughly half to two-thirds of the global yield variance of the world's six top crops was unexplained by temperature and precipitation. For rice and soybeans, however, much of the smaller remainder of the yield variance was explained by "a positive relationship with precipitation," while for the other four crops temperature provided most of the remaining explanatory power.

Based on the yield-temperature relationships they derived, the two researchers calculated that for the increases in growing season temperatures experienced between 1981 and 2002, only wheat, maize and barley showed significant warming-induced decreases in yield, and that the production of these crops in 2002 "would have been roughly 2-3% higher without climate trends since 1981," which for the year 2002 represented a loss of close to five billion US dollars.

What it means
Lobell and Field say their results suggest that "recent climate trends, attributable to human activity [our italics], have had a discernible negative impact on global production of several major crops," which is the message that is touted in most news stories of the research. However, they go on to say that "the impact of warming was likely offset to some extent by fertilization effects of increased CO2 levels." Although this "crumb of concession" sure doesn't sound like much (i.e., "some extent"), we note they accurately report that the approximate 35-ppm increase in the air's CO2 concentration experienced over the period in question would be expected to have boosted the yields of the C3 crops studied by about 3.5%, which clearly more than compensates for the temperature-induced decrease in yield. And if they and others are going to talk about human-induced yield changes, they need to include those due to concomitant increases in atmospheric CO2 concentration; and when this is done, the positive effect of the aerial fertilization effect totally overpowers the negative effect of the observed warming. What is more, there is considerable debate about how much of the warming over the period in question was truly of human origin (as well as how much of it was even real), while there is general agreement that most of the CO2 increase did indeed have an anthropogenic origin.

Once again, therefore, a bad-news story turns out to be a good-news story: environmental changes attributable to human activity have likely had a positive effect on the global production of several major crops since 1981.

References
Leff, B., Ramankutty, N. and Foley, J.A. 2004. Geographic distribution of major crops across the world. Global Biogeochemical Cycles 18: 10.1029/2003GB002108.

Mitchell, T.D. and Jones, P.D. 2005. An improved method of constructing a database of monthly climate observations and associated high-resolution grids. International Journal of Climatology 25: 693-712.