What was done
The authors calculated cropland net primary production (NPP) in the central United States (South Dakota, Nebraska, Kansas, Missouri, Iowa, Minnesota, Wisconsin and Illinois) using USDA information together with crop-specific parameters that convert agronomic data into carbon fluxes for the period 1972-2001.

What was learned
Total cropland area exhibited no temporal trend over the study period.  In contrast, in the words of Hicke and Lobell, "both NPP (flux per unit area) and P (spatially aggregated flux) increased during the study period (46 and 51%, respectively)."

What it means
In spite of the "twin evils" of rising atmospheric CO2 concentration and air temperature that climate alarmists decry so mightily, agricultural productivity in the central U.S. increased dramatically over the three-decade study period.  Possible drivers of the long-term increase in NPP, according to the authors, include "improved cultivars, better fertilizer and pest management, more favorable climate, shifts to productive crop types, and economic influences (Duvick and Cassman, 1999; Evans, 1997; Lobell and Asner, 2003)."  Consequently, it would appear that if either of the twin evils of the climate-alarmist crowd had a negative impact on crop productivity - which is highly unlikely, considering Hicke and Lobell attribute some of the increase in NPP to a "more favorable climate" and that carbon dioxide is an effective aerial fertilizer that also increases plant water use efficiency - that negative impact was miniscule compared to the positive impacts of all of the other factors cited by Hicke and Lobell.  Based on this experience, therefore, we may expect to see more of the same in the future, i.e., increased crop yields, even in the face of (and likely partly because of) continued increases in atmospheric CO2 concentrations and air temperatures.

References
Duvick, D.N. and Cassman, K.G.  1999.  Post-green revolution trends in yield potential of temperate maize in the north-central United States.  Crop Science 39: 1622-1630.

Evans, L.T.  1997.  Adapting and improving crops: The endless task.  Philosophical Transactions of the Royal Society of London, Series B 352: 901-906.

Lobell, D.B. and Asner, G.P.  2003.  Climate and management contributions to recent trends in US agricultural yields.  Science 299: 1032.