@article{Trindade:149826,
      recid = {149826},
      author = {Trindade,   Federico J.},
      title = {U.S. Crop Yields Redux: Weather Effects versus Human  Inputs},
      address = {2013},
      pages = {27},
      year = {2013},
      abstract = {Over the 40-year period from 1960 to 2000 the dramatic  increase in world crop production was the result of land  expansion, but also the result from increasing yields  through  the use of chemicals, fertilizers, pesticides and  water from irrigation systems (Tilman et al., 2002, PNAS).  An additional 50 percent increase in world population and  higher incomes are expected to double food demand by 2050.   Studies show that agriculture production in certain regions  like the cereal production areas of the United States, are  operating near maximum yield potential (Grassini et. al,  2011, FCP). All this seems to indicate that the food  production increases needed to satisfy future demand will  put great pressure on existing cropland and natural  resources. Climate change, a final source of concern, is  likely to aggravate the situation (Schlenker and Roberts,  2009, PNAS). An important step towards understanding the  evolution of agricultural production under different  climate scenarios is to carefully estimate the effect that  different temperatures have on agricultural productivity.   This is the goal of this research.
Most agronomic studies  on the effects of weather on crop yields are based on field  experiments.  These studies account for the biological  effect of different temperatures on specific crops (Ritchie  and Nesmith, 1991, MP&SS.) Schlenker and Roberts (2009)  consider the effect of weather on aggregate farm yields.   They regress corn, wheat and cotton yields in counties east  of the 100º meridian on weather variables during the years  1950-2005 and found that there is an increasing positive  relation between temperatures and crop yield up to 29-32ºC  (depending on the crop.) Temperatures above these  thresholds are found to reduce yields significantly. Their  regressions included precipitation, time trend, soils, and  county effects for location-specific unobserved factors.  There are two important omissions in this study.  First,  they only consider rain-fed counties, those east of the  100º meridian, while production increases have been  directly related to irrigation developments mostly west of  the 100º. Second, their study controls for natural  characteristics like precipitation but does not allow for  purchased inputs. These inputs have had a pivotal role on  increased yields and are under the control of the farmer.  It is important then to understand the degree of  substitution and the contribution of these versus other  inputs to the time trends they estimated. 
This research  develops a county level biomass production function for an  800-mile climatic gradient from the Rocky Mountains to the  Mississippi River (41o N latitude).  A panel data set that  includes 101 counties for the 1960-2008 period is  developed. The quantity of biomass produced per hectare  (from all crops) is hypothesized to result from the use of  traditional inputs under farmers’ control such as land,  fertilizer, chemicals, and irrigation and from  environmental variables such as soil organic matter,  precipitation and temperatures. Indexes are constructed for  all variables at the county level.  Given interest on  climate effects, particular emphasis is placed in the  development of county precipitation and degree-days  indexes. A semi transcendental logarithmic production  specification is jointly estimated with share equations for  purchased inputs using a seemingly unrelated estimation  approach with panel data corrections. The authors do not  know of any other study for this region that provides this  information. 
Results quantify the critical effects that  high temperatures have on agricultural productivity in the  region, after controlling for irrigation, other managed  inputs, soil characteristics, precipitation, and  technological change.  We found a negative and  substantially increasing (nonlinear) effect of temperatures  over 30 ºC on crop yields. While a full day of temperatures  between 30ºC and 35ºC decreases expected yield by 0.5%, a  day of temperatures over 35ºC decreases yields by 36.5%.  Our results are qualitatively similar to the findings in  Schlenker and Roberts but provide additional information.  First, the inclusion of irrigated land seems to diminish  greatly the negative effect of higher temperatures. We  estimate that in irrigated areas the harmful effect of  temperatures above 35ºC is reduced by 62%.  Second, it is  also clear that semi-arid areas like western Nebraska and  eastern Colorado and Wyoming, for example, compensate the  lack of precipitation with high values of irrigation.  Finally, the contribution of fertilizer and chemicals to  yield changes is significant, substantially reducing the  residual time trend due to unspecified factors.  Technological change has been irrigation, fertilizer, and  chemicals using.},
      url = {http://ageconsearch.umn.edu/record/149826},
      doi = {https://doi.org/10.22004/ag.econ.149826},
}