U.S. Agro-Climate in 20 th Century: Growing Degree Days, First and Last Frost, Growing Season Length, and Impacts on Crop Yields.

Significant air temperature changes have occurred globally during the 20th century, which are spatially variable to a considerable degree and these changes can have substantial implications in agroecosystem productivity. The agroclimate indicators that are responsible in these contexts are first fall frost (FFF), last spring frost (LSF), climatological growing season (CGS) length, and heat accumulation (growing degree days, GDD). We explore spatial and temporal trends associated with these indices across the continental U.S. (CONUS) during 1900-2014 using datasets collected at 1218 sites. On average, FFF has been occurring later (by 5.4 days century-1 ), and LSF has been occurring earlier (by 6.9 days century-1 ), resulting in the average lengthening of the CGS (by 12.7 days century-1 ). Annual GDD has been increasing by 50 °C century-1 . We also report trends for agricultural belts and climate regions. We developed relationships between county-level crop yields vs. agroclimate changes and found that all crops (maize, soybean, sorghum, spring wheat, winter wheat, and cotton) responded positively to a lengthened CGS, while responding negatively to increase in GDD, except cotton. Overall, we find that the observed changes in agroclimate, were beneficial for crop yields in the CONUS, albeit some crop and region specific exceptions.