Thermal trends and increased temperature ranges as related to a shorter growing season in portions of the Great Plains

The last decade has had some of the warmest mean annual average temperatures on record for North America. Easterling, D.R. has also shown that there has been an average increase in the growing season for the entire United States of almost 11 days since 1950. However, a large area of the Great Plains has seen a quite different trend. There are locations in the Great Plains which have seen average decreases in the growing season approaching two weeks in length. With the perception that the climate is getting warmer in the Great Plains, and therefore, a parallel assumption that the growing season is getting longer, the reality of a decreasing growing season can have a dramatic unanticipated impact on the planting schedules of agricultural interests.

There were two potential causes for this observed phenomenon which were examined: (1) the possible occurrence of cooler springs and/or autumns and (2) a possible increase in the range of temperatures being observed during the spring and/or autumn transitional seasons. This second possible cause is related to The Intergovernmental Panel on Climate Change (IPCC 1995) which indicated that more extremes in temperature and precipitation can be expected due to human induced climate changes. Therefore, it is quite possible to in fact have a warming trend during these transitional seasons, but with increased ranges in temperature, late spring and early autumn record or near record low sub-freezing temperatures could also occur.

Cooperative climate data were examined (to avoid airport and urban biases) for the 6-state High Plains Climate region for the time period 1950 to present. A contiguous region in the eastern Plains region has seen a decrease in the growing season (especially since the 1970's). Most of this decrease is, however, occurring at the end, and not at the beginning of the growing season. There has been a strong and statistically significant trend toward cooler summers and autumns in this region of the U.S. Record low temperature occurrences (record low minimums and record low maximums) were examined and although there has been a slight increase in the number of minimum records in the autumn, this alone does not explain the decrease in growing season. Finally, the range of temperatures on a variety of time scales ranging from diurnal to 10 day time periods were examined to determine if there is an increase in thermal ranges during the transitional season. For much of the study region, there has been an increase in temperature ranges (both average and smoothed average) in the autumn transitional season which could help explain the recent increased probability of temperatures falling below freezing.