By Scott Schrage
The past century of climate change not only has extended the average U.S. growing season by nearly two weeks, but also has driven annual buildups of yield-stifling heat in the West and Northeast, says new research from the University of Nebraska-Lincoln.
Relying on data from 1900 to 2014, the study assembled a region-by-region composite of how climate change has affected agricultural timelines and the yields of six major crops in the U.S.
The last frost of spring now occurs roughly seven days earlier than it did 100 years ago, the study concluded, and the first frost of the fall hits about five days later, though the trends did vary by region. Those frosts traditionally mark the beginning and end of a growing season, meaning that U.S. farmers have an average of 12 more days to plant, cultivate and harvest crops.
To examine links between temperature changes and crop yields during the past century, Suat Irmak, a professor of biological systems Engineering at UNL, and doctoral student Meetpal Kukal used a measure of annual heat accumulation that factors in the ideal growing temperatures of the six crops. Each crop has a base temperature below which it will not grow. By adding up the differences between an average daily temperature and that base temperature whenever the former exceeded the latter, the researchers calculated how many degrees of a growing season's cumulative temperature spurred crop growth.
According to that metric, the U.S. averages 90 more degrees F of accumulated heat now versus a century ago. But at the national scale, that increase was linked to lower yields among five of the crops — corn, soybean, sorghum, spring wheat and winter wheat — with only cotton responding positively to the trend.
Yet most of the agricultural belts where the study's six crops are grown — especially the Upper Midwest, Ohio Valley and Southeast — saw slight to moderate decreases in heat accumulation during their growing seasons. Corn, soybean, sorghum and winter wheat yields generally rose in those regions, the study reported, while yields of heat-craving cotton fell.
By contrast, cumulative heat increased in the western and northeastern portions of the U.S. That trend especially affected the Southwest, which saw its annual heat accumulation rise by about 500 degrees — by far the largest positive or negative shift of any region. The annual heat accumulation of one site in California increased by nearly 2,300 degrees over the century.
Irmak and Kukal concluded that the longer growing season has influenced crop yields less than the changes in accumulated heat, which more directly and regularly affects the growth of a crop. And because many growers plant well after the last spring frost and harvest well before the first fall frost, the widening window may not have increased yields as much as it could have, the researchers said.
Taking that information into account, Irmak said, could help growers improve the odds of maximizing their planting conditions and potentially save money by reducing the risk of failed plantings.
"For optimal crop growth, development and yield, you have to plant at a certain soil depth, moisture and temperature level," Irmak said. "If you have that right combination when you plant a crop, you are starting off great, which is very critical for the rest of the season. What happens in that short time period is critical for the rest of the season in terms of productivity, quality and economics.
"But in the Midwest, because we have such fast-fluctuating weather conditions — hailstorms or a lot of rain or no rain — you may have to go back and replant. If your growing season is longer, your planting window will be longer, too. You will have more opportunities to look for the right combination of soil temperature and moisture. That's a big benefit, because we lose a lot of yield, money and time in the Midwest through replanting."
The emergence of longer seasons might also get farmers reconsidering which planting strategies make the most sense from both a yield and cost standpoint, Irmak said.
"If my growing season is longer, can I plant two short-season crops? If so, which crops will be viable in different regions? Will that be more economical than just one corn crop or soybean crop? We would be able to do that now, because we have longer seasons," he said. "But is that more economical in terms of inputs and outputs?
"I think one of the advantages of our team is that one of our feet is always in the ground. All of us have spent a lot of time traveling to many locations and collecting data. So when you get to the office and start analyzing that data, part of your brain is still in the field. That helps you couple the scientific data and analyses with real-world conditions, which can make the interpretation of the results more meaningful in practice."
Schrage is science editor at University Communication at UNL, the source of this report.