The past century of climate change has extended the average U.S. growing season by nearly two weeks but driven annual buildups of yield-stifling heat in the West and Northeast, says new research from the University of Nebraska–Lincoln.

Relying on data collected 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 throughout the contiguous United States.

The last frost of the spring season now occurs roughly seven days earlier than it did 100 years ago, the study concluded, and the first frost of the fall strikes 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.

"If you have a longer growing season, you can cultivate longer-maturing crops that yield more than shorter-season crops,” said co-author Suat Irmak, Eberhard Distinguished Professor of Biological Systems Engineering. “But doing that is going to require more water, more nitrogen, perhaps more insecticide, herbicide and fungicide — all these inputs that go into growing crops. More analyses are needed to determine the viability and economics of growing longer-season crops in different regions.”

To examine links between temperature changes and crop yields during the past century, Irmak 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 United States averages 90 more degrees Fahrenheit of accumulated heat now versus a century ago. But at the national scale, that increase was actually 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.

“It’s possible that we exceeded temperature thresholds and stressed the crops too much in some regions, and heat stress can be detrimental for crop productivity,” Irmak said. “That’s perhaps why there’s a yield reduction. But there’s another component: water stress. As you have more (heat accumulation), that means more thermal energy for evaporation and water loss through transpiration. In non-irrigated areas, that evaporation might not be replenished.”

Read More of this article and view maps of changing temperature trends and accumulated heat units.

Irmak and Kukal detailed their findings in the journal Scientific Reports.