The researchers behind the projection believe the change could be broadly good news for US agriculture, allowing new species of crops to be cultivated, although they add that it could also bring more weeds and pests.

"This study underscores the importance of examining changes in extremes as a means of providing a more complete picture of climate change," said Lauren Parker at the University of Idaho in Moscow, US. "This is a growing part of climate change research, particularly with respect to research on climate change impacts."

Many studies have looked at changes in average low temperatures under climate change. In recent years, more scientists have come to realize that the lowest temperatures are equally important.

Parker and her colleague John Abatzoglou investigated the future relationship between average lows and lowest temperatures. They figured out how the changes will affect "cold hardiness zones" – officially defined regions that indicate what crops are likely to survive where.

The researchers began by taking temperature predictions from 20 global climate models, downsized for the US, for two 30 year periods: 1971–2000 and 2041–2070. They calculated the differences between the average minimum temperatures in winter and the average coldest temperatures for the two periods, then assessed the magnitude of change between them.

They found that the rise in lowest temperatures was on average some 40% greater than the rise in average lows. This equated to the lowest temperatures shifting northwards by over 21 km per decade, and a change in many future cold hardiness zones.

"Broadly speaking, warming [of lowest temperatures] or upward shifts in cold hardiness zones can be a good thing for US agriculture, though it is not without its challenges," said Parker. "While warmer winter temperatures may allow for the cultivation of new species of crops or different crop varietals in locations that were previously too cold, that same lack of cold temperatures can exacerbate pest and weed problems. Consequently, growers will have to balance any potential benefits and challenges of warmer winters when making planting and management decisions."

The possibility of cultivating crops, however, is not solely dependent on lowest temperatures, with other factors such as heat stress, frost susceptibility and water availability playing a role. "We are currently seeking to address these factors, among others, to provide a better picture of how future climate changes may shift the geographic distribution of crops," said Parker.

The study is published in Environmental Research Letters (ERL).

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