Warmer temperatures and a longer frost-free growing season
Air temperatures and changes in growing season are closely linked (1). As average air temperatures increase, the daily minimum temperatures have risen faster than daily maximum temperatures (6). In other words, most of the global increase in temperature has happened at night. Night time minimum temperatures in the spring and fall determine first and last frost days. Most areas of the US have experienced fewer frosts overall, as well as longer growing seasons. In the Pacific Northwest, the growing season has increased by 4 days per decade (1).
The US Historical Climatology Network Daily Data includes data from twelve locations that were chosen throughout Idaho (7). All locations had data from 1919 to 2009 and were missing less than 10% of all data during that time. The elevations of these stations range from 640-1807 m (2100-6000 ft) above sea level. The last spring day and first fall day with a minimum temperature less than 0° C (32° F) were used to find the growing season length for each location. The average number of days in the growing season for all 90 years was used to find the difference between each year’s growing season length and the 90 year average. Earlier years generally had shorter growing seasons than the average, while later years generally had longer growing seasons than the average.
The growing season in Idaho is approximately 13 days longer now than it was in the early 20th century. The Idaho growing season is both starting sooner and ending later. The last spring frost is on average, 8 days earlier, and the last fall frost is 5 days later. While some of the benefits of the longer growing season can be seen as positives, negative effects also exist.
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