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Growing-season (freeze-free season) [reference evapotranspiration] ETo exhibited a significant positive trend across multiple temporal windows with a median trend of 7–13mm decade-1 for time periods from 1950 onward [from 4 datasets (USHCN v2.5, PRISM, CRU, NCDC) over the Pacific Northwest (Oregon, Washington, Idaho, western Montana and extreme northwestern Wyoming)] (Fig. 7). This increase was associated with both an increase in the growing season length, as depicted in Fig. 6, and increased vapor pressure deficit from July to September. Despite no long-term change in climatic water deficit for time periods commencing in the 1920s and 1930s (Dobrowski et al. 2013), declines in summer and autumn precipitation concomitant with increased summer ETo resulted in increased deficit in over 90% of all reporting stations and a regional median trend of about 20mm decade-1 from 1980 to 2012.
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Linear trends in the dates of the last spring freeze, first autumn freeze, and length of the freeze-free season [from 4 datasets: USHCN v2.5, PRISM, CRU, NCDC] provide additional support of pervasive warming across the PNW [Oregon, Washington, Idaho, western Montana and extreme northwestern Wyoming] (Fig. 6). The date of the last spring freeze has advanced by, on average, 9 days since 1950, while the date of the first autumn freeze has been delayed by a comparable number of days since 1950. Collectively, these changes have resulted in an extension of the freeze-free season, or growing season by an average of approximately two weeks over the last four decades. Following trends in seasonal temperature, no significant changes in the date of the last spring freeze have been observed from 1980 to 2012, whereas there has been continued delay in the date of the first autumn freeze.
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Trends for the absolute maximum temperature [in the Pacific Northwest: Oregon, Washington, Idaho, western Montana and extreme northwestern Wyoming] were comparable in magnitude to annual mean maximum temperature for periods from 1950 onward [from 4 datasets: USHCN v2.5, PRISM, CRU, NCDC], but are heterogeneous, similar to trends in annual absolute maxima observed globally (e.g., Karl et al. 1991). By contrast, trends in absolute minimum temperature exhibited strong positive trends, including in excess of 1°C decade-1 for 1970–2012 and 1980–2012. Increases in absolute minimum temperature are consistent with updated U.S. Department of Agriculture Plant Hardiness Zone Maps (Daly et al. 2012). While not explicitly examined in this study, Bumbaco et al. (2013) found that the frequency of nighttime minimum temperatures exceeding the 99th percentile for June–September increased substantially in western Washington and Oregon since 1901, whereas trends in the frequency of maximum temperatures exceeding the 99th percentile were not apparent.
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Regional [Pacific Northwest: Oregon, Washington, Idaho, western Montana and extreme northwestern Wyoming] annual mean temperature (Fig. 1a) shows long term warming modulated by interdecadal variability with relatively cool periods 1910–25 and 1945–60 and relatively warm periods around 1940 and since the mid- 1980s [from 4 datasets: USHCN v2.5, PRISM, CRU, NCDC]. The warmest year in the region was 1934, but the warmest 10-yr period was 1998–2007, with very few years since 1980 that had below-average annual mean temperatures. Linear trends in annual mean temperature from 1901 to 2012 were between 0.056° and 0.076°C decade-1, with larger warming found in the CRU dataset.
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v3.2
