From 1940 to 1989, multiyear values are mostly below average, with the notable exception of the 1970 to 1974 period. Between 19, multiyear values for very hot days in eastern Washington are mostly above the long-term average of 14 days, with the exception of the 1900 to 1914 interval. Annual values show year-to-year variability and range from about 3 to 28 very hot days and from 0.8 to about 12 hot days. Y-axis values range from 0 to 30 very hot days and 0 to 12 hot days. Graph of the observed annual number of very hot days for eastern Washington (top) and hot days for western Washington (bottom) from 1900 to 2020 as described in the caption. The highest multiyear values for freezing days occurred in 1920 to 19 to 1934 while the 2000 to 2004 period had the lowest value. Recent multiyear periods, starting with 1995 to 1999, have all been below average. Multiyear values for freezing days in western Washington show a similar pattern of variability, with most periods before 1995 being near or above the long-term average of 3 days, with notable exceptions in 1900 to 19 to 1944. The 2000 to 2004 period has the lowest multiyear value for very cold nights and the 1920 to 1924 period has the highest. Since 1985, multiyear values are all below average. Prior to 1980, they are mostly above the long-term average of 3.7 nights, with the notable exceptions of the 1900 to 1904, 1940 to 1944, 1960 to 1964, and 1965 to 1969 periods. Multiyear values for eastern Washington very cold nights show variability. Annual values show year-to-year variability and range from about 0 to 19 very cold nights for eastern Washington and from 0.2 to 18 freezing days for western Washington. Y-axis values range from 0 to 20 for both panels. Graph of the observed annual number of very cold nights for eastern Washington (top) and freezing days for western Washington (bottom) from 1900 to 2020 as described in the caption. The numbers of very hot days in Eastern Washington and hot days in Western Washington have been quite variable but were both generally above average during the 2015–2020 period, after below average numbers during the 2010–2014 period (Figure 2b). However, the numbers of very warm nights in Eastern Washington and warm nights in Western Washington have both been above average since 1990 (Figure 3). Since 1990, the numbers of very cold nights in Eastern Washington and freezing days in Western Washington have both been below average (Figure 2a). The overall warming trend is evident in an increased number of warm nights. The hottest year on record was 2015, with a statewide average temperature of 50.0☏, which was 3.7☏ above the long-term average. Since the beginning of the 20th century, temperatures in Washington have risen almost 2☏ (Figure 1), and since 1986, all but 5 years have been above the long-term (1895–2020) average. Less warming is expected under a lower emissions future (the coldest end-of-century projections being about 1☏ warmer than the historical average green shading) and more warming under a higher emissions future (the hottest end-of-century projections being about 10☏ warmer than the hottest year in the historical record red shading). Historically unprecedented warming is projected to continue through this century. Observed temperatures are generally within the envelope of model simulations of the historical period (gray shading). Shading indicates the range of annual temperatures from the set of models. Temperatures in Washington (orange line) have risen almost 2☏ since the beginning of the 20th century. Projected changes for 2006–2100 are from global climate models for two possible futures: one in which greenhouse gas emissions continue to increase (higher emissions) and another in which greenhouse gas emissions increase at a slower rate (lower emissions). Figure 1: Observed and projected changes (compared to the 1901–1960 average) in near-surface air temperature for Washington.
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