RESULTS and DISCUSSION

Mean annual temperature (MAT)

        Mean annual temperature in BC fluctuated throughout the 20^th century.  Decadally-averaged MAT declined between 1901 and 1920 and then rose from the 1920s through the 1940s.  Average mean temperatures declined marginally in the 1950, 60s and 70s before rising to unprecedented levels in the 1980s and 1990s (Table 1, Figure 1).  These temperature trends correspond strongly with the Pacific Decadal Oscillations of the 20^th century (“cool” regime from 1890-1924 and from 1947-1976; “warm” regime from 1925-1946 and from 1977 through at least the mid-1990’s; see 20^th Century Climate Trends).  

There was a 1.2°C increase in minimum mean annual temperature between 1901-1910 and 1991-2000, while maximum MAT rose by only 0.8°C during the same time period.  This corresponds with the increasing minimum temperature trends documented by the Ministry of Land, Water and Air Protection.  Standard deviations declined marginally as the century progressed, perhaps correlated to increased distribution and precisions of weather stations temperature records.

Table 1: Decadally-averaged mean annual temperature statistics, 1901-2000

Figure 1: Decadally-averaged Mean Annual Temperature statistics, 1901-2000

Number of frost free days (NFFD)

        The number of frost free days per year in BC increased in a linear fashion between 1901-1910 and 1991-2000, from 131.53 to 150.47 (Table 2, Figure 2).  The maximum number of frost-free days increased by 17 over the course of the century, whereas the minimum number increased by 25.  This is in keeping with the trend towards increased minimum temperatures in BC over the 20^th century.  Standard deviations remained relatively consistent throughout the century.

Table 2: Decadally-averaged Number of Frost-free Days statistics, 1901-2000

Figure 2: Decadally-averaged Number of Frost-free Days statistics, 1901-2000

Summer heat:moisture index (SHM)

        There was a general trend towards decreasing mean SH:M indices throughout the 20^th century in BC, with the notable exception of high means in the 1910s and 1920s.  Minimum SH:M indices remained relatively stable throughout the century, while maximum indices increased dramatically in the 1920s and 30s before starting a moderate decline.  This time period corresponds to the famous western Canada drought of the 1930s.  Similar peaks may be seen in 2001-2010 data records due to the summer droughts of the last few years.  Standard deviations peaked in the 20s and 30s as well, indicating high variability in temperature and/or precipitation during those decades.

Table 3: Decadally-averaged Summer Heat:Moisture Index statistics, 1901-2000

Figure 3: Decadally-averaged Summer Heat:Moisture Index statistics, 1901-2000