Climate Change Canada

 

4.2.2.1: Observed changes

The annual highest daily maximum temperature, averaged across the country, increased by 0.61ºC between 1948 and 2016 (updated from Wan et al., 2018). The largest increases were in northern Canada, while decreases were observed in the southern Prairies (see Figure 4.10a). The highest daily maximum temperature that occurs once in 20 years, on average, also increased (Wang et al., 2014). The annual lowest daily minimum temperature, averaged across the country, increased by 3.3ºC between 1948 and 2016, with the strongest warming in the west (see Figure 4.10b) (updated from Wan et al., 2018). The lowest daily minimum temperature that occurs once in 20 years, on average, increased more strongly (Wang et al., 2014). Overall, extreme cold temperatures increased much more rapidly than the extreme warm temperatures, consistent with greater warming in winter than in summer, as well as greater warming in night temperatures than in day temperatures.

Indices of high temperature, such as hot days and hot nights, are particularly relevant to public health. Hot days, defined as days with maximum temperature above 30ºC, are rarely observed in the regions north of 60º north latitude. In southern Canada, the number of hot days annually increased by about 1 to 3 days at a few stations over the 1948–2016 period (see Figure 4.10c; also see Vincent et al., 2018). Most locations in Canada are not warm enough to have hot nights, defined as nights with daily minimum temperature above 22ºC, and the number of hot nights has significantly increased only at a few stations in southern Ontario and Quebec.

Warming in winter and spring has resulted in a significant decrease in the number of frost days (days with daily minimum temperature of 0ºC or lower) and ice days (days with daily maximum temperature of 0ºC or lower), as well as shortened winter seasons (Vincent et al., 2018). Averaged for the country as a whole, frost days have decreased by more than 15 and ice days by more than 10 days from 1948 to 2016. These changes are consistent across the country. As a result, the frost-free season has been extended by 20 days, starting about 10 days earlier and ending about 10 days later. Heating degree days have decreased while cooling degree-days have increased (see Figure 4.10e and f). The length of growing seasons (see Figure 4.10d) and the number of growing degree days have also increased. The growing season, which starts when there are six consecutive days with daily mean temperature above 5ºC in spring or summer and ends when this condition fails to be met late in the year, started earlier and ended later, resulting in an increase in growing season length of about 15 days between 1948 and 2016. With the longer growing season, the number of growing degree days increased.

 

4.2.2.2: Causes of observed changes

It is very likely that anthropogenic forcing has contributed to the observed changes in the frequency and intensity of daily temperature extremes on the global scale since the mid-20th century (Bindoff et al., 2013; see also Chapter 2, Section 2.3.4). Several detection studies have shown that the annual lowest daily minimum temperature (Zwiers et al., 2011; Min et al., 2013; Kim et al., 2015) and the annual highest daily maximum temperatures (Wang et al., 2017) have been influenced by human activity in three subregions of North America. In Canada, an increase of 3.2ºC in the annual lowest daily minimum temperature was observed from 1948 to 2012 (Wan et al., 2018). Only a small fraction (about 0.5ºC) of this increase can be related to natural internal climate variability, and anthropogenic influence may have contributed as much as 2.8ºC (likely range 1.5º to 4.2ºC) to the warming (see Figure 4.5). In addition, much of the observed warming seen in the annual highest daily maximum temperature may also be attributable to anthropogenic influence. Overall, most of the observed increase in the coldest (likely) and warmest (high confidence) daily temperatures of the year in Canada from 1948 to 2012 can be attributed to anthropogenic influence.

While there is a lack of studies directly attributing observed changes in other temperature indices, there is high confidence that substantial parts of the observed changes in most of these temperature indices are also due to anthropogenic influence. It is more difficult to detect anthropogenic influence in values such as annual lowest daily minimum temperature, which are sampled once a year, than in other temperature indices that integrate information from many data samples in a year. These indices are less affected by natural internal variability, while nevertheless retaining the climate responses to external forcing.

 

4.2.2.3: Projected changes and uncertainties

The models used to make projections of future climate are discussed in Chapter 3, Section 3.3. When using climate model projections for impact studies, it is often important to consider that the model simulated current climate may differ from observed climate — a reflection of model biases (Flato et al. 2013). Many temperature indices are connected to absolute thresholds (like the freezing temperature), and, so, mean biases can substantially alter their usefulness. As a result, where absolute values are important, some form of bias correction is needed. This is a method of correcting the model output to remove, to the extent possible, the influence of model biases. The assessments of projected changes in temperature indices discussed in this subsection are, unless otherwise stated, based on statistically downscaled and bias-corrected data.

Daily extreme temperatures, hot and cold, are projected to increase substantially (see Figure 4.11). Annual highest daily maximum temperature is projected to track the projected changes in summer mean temperature, but at a slightly higher rate (the largest difference between the two is less than 0.5ºC, appearing in 2081–2100 under a high emission scenario [RCP8.5]). Annual lowest daily minimum temperature is projected to warm faster than winter mean temperature over most of Canada, increasing the extreme minimum temperature in southern Canada by about 3ºC by the end of the century under a high emission scenario (RCP8.5). Table 4.3 summarizes projected changes in Canada. For example, averaged over the country, the annual highest daily maximum temperatures are projected to increase by 1.4ºC over the 2031–2050 period under a low emission scenario (RCP2.6), and by 2ºC for the same period under a high emission scenario (RCP8.5) compared with the current climate (1986–2005). The corresponding projected increase in 2081–2100 under the low emission scenario (RCP2.6) is 1.5ºC, only slightly higher than the increases in 2031–2050. A much larger increase, of about 6ºC, is expected in 2081–2100 under the high emission scenario (RCP8.5).

In addition to changes in magnitude, the frequency of certain temperature extremes is also expected to change. Extreme hot temperatures are expected to become more frequent, while extreme cold temperatures less frequent. For example, under a high emission scenario (RCP8.5), the annual highest daily temperature that would currently be attained once every 10 years, on average, will become a once in two-year event by 2050 — a five-fold increase in frequency. The annual highest daily temperature that occurs once every 50 years in the current climate is projected to become a once in five-year event by 2050 — a 10-fold increase in frequency. These projected changes indicate not only more frequent hot temperature extremes, but also relatively larger increases in frequency for more rare events (e.g., 10-year extreme versus 50-year extreme; see also Kharin et al., 2018).

The projected increase in the number of hot days is substantial. In regions that currently experience hot days, the increase may be more than 50 days by the late century under RCP8.5 (see Figure 4.13a). Areas with hot days will progressively expand northward, depending on the level of global warming. The number of frost days and ice days is projected to decrease, with projections ranging from about 10 fewer days in 2031–2050 under the low emission scenario (RCP2.6) to more than 40 fewer days in 2081–2100 under the high emission scenario (RCP8.5) (see Table 4.3) The length of the growing season (see Figure 4.13b) and the number of cooling degree days (see Figure 4.13c) are projected to increase, while heating degree days (see Figure 4.13d) are projected to decrease (see Table 4.3).