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).
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