4.2: Temperature
It is
virtually certain that Canada’s climate has warmed and that it will warm
further in the future. Both the observed and projected increases in mean
temperature in Canada are about twice the corresponding increases in the global
mean temperature, regardless of emission scenario.
Annual and
seasonal mean temperatures across Canada have increased, with the greatest
warming occurring in winter. Between 1948 and 2016, the best estimate of mean
annual temperature increase is 1.7ºC for Canada as a whole and 2.3ºC for
northern Canada.
While both
human activities and natural variations in the climate have contributed to the
observed warming in Canada, the human factor is dominant. It is likely13 that
more than half of the observed warming in Canada is due to the influence of
human activities.
Annual and
seasonal mean temperature is projected to increase everywhere, with much larger
changes in northern Canada in winter. Averaged over the country, warming
projected in a low emission scenario is about 2ºC higher than the 1986–2005
reference period, remaining relatively steady after 2050, whereas in a high
emission scenario, temperature increases will continue, reaching more than 6ºC
by the late 21st century.
Future
warming will be accompanied by a longer growing season, fewer heating degree
days, and more cooling degree days.
Extreme
temperature changes, both in observations and future projections, are
consistent with warming. Extreme warm temperatures have become hotter, while
extreme cold temperatures have become less cold. Such changes are projected to
continue in the future, with the magnitude of change proportional to the magnitude
of mean temperature change.
Temperatures
referred to in this chapter are surface air temperatures, typically measured
2 m above the ground, which have an immediate effect on human comfort and
health, play an important role in determining the types of crops a farmer can
grow, and influence the functioning of local ecosystems. Temperatures in Canada
vary widely across the country. The lowest temperature on record is −63ºC,
observed at Snag, Yukon, on February 3, 1947. The highest temperature on record
is 45ºC, observed at Midale and Yellow Grass, Saskatchewan, on July 5, 1937.
Annual mean temperature provides a simple measure of the overall warmth of a
region: it varies from about 10ºC in some southern regions to about −20ºC in
the far north. Seasonally, this variability is even more pronounced. Winter
averages range from −5ºC in the south to about −35ºC in the far north, while
summer averages vary from about 22ºC in the south to 2ºC in the far north
(Gullett and Skinner, 1992).
In some
locations in Canada, temperatures have been observed for a long time. For
example, an observing site in Toronto has provided continuous daily temperature
records since 1840. Multiple sites have temperature records that date back a
century or longer. However, the availability of temperature data is unevenly
distributed across the country or over different time periods. Observation
sites are relatively densely distributed in the populated portion of southern
Canada, while, for much of Canada, especially northern Canada, observations are
sparse (see Figure 4.1), and very few observation sites predate 1948. As a
result, the analysis of past changes in temperature for Canada as a whole is
limited to the period since 1948, while 1900 can be used as a starting point
for records in southern Canada (Vincent et al., 2015; DeBeer et al., 2016).
Temperature
is also a key indicator of the climate response to human emissions of
greenhouse gases (GHGs), as increasing GHG concentrations result in warming of
the lower atmosphere (see Chapter 2, Section 2.3). While the original purpose
of historical observations was to monitor daily to seasonal climate variability
and support weather prediction, today these observations also support climate
change impact studies and climate services. Monitoring instruments,
observational sites, and their surrounding environment, as well as observation
procedures, have undergone changes over the past century to meet new needs and
to introduce new technology. These changes also introduce non-climatic changes,
referred to as “data inhomogeneities,” in data records. Inhomogeneities affect
the reliability of long-term trend assessment if not accounted for (Milewska
and Vincent, 2016; Vincent et al., 2012, see Box 4.1). In particular, the
reduction in the number of manned observational sites, with many being
converted to automatic stations, has necessitated the integration of data from
these different sources, which has proven challenging. Changes identified in
the historical data archive reflect changes in both climate and data
inhomogeneity (Vincent et al., 2012). Techniques for removing climate data
inhomogeneity (“climate data homogenization”) have been developed to identify
such artifacts in climate records and remove them (see Box 4.1; Vincent et al.,
2002, 2012, 2017; Wang et al., 2007, 2010).
4.2.1: Mean temperature
4.2.1.1: Observed changes
The annual
average temperature in Canada increased by 1.7ºC (likely range 1.1ºC –2.3ºC17)
between 1948 and 2016 (updated from Vincent et al., 2015; Figure 4.3 and Table
4.1), roughly twice the increase observed for the Earth as a whole (0.8ºC for
1948–2016 according to the global mean surface temperature dataset produced by
the Met Office Hadley Centre and the Climatic Research Unit at the University
of East Anglia, UK, HadCRUT4 [Osborn and Jones, 2014]). Warming was not uniform
across seasons, with considerably more warming in winter than in summer. The
mean temperature increased by 3.3ºC in winter, 1.7ºC in spring, 1.5ºC in
summer, and 1.7ºC in autumn between 1948 and 2016 (see Figure 4.4 and Table
4.1). The changes in temperatures are significant at the 5% level (i.e., there
is only a 5% possibility that such changes are due to chance). As well, warming
was unevenly distributed across the country. The largest increases in the
annual mean temperature were in the northwest, where it increased by more than
3ºC in some areas. Annual mean temperature over northern Canada increased by
2.3ºC (likely range 1.7 ºC–3.0ºC) from 1948 to 2016, or roughly three times the
global mean warming rate. Warming was much weaker in the southeast of Canada,
where average temperature increased by less than 1ºC in some maritime areas.
Winter warming was predominant in northern British Columbia and Alberta, Yukon,
Northwest Territories, and western Nunavut, ranging from 4ºC to 6ºC over the
1948–2016 period. Spring had a similar warming pattern, but with smaller
magnitude. Summer warming was much weaker than that in winter and spring, but
the magnitude of the warming was generally more uniform across the country than
during other seasons. During autumn, most of the warming was observed in the
northeast regions of Canada (mainly in northern Northwest Territories, Nunavut,
and northern Quebec). In addition to higher temperatures, the reduction in snow
cover (see Chapter 5) and earlier snowmelt (see Chapter 6) also indicate Canada
has warmed.
In southern
Canada, annual mean temperature increased by 1.9ºC between 1900 and 2016
(updated from Vincent et al., 2015). This warming is significant at the 5%
level. This temperature did not rise steadily over time. Temperature increased
until about the 1940s, decreased slightly until 1970, and then increased
rapidly through 2016. This long-term behaviour of temperature is consistent
with that observed globally (see Chapter 2, Section 2.2.1; Hartmann et al.,
2013), but the magnitude of warming in Canada is larger. Mean temperature in
southern Canada increased by 2.8ºC in winter, 2.2ºC in spring, 1.7ºC in summer,
and 1.6ºC in autumn during the same period.
4.2.1.2: Causes of observed changes
It is
extremely likely that human activities have caused more than half of the
observed increase in global mean surface temperature from 1951 to 2010 (Bindoff
et al., 2013). This causal effect was established through detection and
attribution analysis, comparing the observed changes with the natural internal
climate variability and with the expected climate responses to human activities
(see Chapter 2, Section 2.3.4). Changes in the climate become detectable if
they are large when compared with natural internal climate variability, and the
change is attributed to human activity if it is (1) consistent with the expected
“fingerprint” of human-caused change, as simulated by climate models (see
Chapter 3); and (2) inconsistent with other plausible causes. For Canada and
the Arctic, where natural internal variability of temperature is high,
attribution of observed warming is more difficult than it is on a global scale.
Nevertheless, evidence of anthropogenic influence on Canadian temperature has
emerged (Gillett et al., 2004; Zhang et al., 2006; Wan et al., 2018), with a
detectable contribution to warming in annual and seasonal temperatures and in
extreme temperatures.
Two modes of
natural internal climate variability that affect temperatures in Canada are the
Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) (see
Chapter 2, Box 2.5). About 0.5ºC of the observed warming of 1.7ºC over the
1948–2012 period can be explained by a linear relationship between the PDO and
the NAO. Assuming this is completely due to natural climate variability,
roughly 1.1ºC (likely range 0.6ºC–1.5ºC) of the observed 1.7ºC increase in
annual mean temperature in Canada from 1948–2012 can be attributed to human
influence (see Figure 4.5; Wan et al., 2018). There is a 33% probability that
anthropogenic influence increased Canadian temperature by at least 0.9ºC. It is
likely that more than half of the observed warming in Canada is due to human
influence. The effects of natural internal climate variability on Canadian
temperature trends differ in different parts of Canada, enhancing the warming
trend in the western Canada and reducing the warming trend in eastern Canada
over the past half of the 20th century (Vincent et al., 2015). The detection of
anthropogenic influence on Canadian temperature is also corroborated by other
independent evidence, including the attribution of Arctic temperature change to
the influence of GHGs and aerosols (Najafi et al., 2015). The reduction in
spring snow pack and the ensuing reduction in summer streamflow in British
Columbia have been attributed to anthropogenic climate change (Najafi et al. 2017a,
2017b; see Chapter 6, Section 6.2.1). Anthropogenic warming has also increased
fire risk in Alberta.
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