Climate Change Canada

 

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.