Climate Change Canada

 

4.3.1.2: Causes of observed changes

There is medium confidence that there is a human-caused contribution to observed global-scale changes in precipitation over land since 1950 (Bindoff et al., 2013). Much of the evidence of human influence on global-scale precipitation results from precipitation increases in the northern mid- to high latitudes (Min et al., 2008; Marvel and Bonfils, 2013; Wan et al., 2014). This pattern of increase is clear in climate model simulations with historical forcing (e.g., Min et al., 2008) and in future projections (Collins et al., 2013). Observed precipitation in northern high latitudes, including Canada, has increased and can be attributed — at least in part — to external forcing (Min et al., 2008; Wan et al., 2014). Atmospheric moisture increases with warming in both observations and model simulations. Natural internal climate variability from decade to decade contributes little to the observed changes (Vincent et al., 2015). This evidence, when combined, leads us to conclude that there is medium confidence that the observed increase in Canadian precipitation is at least partly due to human influence.

 

4.3.1.3: Projected changes and uncertainties

Multi-model projections of percentage changes (relative to 1986–2005) in winter, summer, and annual precipitation in Canada are shown in Figures 4.17, 4.18, and 4.19. The figures include maps of change for the low emission scenario (RCP2.6) and high emission scenario (RCP8.5) for the near term (2031–2050) and late century (2081–2100), and a national average time series of the normalized local changes for Canada as a whole for the period 1900–2100. Unlike for temperature, which is projected to increase everywhere in every season, precipitation has patterns of increase and decrease. In the near term, a small (generally less than 10%) increase in precipitation is projected in all seasons, with slightly larger values in northeastern Canada. In the late century (2081–2100), under the high emission scenario, the changes are much larger, with extensive areas of increased precipitation in northern Canada (more than 30% of the annual mean in the high Arctic). Since annual mean precipitation is low in the Arctic, even modest changes in absolute amount translate into a large percentage change. In contrast, large areas of southern Canada are projected to see a reduction in precipitation in summer under the high emission scenario (RCP8.5); for example, a median reduction of more than 30% is projected for southwestern British Columbia (see Figure 4.18). The projected decrease in summer precipitation (also projected in other parts of the world) is a consequence of overall surface drying and changes in atmospheric circulation (Collins et al., 2013).

As was the case for temperature, the national average time series for precipitation in the lower panels of the three figures show relatively small differences between the low emission scenario (RCP2.6) and high emission scenario (RCP8.5) in the near term (2031–2050). The winter season precipitation changes projected under the two scenarios diverge somewhat by the late century, while the summertime changes are near zero over the entire century, regardless of emission scenario. This small change in national average of locally normalized precipitation hides the fact that summertime precipitation changes are projected to be large (and hence impactful) in many areas of Canada. The large percent increases in northern Canada are generally offset by the large percent decreases in southern Canada, so that the average of percent changes for Canada as a whole in the time series plots shows little overall change in summer precipitation. As mean precipitation is much larger in southern Canada than in northern Canada, the absolute amount of precipitation decrease in the southern Canada is larger than the absolute value of precipitation increase in northern Canada. Regional differences are clearly important for impact studies, and quantitative information at the regional level is provided in Table 4.5. In general, changes in precipitation exhibit more temporal and regional variation than changes in temperature, and, so, projection results for precipitation have less confidence than projection results for temperature.

As the climate warms, particularly in northern Canada, there will inevitably be an increased likelihood of precipitation falling as rain rather than snow. This is consistent with the observed changes in the snowfall fraction noted earlier. Although there has not been a systematic analysis for Canada, one analysis projected a decrease in the fraction of precipitation falling as snow, especially in the autumn and spring, for southern Alaska and eastern Quebec (Krasting et al., 2013). In addition, regional climate model projections show a general increase in rain-on-snow events over the coming century (Jeong and Sushama, 2017).

These results for changes in mean precipitation are consistent with the IPCC Fifth Assessment, in that the high latitudes are projected to experience a large increase in annual mean precipitation by the late of this century under the high emission (RCP8.5) scenario. The projected increase in annual mean precipitation in the high latitudes is a common feature of generations of climate models. It can be explained by the expected warming-induced large increase in atmospheric water vapour (Collins et al. 2013). Over the historical period, an increase in annual total precipitation in the high latitudes has been detected and can be attributed to human influence (Min et al., 2008; Wan et al., 2014). There is high confidence in the projected increase in annual mean precipitation. Confidence in projected changes in seasonal mean precipitation is lower. It should be noted that models generally project less summertime precipitation for southern Canada under a high emission scenario.

 

4.3.2: Extreme precipitation

Mean precipitation over a day or less can cause localized damage to infrastructure, such as roads and buildings, while heavy multi-day episodes of precipitation can produce flooding over a large region. This section assesses only changes in short-duration (a day or less) extreme precipitation, for which there is relatively more data and research than for longer-duration extremes.

 

4.3.2.1: Observed changes

There do not appear to be detectable trends in short-duration extreme precipitation in Canada for the country as a whole based on available station data. More stations have experienced an increase than a decrease in the highest amount of one-day rainfall each year, but the direction of trends is rather random over space. Some stations show significant trends, but the number of sites that had significant trends is not more than what one would expect from chance (Shephard et al., 2014; Mekis et al., 2015; Vincent et al., 2018). This seems to be inconsistent with global results (Westra et al., 2013) and the results for the contiguous region of the United States (Barbero et al., 2017). The number of days with heavy rainfall18 has increased by only 2 to 3 days since 1948 at a few locations in southern British Columbia, Ontario, Quebec, and the Atlantic provinces (Vincent et al., 2018). The number of days with one hour total rainfall greater than 10 mm, with 24-hour total rainfall greater than 25 mm, or with 48-hour total rainfall greater than 50 mm also did not show any consistent change across the country (Mekis et al., 2015). Days with heavy snowfall19 have decreased by a few days at numerous locations in western Canada (British Columbia to Manitoba), while they have increased at several locations in the North (Yukon, Northwest Territories, and western Nunavut). The highest one-day snowfall amount has decreased by several millimetres (snow water equivalent) at several locations in the southern region of British Columbia and Alberta (Mekis et al., 2015; Vincent et al., 2018).

The lack of a detectable change in extreme precipitation in Canada is not necessarily evidence of a lack of change. On one hand, this is inconsistent with the observed increase in mean precipitation. As the variance of precipitation is proportional to the mean, and as there is a significant increase in mean precipitation, one would expect to see an increase in extreme precipitation. On the other hand, the expected change in response to warming may be small when compared with natural internal variability. Warming has resulted in an increase in atmospheric moisture, which is expected to lead to an increase in extreme precipitation if other conditions, such as atmospheric circulation, do not change. On the global scale, observations indicate an increase in extreme precipitation associated with warming. Moreover, the increase can be attributed to human influence (Min et al., 2011; Zhang et al., 2013). The median increase in extreme precipitation is about 7% per 1ºC increase in global mean temperature, consistent with the increase in the water-holding capacity of the atmosphere due to warming (Westra et al., 2013). Compared with the natural internal variability of precipitation, this amount of increase would be too small to be detectable at individual locations. Only about 8.5% of all stations over global land areas with more than 30 years of data show an increase in extreme precipitation at the 5% significance level, which is slightly higher than the rate of stations showing an increase (5%) that could be expected from chance (Westra et al., 2013). The detection of the increasing intensity of extreme precipitation over lands on Earth is possible because of the vast amount of data available. On the regional scale, there is much less information, which is the case for Canada, where long-term observations are very limited, and detection becomes more difficult.