Climate Change Canada

 

4.3: Precipitation

There is medium confidence that annual mean precipitation has increased, on average, in Canada, with larger percentage increases in northern Canada. Such increases are consistent with model simulations of anthropogenic climate change.

Annual and winter precipitation is projected to increase everywhere in Canada over the 21st century, with larger percentage changes in northern Canada. Summer precipitation is projected to decrease over southern Canada under a high emission scenario toward the end of the 21st century, but only small changes are projected under a low emission scenario.

For Canada as a whole, observational evidence of changes in extreme precipitation amounts, accumulated over periods of a day or less, is lacking. However, in the future, daily extreme precipitation is projected to increase (high confidence).

 

Precipitation, as the ultimate source of water for our lands, lakes, and rivers, plays an important role in human society and in shaping and sustaining ecosystems. Human society and natural systems have evolved and adapted to variable precipitation in the past. However, shifts in precipitation beyond its historical range of variability could have profound impacts.

The amount of precipitation varies widely across Canada. The Pacific Coastal and Rocky Mountain ranges of western Canada block much of the moisture brought by westerly winds from the Pacific. As a result, some locations on the west coast receive an average of 3000 mm of precipitation or more in a year. In contrast, the annual mean precipitation can be as low as 300 mm in parts of the Prairies. Because warm air can hold more moisture, the amount of precipitation decreases from south to north, with annual precipitation of only about 200 mm in the far north (Environment Canada, 1995).

Precipitation records for some locations in Canada extend back for more than a century. While the Meteorological Service of Canada has many observational stations at any given time, including more than 2500 stations currently active, only a few hundred stations have continuous long-term records. As with temperature observations, there have been significant changes in observing instruments and/or procedures, including many manned stations having been replaced by automated observing systems. Integrating the data from the manned and automatic observations into one continuous series is challenging, as it requires the accumulation of sufficient data from the new systems to fully understand their characteristics (Milewska and Hogg, 2002). Precipitation measurements have additional challenges when compared with temperature measurements, as they are affected by weather conditions at the time of observation. This is because thermometers are placed in well-protected screens, while precipitation gauges are in the open air. In general, precipitation gauges catch only a portion of precipitation if it is windy, and they become less efficient as wind speed increases (Mekis and Vincent, 2011; Milewska et al., 2018). Additionally, a small amount of precipitation is lost due to evaporation and wetting of the inside of the gauge. Precipitation in the form of snowfall is particularly difficult to observe. A gauge can catch only a small fraction of total snowfall; drifting snow makes it even more complicated to measure snowfall amount. The introduction, over time, of new precipitation gauges has unintentionally introduced data inhomogeneity into the records. The effect of weather conditions and the use of different gauges on observational data need to be carefully adjusted for, to reflect the actual amount of precipitation at a particular site.

Monitoring precipitation over a region is challenging because a gauge measurement is a point observation and thus may not represent precipitation conditions over a large area. As precipitation is sporadic in time and space, point observations of precipitation amount in a day can represent only a very small area surrounding the observational site. However, station observations of precipitation amounts accumulated over longer time periods (a month or a year) can represent larger areas. For example, total precipitation for a season may be interpolated for a location without observations with reasonable accuracy, if the location is within 20 to 120 km from the observational sites, depending on the season (Milewska and Hogg, 2001). Factors such as topography, season, and dominant weather systems all affect the spatial representativeness of point observations of precipitation.

In general, there is insufficient station density in Canada to compute national average precipitation with desirable accuracy; thus, there is low confidence in quantifying regional or national total amounts of precipitation. This is because the distance between observational stations with long-term records (see Figure 4.1) is generally greater than 120 km and because there is a large variation in precipitation over space. In northern Canada, the distance between stations is often more than 1000 km. Locally normalized precipitation (the amount of precipitation divided by its long-term mean) has been used in the past as one alternative. This measure is less variable over space than precipitation amount. As a result, its value at a point location can represent the average over a larger area. Stations with long-term records can provide regional averages for normalized precipitation across southern Canada with reasonable accuracy, although this is not the case for northern Canada (Milewska and Hogg, 2001). As a result, much of the assessment of national or regional changes in precipitation is based on locally normalized precipitation, expressed as a percentage. While this makes it possible to compute some form of national and regional averages, such averages should not be interpreted as normalized spatial averages of precipitation. This is because the local normalization factor is not constant in space.

 

4.3.1: Mean precipitation

4.3.1.1: Observed changes

Averaged over the country, normalized precipitation has increased by about 20% from 1948 to 2012 (Vincent et al., 2015; Figure 4.15 and Table 4.4). The percentage increase was larger in northern Canada — including Yukon, Northwest Territories, Nunavut, and northern Quebec — than in southern Canada. Nonetheless, significant increases were experienced in parts of southern Canada, including eastern Manitoba, western and southern Ontario, and Atlantic Canada. As mean precipitation is typically higher in southern Canada, the absolute amount of precipitation increase is higher in the south, even though the increase in normalized precipitation is smaller in the region. The regional average of normalized precipitation based on the few available long-term data from stations in northern Canada shows an increase of about 30% from 1948 to 2012 (Vincent et al,.2015); confidence in the regional average is low, however. As trends from individual locations in northern Canada are all increasing, there is a medium confidence that annual mean precipitation has increased in this region. Taken together, there is medium confidence that annual precipitation has increased for Canada as a whole. Additionally, the percentage increase in normalized precipitation is larger than what might be expected from the warming-induced increase in water-holding capacity of the atmosphere, leading to doubt over the magnitude of historical trends. There is low confidence in the estimate of the magnitude of the trend.

Precipitation has increased in every season in northern Canada. In southern Canada, precipitation has also increased in most seasons but the increase is generally not statistically significant. However, a statistically significant decrease in winter precipitation has been observed in British Columbia, Alberta, and Saskatchewan.

For long-term observed trends, at the century scale, changes in precipitation can be assessed only for southern Canada, due to the lack of data for northern Canada. An increase was observed over all regions of southern Canada since 1900 and is statistically significant at that spatial scale at the 5% level. Warming has resulted in the proportion of the amount of precipitation falling as snow (i.e., the ratio of snowfall to total precipitation) steadily and significantly decreasing over southern Canada, especially during spring and autumn (Vincent et al., 2015). This is also the case for the Arctic region. There is a pronounced decline in summer snowfall over the Arctic Ocean and the Canadian Arctic Archipelago, and this decline is almost entirely caused by snowfall being replaced by rain (Screen and Simmonds, 2012). Such a change in the form of precipitation, from snow to rain, has profound impacts in other components of the physical environment, such as river flow, with the spring freshet becoming significantly earlier).