Climate data and scenarios: synthesis of recent observation and modelling results, chapter 2

2. Historical climate change and variability in Canada

Climate everywhere varies from season to season, year to year, and decade to decade. This is a natural consequence of the complex interactions between processes in the atmosphere, ocean, and on land. Superimposed on this natural variability is the long-term shift or change in the mean state of the climate (what is commonly referred to as “climate change”). Long-term climate change is driven by both natural and human-caused, or anthropogenic, factors. The key anthropogenic contributors to long-term climate change are changes in atmospheric greenhouse gas concentrations and aerosol loadings. The Earth’s climate has experienced long-term changes in the past. However, it is “extremely likely that human influence has been the dominant cause of the observed warming since the mid-20^th century” (IPCC, 2013).

Averaged globally, temperature has increased by approximately 0.85°C, over the period 1880 to 2012 (IPCC, 2013), although the warming has not been uniform in time or in space. Of particular note is that warming has been greater over high latitudes including Canada and Eurasia. Globally, as climate has warmed, extreme temperatures have also changed with increases in the frequency of hot days and heat waves and decreases in cold days (IPCC, 2013).

Because of natural variations on different time scales, historical changes in the climate need to be assessed over a long period of time. Changes in measurement techniques and instruments, in observing procedures, and in siting of the instruments do occur from time to time and can be reflected in the original climate records. As a result, the proper characterization of past climate change requires the use of homogenized climate data which have been adjusted to address artificial discontinuities which may be present in original historical records. Homogenized climate data sets account for possible artificial shifts imposed by non-climatic factors. For Canada, the adjusted data for some climate variables, including temperature and precipitation, are updated annually and are available publicly:

Adjusted and Homogenized Canadian Climate Data (AHCCD) for daily and monthly temperature and precipitation
Canadian Blended Precipitation, version 0 (CanBPv0)
Canadian Gridded Temperature and Precipitation Anomalies (CANGRD) at 50 km resolution

Additionally, Environment Canada’s Climate Trends and Variations Bulletin (CTVB) summarizes recent Canadian climate data and presents it in a historical context. The CTVB makes use of the adjusted and homogenized Canadian climate datasets to present seasonal, annual, and long-term temperature and precipitation trends on the national and regional scales. The CTVB can be accessed from the climate trends and variations section of Environment Canada’s website.

In Canada, sufficient observations to generate national temperature estimates are available from 1948 onward, and a summary is shown in Figures 1 and 2. These results, when compared with global temperature trends calculated over the same time period, indicate that the rate of warming in Canada as a whole has been more than double that of the global mean, and that warming in northern Canada (i.e., north of 60°N) has been roughly three times the global mean. Longer term trends are available for some locations, especially for southern Canada, with data records extending back more than 100 years.

Figure 1 - Annual mean temperature anomalies and linear trends for the globe, all of Canada, southern Canada (i.e., south of 60°N), and northern Canada (i.e., north of 60°N) over the period 1948-2013 (relative to the 1961-1990 average). See inset for colour scheme. Global temperature anomalies were computed using HadCRUTv4. Canadian mean temperatures were computed using the CANGRD data set (updated from Zhang et al., 2000), which is based on homogenized temperature data from 338 stations in Canada.

Figure 2 - Linear trends in annual mean temperatures (°C) in Canada over the period 1948-2013, as computed from CANGRD data (updated from Zhang, et al., 2000). Note that the northern region has lower station density and as such higher uncertainty in gridded temperature anomalies.

To illustrate long-term changes in temperature at the local level, Table 1 provides estimates of linear trends in annual, summer, and winter mean temperatures for the 1900-2013 period for 16 selected Canadian cities where sufficient data is available (data is available from 1942, 1942, and 1946 for Whitehorse, Yellowknife, and Iqaluit, respectively, and trends for these cities are calculated accordingly). The cities were selected to include Canada’s three largest cities, the national capital, and all provincial and territorial capitals.

Precipitation totals have also changed in Canada as illustrated in Figure 3, with most of the country (particularly the North) having experienced an increase in precipitation over the past century. There are regional exceptions however, such as the lack of significant change over the southern Prairies and northeastern Ontario. Seasonally, total precipitation has increased mainly in the north. In winter, decreasing trends are dominant in the southwestern part of the country (British Columbia, Alberta, and Saskatchewan). There is less evidence of significant changes in the south during spring, summer, and autumn. It should be noted that changes in annual precipitation do not directly relate to changes in water availability, particularly in critical summer periods (e.g., an increase in precipitation does not necessarily translate directly to an increase in water availability, as other factors are also involved).

Figure 3 - Linear trends in annual total precipitation (expressed as percent change relative to the 1961-1990 climatology) for the period 1948-2012 for all of Canada (upper left) and for the period 1900-2012 for southern Canada (lower left). Trends are computed based on CANGRD datasets (updated from Zhang, et al., 2000). Note that the northern region has lower station density and as such higher uncertainty in gridded precipitation anomalies. Also note that precipitation climatology in the north is much smaller than in the south (i.e., the north receives much less precipitation, on average, than the south). As such, a large percentage increase in the north may only represent a small change in total precipitation amounts. The right panels show time series and their 11-year moving averages for Canada (upper right) and for southern Canada (lower right).

The role of anthropogenic forcing in observed warming at global and continental scales has been a subject of intense study for many years. The most recent findings indicate that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20^th century”, that “it is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20^th century”, and that there is medium confidence that “anthropogenic influences have contributed to… intensification of heavy precipitation over land regions where data are sufficient” (IPCC, 2013).