Because air in the Earth’s atmosphere and water in the global oceans flow freely, Canada’s climate is intimately linked to the global climate. Thus, changes in Canada’s climate are a manifestation of changes in the global system, modulated by the effects of Canada’s mountains, coastlines, and other geographical features. For example, a robust feature of both observed and projected global-scale climate change is the amplification of warming at high northern latitudes (so-called Arctic amplification), which means Canada’s climate is expected to warm more than the global average (see Chapter 2, Section 2.2 and Chapter 3, Section 3.3). Canadian temperature has increased, and is projected to increase further, at almost double the rate of global mean temperature (see Chapter 2, Section 2.2 and Chapter 4, Section 4.2). Precipitation changes in Canada are also closely linked to global-scale changes, such as the overall intensification of the global water cycle, the increase in high-latitude precipitation, and the intensification of precipitation extremes that are projected as a result of greenhouse gas increases. Precipitation has increased in Canada since mid-century, particularly in northern parts of the country. Many other aspects of climate that are important to Canadians are also changing as a consequence of global-scale climate change. These changes include the extent and duration of snow and ice cover, permafrost temperatures, freshwater availability, fire weather, other extremes of weather and climate, sea level, and other properties of the oceans surrounding Canada (Chapters 4 to 7). Locations of places in Canada referred to in this chapter are shown in Figure 8.1.
There is no doubt that Canada’s climate has warmed. Temperature has increased in all regions of the country and in the surrounding oceans. Since 1948, Canada’s annual average surface air temperature over land has warmed by 1.7°C (best estimate), with higher temperature increases observed in the North, the Prairies, and northern British Columbia (see Chapter 4, Section 4.2.1). The greatest warming has occurred in winter. Human influence is likely the main cause of the observed increase in Canada’s temperature, as more than half of the observed warming in annual temperature in Canada can be attributed to human influence (Chapter 4, Section 4.2.1). Temperature extremes are also changing, consistent with the increase in mean temperature. Extreme warm temperatures have become hotter, while extreme cold temperatures have become less cold (see Chapter 4, Section 4.2.2). Overall, there is high confidence that most of the observed increase in the coldest and warmest daily temperatures in Canada (1948–2012) can be attributed to anthropogenic influence. Warming has also been demonstrated to have led to an increased risk of extreme fire weather in parts of western Canada (see Chapter 4, Section 4.3 and Box 4.2).
Observed changes in snow and ice features across Canada provide a coherent picture of a warming climate: fall and spring snow cover and summer sea ice extent are decreasing; glaciers are losing extent and mass; and permafrost is warming (see Chapter 5, Sections 5.2.1, 5.3.1, 5.4.1, and 5.6.1). Changes in relative (or local) sea level at locations along Canadian coastlines are driven primarily by the observed rise in average global sea level — a response to global-scale warming — and by vertical land motion (i.e., land uplift or subsidence). As a result, some coastal regions have experienced a larger relative sea-level rise compared to average global sea level, while others have experienced more modest increases or even decreases in relative sea level (see Chapter 7, Section 7.5).
Canada’s climate will warm further, with warming projected in all seasons. Projected warming for Canada as a whole is almost double that of the global average, regardless of the emission scenario (see Chapter 3, Section 3.3.3, and Chapter 4, Section 18.104.22.168). Country-wide annual average temperature projections for the late century (2081–2100) range from an increase of 1.8°C (1.1, 2.5) for a low emission scenario (RCP2.6) to 6.3°C (5.6, 7.7) for a high emission scenario (RCP8.5), compared to the base period 1986–2005. Further warming of extreme warm and cold temperatures is projected to be substantial (see Chapter 4, Section 22.214.171.124). In the future, higher temperatures will contribute to an increased risk of extreme fire weather across much of Canada. It is very likely that snow cover duration will decline to mid-century across Canada due to increases in temperature under all emission scenarios. Projections with a high emission scenario show continued snow loss after mid-century (high confidence) (see Chapter 5, Section 5.2.2). Oceans surrounding Canada are projected to continue to warm over the 21st century, in response to past and future emissions of greenhouse gases, with the size of the increase depending on the emission scenario. The warming in summer will be greatest in the ice-free areas of the Arctic and off southern Atlantic Canada, where subtropical water is projected to shift further north (medium confidence). Atlantic Canada will be the region of Canada’s oceans that will warm the most during winter (medium confidence) (see Chapter 7, Section 7.2.2).
Canada’s annual precipitation has increased in all regions since 1948, with relatively larger percentage increases in northern Canada and parts of Manitoba, Ontario, northern Quebec, and Atlantic Canada, although there is low confidence in observed regional precipitation trends. Mean precipitation has also increased in all seasons, except during winter in British Columbia and the western Prairies (see Chapter 4, Section 4.3.1). As a result of warming, snowfall has been reduced as a proportion of total precipitation in southern Canada. Seasonal snow accumulation has declined over the period of record (1981–2015) on a country-wide basis (medium confidence) (see Chapter 5, Section 5.2.1). The most significant observed changes in freshwater availability are in the seasonal distribution of streamflow in many snow-fed catchments: winter flows have become higher the timing of spring peak flows has become earlier, and there has been an overall reduction in summer flows (high confidence) (see Chapter 6, Sections 6.2.1, 6.2.2, and 6.2.3). However, many other indicators — annual streamflow magnitudes, surface and shallow groundwater water levels, soil moisture content and droughts — have, for the most part, been variable, with no clear increasing or decreasing trends (see Chapter 6, Sections 6.2.1, 6.3, 6.4, and 6.5). This variability corresponds to observed year-to-year and multi-year variations in precipitation, which are influenced by naturally occurring large-scale climate variability (see Chapter 2, Box 2.5).
In the future, annual and winter precipitation is projected to increase in all regions, with larger relative changes for the North. Summer precipitation shows relatively smaller changes and is projected to decrease in southern regions of Canada by the end of the century under a high emission scenario (see Chapter 4, Section 4.3.1). Daily extreme precipitation (that is, changes in extreme precipitation amounts accumulated over a day or less) is projected to increase; thus, there is potential for a higher incidence of rain-generated local flooding, including in urban areas (high confidence) (see Chapter 6, Section 6.2.4). Significant reductions in seasonal snow accumulation are projected through to mid-century for much of southern Canada due to warming surface temperatures, while only small changes are projected for northern Canada because winter temperatures will remain sufficiently cold despite overall warming (see Chapter 5, Section 5.2.2). In association with warmer temperatures, seasonal changes in streamflow are expected to continue, including shifts from more snowmelt-dominated regimes toward rainfall-dominated regimes. Shifts toward earlier snowmelt-related floods, including those associated with spring snowmelt, ice jams, and rain-on-snow events, are also anticipated. However, changes to the frequency and magnitude of future snowmelt-related floods are uncertain (see Chapter 6, Section 6.2.1, 6.2.3, and 6.2.4). Freshening of the ocean surface is projected in most Canadian waters over the rest of this century due to increases in precipitation and melting of land and sea ice. However, salinity is expected to increase in waters off the continental shelf south of Atlantic Canada as a result of a northward shift of subtropical water. The freshening in the upper layers of the ocean, along with warming, is expected to increase the “vertical stratification” (changes in density of ocean water at greater depths), which will affect the oceans’ ability to sequester greenhouse gases, dissolved oxygen levels, and marine ecosystems (see Chapter 7, Section 7.3.2).