Climate Change Canada

 

2.2: Observed Changes In The Global Climate System

Warming of the climate system during the Industrial Era is unequivocal, based on robust evidence from a suite of indicators. Global average temperature has increased, as have atmospheric water vapour and ocean heat content. Land ice has melted and thinned, contributing to sea level rise, and Arctic sea ice has been much reduced.

The global climate system comprises a number of interacting components, encompassing the atmosphere, hydrosphere (liquid water in oceans, lakes, rivers, etc.), cryosphere (snow, ice, and frozen ground), biosphere (all living things on land and in water), and the land surface. Long-term changes that are consistent with an overall warming of the climate system can be observed throughout the various components of the system. In this section, observed changes in global mean surface temperature (GMST), precipitation, the cryosphere, and oceans are reviewed. These changes are summarized from Chapters 2, 3, and 4 of the IPCC AR5 (Hartmann et al., 2013; Rhein et al., 2013; Vaughan et al., 2013). More recent observations indicate a general continuation of warming and related changes, with short-term year-to-year variability evident, as it is in the earlier record (Blunden and Ardnt, 2017; USGCRP, 2017).

“Climate” can be considered the average, or expected, weather and related atmospheric, land, and marine conditions for a particular location. Climate statistics are commonly calculated for 30-year periods, as recommended by the World Meteorological Organization. “Climate change” refers to a persistent, long-term change in the state of the climate, measured by changes in the mean state and/or its variability (IPCC, 2013c). Measuring climate change therefore requires long-term observations of climate parameters so that long-term trends can be distinguished from shorter-term variations.

Changes in the frequency, intensity, and duration of climate and weather extremes4 are expected to accompany a changing climate. These changes can have large impacts on human and natural systems. For some types of extremes (e.g., hot and cold days/nights), changes in frequency are a natural consequence of a shift toward a warmer climate on average. For other extremes, the factors underlying expected changes are more complicated and can involve changes in the water cycle, ocean temperatures, atmosphere-ocean circulation, and other factors.

Quantifying changes in many extremes of climate and weather is more challenging than quantifying changes in mean climate conditions, for several reasons (IPCC, 2012). By definition, extremes occur infrequently. Therefore, observational data spanning many decades or longer are needed to derive adequate statistics about the historical occurrence rate of extremes, but these are often lacking.

 

2.2.1: Global annual and extreme temperature changes

Global-scale records of surface temperatures, based on thermometer observations of surface air temperatures over land and measurements of sea surface temperatures, are available from the late 19th century onwards. From these observations, various research groups have developed global temperature datasets using different procedures for processing the available raw data, such as the treatment of gaps in observations. Based on these independently produced global temperature datasets, a best estimate of GMST has been calculated, which represents changes over both land and ocean. This estimate shows that GMST rose 0.85ºC over the period 1880–2012 (based on a linear trend, with a 90% uncertainty range between 0.65ºC and 1.06ºC) (Hartmann et al., 2013). The last three full decades (1980–2010) have been the warmest on record, with the longest dataset extending back to 1850 (Hartmann et al., 2013). Global temperatures in the last three years with complete records (2015, 2016, and 2017) are the three warmest years on record for the globe on average (WMO, 2018), at more than 1ºC above pre-industrial average levels (Blunden and Arndt, 2016, 2017; WMO, 2017, 2018; Hawkins et al., 2017).

Annual GMST has not increased in a steady linear progression since the late 19th century. During several periods, warming was more pronounced (e.g., 1900−1940 and 1970 onwards) or less pronounced (e.g., 1940−1970). These fluctuations arise from natural variations within the climate system (internal climate variability) and outside (external) forces, including human factors.

Almost the entire globe experienced warming on a century scale (1901–2012). This warming was not uniform from one region of the Earth to another, owing to a range of factors, including internal climate variability, and regional variations in climate feedbacks and heat uptake (Hartman et al., 2013). In general, warming has been strongest at high northern latitudes and stronger over land than oceans. Since Canada has a large land mass, much of which is located at high northern latitudes, warming across Canada has been about twice the global average.

Cold and warm extremes of temperature can have large impacts on human and natural systems. Based on multi-decadal observational datasets and rigorous statistical analysis, the IPCC AR5 reports that, for global land area as a whole, the number of warm days and nights5 very likely increased and the number of cold days and nights very likely decreased over the period 1951–2010. Robust statistical assessment of heat waves and warm spells is more challenging. The IPCC AR5 assesses with medium confidence that, since the mid20th century, the length and frequency of warm spells, including heatwaves,6 has increased for global land areas as a whole (Hartmann et al., 2013). At the continental scale, it is likely that heatwave frequency has increased in some regions of Europe, Asia, and Australia over this period. For North America and Central America, there is medium confidence that more regions have experienced increases in heatwaves and warm spells than have experienced decreases (Hartmann et al., 2013).

 

2.2.2: Global annual and extreme precipitation and related hydrological changes

Increasing global temperatures have impacts on the hydrological (water) cycle. The amount of moisture the atmosphere can hold increases with rising temperatures (about 7% per degree Celsius of warming). It is very likely that global specific humidity — a measure of the amount of water vapour in the air — near the surface and in the troposphere7 has increased since the 1970s, consistent with the observed temperature increase over this period (Hartmann et al., 2013).

The effects of increasing atmospheric concentrations of greenhouse gases (GHGs) on the hydrological cycle and precipitation are more complex than for temperature. Precipitation varies substantially over time and space, to a greater extent than does temperature. Long-term precipitation trends are smaller, compared to the range of precipitation variability, than are temperature trends relative to the range of temperature variability. Therefore, a greater density of monitoring stations with long records of precipitation is required for robust assessment of precipitation trends than is the case for temperature. Owing to a lack of data, there is low confidence in estimates of precipitation changes over land at the global scale before 1951 and medium confidence thereafter. Annual average precipitation for global land areas increased slightly over the period 1901–2008, and different datasets vary in the magnitude of observed changes (Hartmann et al., 2013). It remains a challenge to determine long-term trends in precipitation for the global oceans. At the regional scale, average annual precipitation for the mid-latitude land area in the Northern Hemisphere shows a likely overall increase since 1901, with medium confidence before 1951 and high confidence after that date (Hartmann et al., 2013). The changes in precipitation across Canada are discussed in Chapter 4.

As climate warming has made more moisture available in the atmosphere, this additional atmospheric moisture can lead to increased intensity of extreme precipitation events that varies by location. Observed changes in extreme precipitation are generally larger than those in total annual precipitation. At the global scale, extreme rainfall over land, measured as the number of heavy precipitation events, has likely increased in more regions than it has decreased since the 1950s. There is large variability among regions and between seasons, but the highest confidence in observed results is for central North America, where there was very likely a trend toward heavier precipitation events since the 1950s (Harman et al., 2013).

While changes in precipitation patterns may be expected to contribute to changes in drought and floods, there is low confidence in global-scale trends for both of these hazards (Hartmann et al., 2013). However, regional-scale trends are evident in some areas, with a likely increase in frequency and intensity of drought in the Mediterranean and West Africa and a likely decrease in central North America (mainly central United States but including parts of southern Canada) since 1950. Perspectives on changes in the frequency and magnitude of droughts and floods in a Canadian context are provided in Chapter 6 (see also Chapter 4, Section 4.4 for a discussion of the 2013 Alberta flood).

 

2.2.3: Ocean changes

A number of changes observed over the past century provide evidence of a warming global ocean (Rhein et al., 2013). Comprehensive estimates of global mean temperatures in the upper ocean (to a depth of 700 m) reveal that warming since the early 1970s is virtually certain. The global average warming for the upper 75 m of the ocean over the period 1971–2010 was an estimated 0.11ºC (90% uncertainty range from 0.09ºC to 0.13ºC) per decade. There is greater uncertainty in measurements of ocean temperatures before 1971 due, in part, to the scarcity of observations, but the IPCC AR5 reports that global average ocean warming (0−700 m) from the 1870s to 1971 was likely. Warming has also been observed deeper in the oceans, although the trends are not as strong. The IPCC AR5 reports that the increasing ocean heat content (absorbed heat that has been stored in the ocean; see Figure 2.2) accounts for roughly 90% of the energy accumulated globally over the period 1971–2010 (high confidence). This accumulation of energy in the ocean is strong evidence of excess energy in the Earth system, with less energy leaving the Earth system than entering. In addition to absorbing excess heat, the Earth’s oceans have also been absorbing excess carbon dioxide (CO2) from the atmosphere, increasing their acidity.

Global sea level rises primarily as a result of the expansion of ocean waters due to warming (thermal expansion) and the addition of water to the ocean from land ice (glacier and ice sheets) that is delivered to the oceans by melting and increased ice discharge. Tide-gauge records from around the world and, more recently, satellite altimeter data, indicate that the global mean sea level has been rising since the late 19th century (see Figure 2.2). The level has risen by an estimated 0.19 m (90% uncertainty range from 0.17 m to 0.21 m), based on a linear trend over the period 1901–2010, and the rate of this sea level rise has likely increased since the early 20th century (Rhein et al., 2013).

Both rising global sea level and increasing ocean heat content are strong evidence of a warming world. Influences of these global changes on the oceans surrounding Canada are detailed in Chapter 7.

 

2.2.4: Changes in the cryosphere

The cryosphere refers to portions of the Earth with sufficiently cold temperatures for water to freeze, and includes snow, sea ice, land ice (glaciers and ice caps), freshwater ice (lake and river ice), permafrost, and seasonally frozen ground. The IPCC AR5 assessed changes in the cryosphere around the globe and found, with very high confidence, that almost all glaciers worldwide have continued to shrink and that the Greenland (very high confidence) and Antarctic (high confidence) ice sheets have lost mass (based on two decades of data) (Vaughan et al., 2013). The IPCC AR5 reported that, over the period 2003−2009, the greatest losses of glacier ice were from glaciers in Alaska, the Southern Andes, Asian mountains, the periphery of the Greenland ice sheet, and the Canadian Arctic (Vaughan et al., 2013).

There is very high confidence that the extent of Arctic sea ice (both newly formed annual ice and multi-year ice) declined over the period 1979–2012, and that declines occurred in all seasons but were most pronounced in summer and autumn (high confidence). Annual mean sea ice extent in the Arctic very likely declined at a rate of 3.5%–4.1% per decade over this period. Antarctic sea ice extent very likely increased over the same period at a rate of 1.2%–1.8% per decade. The causes of variations in Antarctic sea ice properties and trends remain less well known than those for the Arctic, and the World Meteorological Association (2018) reports that, since the increase was reported in 2013, Antarctic sea ice extent was at or near record low levels throughout 2017. There is also very high confidence that snow cover extent has declined in the Northern Hemisphere (especially in spring) and high confidence that permafrost temperatures have increased in most regions since the 1980s, which is related to regional warming. Overall, the net loss in mass of ice from the global cryosphere (due to changes in glaciers, ice sheets, snow cover, sea ice extent, melt period, and ice thickness) is evidence of strong warming at high latitudes (see Figure 2.2) (Vaughan et al., 2013). Further details on these changes and implications from a Canadian perspective are found in Chapter 5.