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.
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