2.3: Understanding The Causes Of
Observed Global climate Change
Warming has
not been steady over time, as natural climate variability has either added to
or subtracted from human-induced warming. Periods of enhanced or reduced
warming on decadal timescales are expected, and the factors causing the early
21st century warming slowdown are now better understood. In the last several
years, global average temperature has warmed substantially, suggesting that the
warming slowdown is now over.
The
heat-trapping effect of atmospheric greenhouse gases is well-established. It is
extremely likely2 that human activities, especially emissions of greenhouse
gases, are the main cause of observed warming since the mid-20th century.
Natural factors cannot explain this observed warming. Evidence is widespread of
a human influence on many other changes in climate as well.
2.3.1: Factors determining global
climate
Scientists have
understood the basic workings of Earth’s climate for almost 200 years. Studies
in the 19th century had already identified the key role of Earth’s atmosphere
and of CO2 in raising the temperature of the planet (Fourier, 1827; Tyndall,
1859; Arrhenius, 1896). The fundamentals of the climate system, including
factors that determine climate and that can drive climate change, have been
included in every major IPCC assessment as foundational background information
(IPCC, 1990, 1996, 2001, 2007, 2013a).
Earth’s
long-term climate and average temperature are regulated by a balance between
energy arriving from the sun (in the form of shortwave radiation) and energy
leaving the Earth (in the form of longwave radiation). When this balance is
disrupted in a persistent way, global temperature rises or falls. Factors that
disrupt this balance are called “climate drivers” or “climate forcing agents,”
evoking their influence in forcing climate toward warmer or cooler conditions.
Their effect on Earth’s energy balance is called “radiative forcing,” which is
defined as the net change in the energy balance of the Earth system due to an
external perturbation. The strength of radiative forcing is measured in units
of watts per square metre (W/m2). Positive radiative forcing indicates excess
energy is being retained in the climate system — less energy is leaving than is
entering the system — leading to a warmer climate, whereas negative radiative
forcing indicates more energy is leaving the climate system than entering it,
leading to a cooler climate (Le Treut et al., 2007; Cubasch et al., 2013).
Radiative forcing provides a useful means of comparing and/or ranking the
influence of different climate drivers.
Climate
drivers can be either natural or anthropogenic — resulting from human
activities. The fact that Earth’s average temperature and climate have varied
significantly over geologic time indicates that natural factors have varied in
the past. On shorter timescales of decades to centuries, the main climate
drivers are changes in solar irradiance, volcanic eruptions, changes in
atmospheric composition, and changes to the land surface. The latter two are
influenced by human activities. How changes in these climate drivers influence
incoming or outgoing radiation is described below.
Solar
irradiance, the strength of solar radiation received at the Earth’s surface,
fluctuates by a small amount over a solar cycle of approximately 11 years, and
these fluctuations can explain global temperature variations of up to
approximately 0.1ºC between the strongest and weakest parts of the cycle.
Small, multi-decadal trends (increasing and decreasing) in solar irradiance can
also occur, with similarly small effects on global climate (Masson-Delmotte et
al., 2013).
Volcanic
eruptions periodically eject large volumes of gases and dust into the
stratosphere (upper atmosphere). Sulphate aerosols (tiny airborne particles)
that form from these gases reflect solar radiation and thereby induce a cooling
effect.8 Since volcanic eruptions are episodic, and sulphate aerosols remain in
the stratosphere for only a few years, the cooling effects are short-lived. The
global cooling effect of large volcanic eruptions, such as the eruption of
Mount Pinatubo in the Philippines in 1991, is clearly evident in the global
temperature record (see Section 2.3.3 and Figure 2.9).
Human
activities affect Earth’s reflectivity (albedo) by changing the atmospheric
composition and the land surface. For example, the combustion of fossil fuels
emits a variety of pollutants, in addition to GHGs, into the lower atmosphere,
where they form aerosols of various chemical compositions. These aerosols may
either reflect or absorb solar radiation and are important drivers of climate
change. Aerosols in the lower atmosphere also serve as particles on which water
vapour can condense to form clouds (cloud condensation nuclei). Changes in
aerosol concentrations can therefore induce changes in cloud properties which,
in turn, can affect Earth’s albedo. While the interactions between aerosols and
clouds are complex and involve a number of different processes, an increase in
aerosol concentration is known to produce brighter clouds, which reflect more
solar radiation, inducing a cooling effect. Human alterations of the land
surface also tend to increase albedo. When forested lands are cleared for
cultivation this tends to produce more reflective land surfaces (Le Treut et
al., 2007; Cubasch et al., 2013).
Changes in
solar irradiance, volcanic eruptions, and changes in albedo affect Earth’s
energy balance by altering the amount of incoming energy available to heat the
Earth, but the primary driver of the amount of heat leaving the Earth is
changes to the chemical composition of the atmosphere. While the two most
abundant gases in Earth’s atmosphere — nitrogen (78%) and oxygen (21%) — are
transparent to outgoing longwave radiation, allowing this heat to escape to
space, some trace gases absorb longwave radiation, creating the greenhouse
effect, and are referred to as GHGs. GHGs have both natural and human sources.
The main GHGs are water vapour, CO2, methane (CH4), ozone (O3), nitrous oxide
(N2O), and groups of synthetic chemicals referred to as halocarbons. Changes to
the atmospheric concentrations of GHGs affect the transparency of the
atmosphere to outgoing heat. Individual GHGs differ in their capacity to trap
heat, and most are more powerful GHGs than CO2. However, CO2 is by far the most
abundant GHG (Myhre et al., 2013) aside from water vapour. The build-up of
atmospheric GHGs has reduced heat loss to space and is therefore a positive
radiative forcing, with a warming effect on the climate system (Le Treut et
al., 2007; Cubasch et al., 2013).
Determining
the relative contribution of different forcing agents perturbing the Earth’s
energy balance provides a useful first-order assessment of the causes of
observed climate change. However, the climate system does not respond in a
straightforward way to changes in radiative forcing. An initial perturbation
can trigger feedbacks in the climate system that alter the response. These
climate feedbacks either amplify the effect of the initial forcing (positive
feedback) or dampen it (negative feedback). Therefore, positive feedbacks in
the climate system are cause for concern because they amplify the warming from
an initial positive forcing, such as increases in atmospheric concentrations of
GHGs.
There are a
number of feedbacks in the climate system, operating on a wide range of
timescales, from hours to centuries (Cubasch et al., 2013; see, in particular,
Fig 1.2 and associated text in this reference). Important positive feedbacks
that have contributed to warming over the Industrial Era include the water vapour
feedback (water vapour, a strong GHG, increases with climate warming) and the
snow/ice albedo feedback (snow and ice diminish with climate warming, decreasing
surface albedo). There is very high confidence that the net feedback — that is,
the sum of the important feedbacks operating on century timescales — is
positive, amplifying global warming (Flato et al., 2013; Fahey et al., 2017).
Some feedbacks are expected to become increasingly important as climate warming
continues this century and beyond. These include feedbacks that change how
rapidly the land and ocean can remove CO2 from the atmosphere and those that
may lead to additional emissions of CO2 and other GHGs, such as from thawing
permafrost (Ciais et al., 2013; Fahey et al., 2017).
2.3.2: Changes in greenhouse gases
and radiative forcing over the Industrial Era
The
Industrial Era refers to the period in history, beginning around the mid-18th
century and continuing today, marked by a rapid increase in industrial activity
powered by the combustion of fossil fuels. Burning these carbon-based fuels
releases CO2, as well as other gases and pollutants, to the atmosphere. The
Industrial Era is recognized as the period when human activity has
substantially affected the chemical composition of the atmosphere by increasing
the concentration of trace gases, including GHGs (Steffen et al., 2007)
2.3.2.1: Changes in greenhouse gas
concentrations over the Industrial Era
GHGs are
emitted to the atmosphere from both natural and human sources (see Box 2.2) and
are also removed from the atmosphere, primarily through natural processes
referred to as natural “sinks.” Atmospheric concentrations of GHGs increase
when the rate of emission to the atmosphere exceeds the rate of removal. Even a
small annual imbalance, in which emissions exceed removals, can lead to a large
build-up of the gas in the atmosphere over time (in the same way that a small
annual deficit in a financial budget can lead to a large accumulation of debt
over time). Sinks and imbalances differ for different GHGs. CH4 is removed from
the atmosphere primarily through photochemical reactions that destroy it
chemically. These reactions remove almost as much CH4 each year as is emitted
from both natural and human sources, leaving a small annual excess of emissions
(Ciais et al., 2013; Saunois et al., 2016). In contrast, only about half of the
CO2 emitted from human activities each year is removed from the atmosphere
through land sinks (mainly uptake by plants during photosynthesis) and ocean
sinks (mainly through CO2 dissolving into the ocean) (Ciais et al., 2013; Le
Quéré et al., 2016). The ongoing annual excess of human-emitted CO2 is the
cause of the observed rise in atmospheric CO2 concentrations.
Well-mixed
GHGs are those that persist in the atmosphere for a sufficiently long time for
concentrations to become relatively uniform throughout the atmosphere. For such
substances, emissions anywhere affect atmospheric concentrations everywhere.
Global average concentrations of well-mixed GHGs can be determined from
measurements taken at only a few monitoring locations around the globe. Canada
monitors GHG concentrations at a number of locations, and these data are used,
along with those from other monitoring stations, to determine global average
GHG concentrations.
Long-term
records of changes in atmospheric concentrations of the three main well-mixed
GHGs — CO2, CH4, and N2O — are compiled from direct atmospheric measurements
(beginning in the late 1950s for CO2 and in the late 1970s for CH4 and N2O) and
from ice-core measurements, which extend the time period of analysis back
hundreds of thousands of years. The evidence clearly shows that the
concentrations of these GHGs have increased substantially over the Industrial
Era, by 40% for CO2, 150% for CH4, and 20% for N2O (Hartman et al., 2013) (see
Figure 2.7). Global concentrations of the main GHGs in 2015 were about 400
parts per million for CO2, 1845 parts per billion for CH4, and 328 parts per
billion for N2O (WMO, 2016). These concentrations exceed the highest
concentrations during the past 800,000 years recorded in ice cores
(Masson-Delmotte et al., 2013).
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