Climate Change Canada

 

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).