Climate
Change
Module 2
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Greenhouse Effect & Gases
Greenhouse gases are the main reason you are here reading this online course. Their presence in the atmosphere is the cause of global warming. They have been increasing the global average temperature since the industrial revolution.
Learning Outcomes
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Understand how the greenhouse effect works
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Sources and sinks of greenhouse gases
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Understand why CO2 is the most important greenhouse gas
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Learn how to reduce greenhouse emission
2.1 Greenhouse Effect?
The greenhouse effect is a natural process where atmospheric gases trap heat rather than letting it escape into space, which warms the Earth's surface. While 30 percent of the solar energy that reaches the Earth is reflected back to space, about 70 % passes through the atmosphere to the earth’s surface, where it is absorbed by the land, oceans, and atmosphere, to heat up the planet. This earth surface also radiates heat back toward space in the form of invisible infrared light. Some of this radiated heat continues into space, however, most of it gets absorbed by atmospheric gases, known as greenhouse gases, and returned toward the earth's surface, causing further warming (Fig 2.1).
The greenhouse effect is beneficial to our planet. It increases the planet’s temperature to an average of 15˚C (59˚F) which makes life habitable on earth. Without the greenhouse effect, the global average temperature would be as low as −18˚C (0˚F). Since the industrial revolution, the greenhouse effect has been growing stronger as human activities such as deforestation and the burning of fossil fuels release more and more greenhouse gases into the atmosphere (Fig 2.2).
2.2 Greenhouse Gases
Atmospheric gases that cause greenhouse effect are called GREENHOUSE GASES. They act like a blanket insulating the Earth. They have been increasing since the industrial revolution and the climate system has not seen any of the current values of these gases. Due to the human enhanced greenhouse gases (effect), the global temperature has increased more than 1˚C (1.8˚F) above its nineteenth-century value. According to the IPCC, the global temperature will likely increase by 4.1°C – 4.8°C above its nineteenth-century values by the end of the century, without a significant reduction in greenhouse gas emissions.
There are five major types of greenhouse gases. Here, we are going to learn about the main greenhouse gases, their emissions and removals from the atmosphere, concentration in the atmosphere and how long they stay in the atmosphere. The major types of greenhouse gases are;
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Carbon dioxide (CO₂)
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Methane (CH4)
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Nitrous Oxide (N₂O)
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Fluorinated Gases
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Water Vapor (H₂O)
2.2.1 Carbon Dioxide
The atmosphere contains CO₂ gas molecules that are part of the Earth’s carbon cycle. Naturally, there is a balance between CO₂ emission and removal as its cycles among the atmosphere, ocean, and land surface through many microorganisms, plants, and animals. However, human activities are altering the amount of atmospheric CO₂ rapidly (show keeling curve). The figure shows the monthly mean of CO₂ measured at Mauna Loa, Hawaii. The seasonal behavior is due to plant photosynthesis (growing season). CO₂ accounts for the largest (76%) of the total global greenhouse gas emission. In March 2021, the CO₂ level in the atmosphere is 417.64 ppm.
Natural Sources: Ocean release, animals and plants respiration, decomposition of organic matter, volcanic eruption, weathering of carbonate rocks and forest fires
Human Sources: Combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, land-use changes and other industrial processes. The main source of human-caused CO₂ emissions is the combustion of fossil fuels.
Sinks: Ocean (biological pump) and terrestrial (photosynthesis and Weathering) sinks. The ocean is the largest sink for CO₂ by absorbing about 25% of all CO₂ emissions.
How long does it stay in the atmosphere? 300 to 1000years. This means there is a 100% chance that CO₂ emitted by humans will outlive the human.
In summary, atmospheric accumulation of CO₂ is the balance between fossil fuel and land-use change emissions, and the uptake due to oceanic and terrestrial sinks
2.2.2 Methane
Methane is the second most important greenhouse gas in terms of concentration and impact on the climate. Humans are responsible for ~60% of methane emissions in the atmosphere. The impact of a molecule of methane on climate change over 20 years is 84 times greater than that CO2; over a 100-year period it is 28 times greater. In December 2020, CH4 level in the atmosphere is 1892.3 ppb.
Natural Sources: Wetland, Termites digestion and Oceans by CH4 producing microbes
Human Sources: Agriculture such as rice cultivation and ruminant livestock; coal mining; oil and gas production and distribution; biomass burning; and decay of organic waste in municipal waste landfilling.
Sinks: Oxidation by atmospheric hydroxyl (OH) radicals
Agriculture is the key emitting sector of methane emissions. It is responsible for about 40%
How long does it stay in the atmosphere? 12 years.
2.2.3 Nitrous Oxide
Nitrous oxide is also naturally present in the atmosphere as part of the Earth's nitrogen cycle, and has a variety of natural sources. Nitrous oxide is emitted during agricultural and industrial activities, combustion of fossil fuels and solid waste, as well as during treatment of wastewater.
Globally, about 40 percent of total N₂O emissions come from human activities.2 Nitrous oxide is emitted from agriculture, transportation, industry, and other activities, described below. In terms of per unit mass, N₂O on warming impact on the atmosphere is almost 300 times that of CO₂.
N₂O emissions from natural sources have not changed much in recent decades. But emissions from human sources have increased rapidly. In November 2020, the N₂O level in the atmosphere is 333.6 ppb.
Natural sources: Ocean and soils under natural vegetation.
Human Sources: Agriculture, including rice cultivation and ruminant livestock; coal mining; oil and gas production and distribution; biomass burning; and decay of organic municipal waste.
Sinks: Solar radiation destroys N₂O in the upper atmosphere. However, humans activities are emitting N₂O faster than it’s being destroyed, so it’s accumulating in the atmosphere.
How long does it stay in the atmosphere? 114 years
N20 can also damage the ozone layer which helps to prevent ultraviolet radiation from the Sun from reaching the Earth’s surface.
2.2.4 Fluorinated Gases
They are a family of gases that contain fluorine. This is a special kind of greenhouse gas because it has no natural sources and exclusively comes from human activities. There are three main types of Fluorinated gases namely;
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Hydrofluorocarbons (HFCs): Major source of emission is through their use as refrigerants. They were replaced with CHCs and HCFCs because they do not deplete the ozone layer. Ozone layer helps to prevent ultraviolet radiation from the Sun from reaching the Earth’s surface.
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Perfluorocarbons (PFCs): They are produced as a byproduct of aluminum production and are used in the manufacturing of semiconductors. PFCs generally have long atmospheric lifetimes
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Sulfur hexafluoride (SF6): Sulfur hexafluoride major is through its use as an insulating gas in electrical transmission equipment and circuit breakers
Destruction by solar radiation in the upper atmosphere removes fluorinated gases. With Varying atmospheric lifetimes, fluorinated gases are the most potent and longest-lasting type of greenhouse gases emitted by human activities. SF6 is the greenhouse gas with the highest warming potential according to the Intergovernmental Panel on Climate Change (IPCC). In November 2020, the SF6 level in the atmosphere is 10.39 ppt.
2.2.5 Water Vapor
Water vapor is an important component of the atmosphere and also acts as a powerful greenhouse gas. As the Earth’s temperature increases due to an increase in the aforementioned greenhouse gases, also known as non-condensable greenhouse gases, this increases the atmospheric water vapor that further increases the temperature. This is an example of positive feedback. Without increasing the non-condensable greenhouse gases, the amount of water vapor in the atmosphere would remain the same.
Therefore, the addition of water vapor to the atmosphere is indirectly through human activities. While CO2 is the most important greenhouse that causes global warming, water vapor is the most important greenhouse gas that controls the Earth’s temperature. It is the largest contributor to the greenhouse effect and it accounts for about 60% of the total warming effect. Due to this, it is often regarded as the main driver of climate change.
However, as water vapor increases in the atmosphere, more of it condenses into clouds, which contradicts the water vapor warming effect by its ability to reflect incoming solar radiation. When compared to non-condensable greenhouse gases, water vapor stays in the atmosphere for a much shorter period of time. Water vapor generally stays in the atmosphere for a few days before precipitating out.
2.3 Why is CO2 Important?
Three characteristics of atmospheric gases determine the strength of their greenhouse effect. The first is their warming potential (ability to absorb and radiate energy) which is the absorption impact of a unit mass on outgoing radiation. The second is the atmospheric lifetime, which is how long they stay in the atmosphere before natural processes remove it. Lastly, the amount of concentration in the atmosphere.
While CO2 has the lowest warming potential, its huge emission (high concentration) through human activities and long atmospheric lifetime make it the highest contributing greenhouse gas to global warming since the industrial revolution. CO2 is the highest contributor of radiative forcing (a net warming influence) since 1750.
According to the IPCC, radiative forcing due to an externally imposed perturbation(say changes in greenhouse gases) is the change in Earth’s energy balance between incoming solar radiation energy and outgoing thermal IR emission energy while all other factors are held constant.
