9.3 The Greenhouse Effect

The Greenhouse Effect

The greenhouse effect is the process by which energy from the sun is trapped in the form of heat by various types of gas. It is a natural process that helps regulate the temperature of the Earth's surface!

Although the sun is incredibly hot and releases both thermal and light energy, the energy that actually reaches the Earth is visible light and ultraviolet radiation. When the sun's energy hits the Earth's surface, some of it is absorbed and re-emitted as lower-energy infrared radiation. That energy then begins to move away from the Earth and back into space.

However, the Earth's atmosphere is composed of a number of gases, including water vapor, carbon dioxide, methane, and nitrous oxide, known as greenhouse gases. So when the infrared radiation sent back from Earth's surface reaches the atmosphere, a portion of it is trapped by gases instead of being lost. This phenomenon allows the surface of the Earth to rise in temperature. 

Greenhouse gases absorb infrared radiation and convert it to heat energy, holding it primarily in the troposphere. The greenhouse effect is a vital process that keeps the Earth warm enough for life to exist. Without these gasses converting and trapping energy close to Earth's surface, it would be too cold for modern life to exist. 

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Greenhouse Effect in Review ☀️

1. The sun emits energy in the form of light and heat, which travels through space and reaches the Earth's atmosphere.
2. Some of this energy is absorbed by the Earth's surface and re-emitted as infrared radiation, a type of heat.
3. The Earth's atmosphere contains greenhouse gases.
4. Infrared radiation emitted by the Earth is absorbed by greenhouse gases in the atmosphere, which trap heat and cause the temperature of the surface to rise.

Greenhouse Gases and Thermal Retention 🔥

Certain gases have what is known as a higher thermal retention property, meaning that they are able to hold onto heat energy at a greater rate than others. 

Greenhouse gases have a high thermal retention property. They include:

·      Carbon dioxide (CO2)

·      Methane

·      Water vapor

·      Nitrous oxide

·      Chlorofluorocarbons (CFCs)

While water vapor is a greenhouse gas and has thermal retention, it has a short residence time in the atmosphere, which means that it does not contribute significantly to global climate change. In fact, different greenhouse gases have varying potency when it comes to their potential impact on climate change. 

Global Warming Potential 🌎

The global warming potential (GWP) of a gas is used to distinguish its potential impact on global climate change. GWP estimates how much a molecule of any compound could contribute to global warming within 100 years in comparison to carbon dioxide. Carbon dioxide has a GWP of 1, which is why it is often used as a comparison point for other glasses. Chlorofluorocarbons (CFCs) have the highest GWP (13,000), followed by nitrous oxide (300), then methane (25).

The GWP of a gas is calculated by multiplying the gas's warming effect by its atmospheric lifetime and the ratio of its global warming effect to that of CO2. For example, since the GWP of methane is 25, this means that one ton of methane has the same warming effect as 25 tons of CO2 over a 100-year time period. 

Why care about GWP? GWP is an important measure because it helps policymakers and researchers understand the relative impact of different greenhouse gases on global warming. It is used to set targets and policies to reduce greenhouse gas emissions and mitigate the impacts of global warming.

Sources of Greenhouse Gases

Carbon Dioxide

Natural sources of carbon dioxide emissions:

• Carbon dioxide naturally occurs as a byproduct of the metabolic reaction of cellular respiration. - Volcanic eruptions release a large amount of CO2 and ash, which upon release can have significant short-term effects on climate. Historic large-scale eruptions have been at fault for reducing the global average temperature by blocking solar radiation. 🌋- The decay of organic matter is another natural source of CO2. As plants and animals die, they decompose, releasing CO2 into the atmosphere. ☠️

Human-induced sources of increased CO2 emissions include:

• Carbon dioxide is produced during the combustion of fossil fuels. Burning of fossil fuels, including the use of coal, oil, and natural gas for electricity generation, transportation, and industrial processes, is the largest source of CO2 emissions into the atmosphere. 🛢️
• Deforestation is another human-induced source of CO2 emission issues. Trees absorb CO2 from the atmosphere as they grow and photosynthesize. When trees are cut down, the carbon dioxide intake process is disrupted, leading to an increase in atmospheric CO2. An example of this is mass deforestation in the Amazon Rainforest, which is a terrestrial carbon dioxide sink. 🌳
• Land use changes feed into deforestation and CO2 emissions. Changes in land use, such as the conversion of forests to agricultural land can increase the release of CO2 into the atmosphere. 🌾
• Industrial manufacturing and processes also emit carbon dioxide. From cement production to waste decomposition, industries also contribute to increased carbon dioxide.

Methane

Natural sources of methane emissions include:

• Wetlands, such as swamps and marshes, produce methane through the decomposition of organic matter.
• In the guts of animals, methane can be produced, meaning animals can create methane through the digestion of their food.
• Wildfires can also release methane into the atmosphere. 🔥

Human-induced sources of methane include:

• Fossil fuel extraction and transportation can contribute to methane emissions. The extraction and transportation of fossil fuels, including coal, oil, and natural gas, can release methane into the atmosphere.
• Landfills are a significant source of methane emissions, as organic waste breaks down in low-oxygen conditions and releases methane gas into the atmosphere.
• Agricultural activities are a significant source of increased atmospheric methane. The raising of livestock, including cows, sheep, and goats, is a major source of methane emissions due to the digestion of their food and the decomposition of manure. Increased cattle farming created a large-scale methane increase. 🐄
• Some industrial processes, such as the production of iron and steel, also emit methane.

Water Vapor

Natural sources of water vapor include:

• Water vapor is released into the atmosphere through the process of evaporation, which occurs when the sun's energy heats the surface of oceans, lakes, and other bodies of water. 💧
• Plants release water vapor into the atmosphere through a process called transpiration, which occurs when water is absorbed by the roots and transported to the leaves, where it is released into the air through small pores called stomata. 🌿
• Animals release water vapor into the atmosphere through the process of cellular respiration, which occurs when they breathe out air that contains water vapor.

Human-induced sources of increased water vapor include:

• Some industrial processes, such as the production of electricity, can release water vapor into the atmosphere as a byproduct of burning fossil fuels. 🏭
• Agricultural activities, such as irrigation and the application of fertilizers, can also release water vapor into the atmosphere.

Nitrous Oxide

Natural sources of nitrous oxide include:

• Nitrous oxide is naturally produced in soil through the decomposition of organic matter and denitrification during the nitrogen cycle, especially in low-oxygen environments like wetlands and the oceans.
• Lightning also can break down nitrogen molecules and produce nitrous oxide in the atmosphere. ⚡

Human-induced sources of increased atmospheric nitrous oxide:

• Agricultural activities are the main source of increased nitrous oxide. Nitrous oxide is produced in agriculture through the use of nitrogen-based fertilizers and the application of animal manure to fields. 🌽
• The industrial production of nitric acid and the burning of fossil fuels, especially the burning of coal for electric power generation, can also release a significant amount of nitrous oxide into the atmosphere.
• The decomposition of organic waste in landfills and sewage treatment plants can also release nitrous oxide into the atmosphere.

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons (CFCs) are a group of man-made chemicals that were once widely used as aerosol propellants. CFCs are highly potent greenhouse gases that contribute to the depletion of the ozone layer in addition to the warming of the Earth's atmosphere.

All of the main sources of CFCs are human-induced:

• CFCs were widely used as refrigerants in refrigerators, air conditioners, and other cooling systems. ❄️
• Solvents to clean electronic equipment and manufacture metal.
• CFCs were used as foam-blowing agents in the production of foam products, such as insulation and packaging materials.

CFCs are no longer produced or used in many countries due to the negative impacts they have on the ozone layer and their exceptionally high GWP. In 1987, the Montreal Protocol was signed, which called for the phasing out of CFC production. As a result, CFC emissions have significantly decreased in recent years. However, CFCs that have already been released into the atmosphere continue to impact the environment.

Learning Summary ✏️

The greenhouse effect is important because it helps to regulate the Earth's temperature and make it habitable for living things. Without the greenhouse effect, the Earth's average temperature would be much colder, making it difficult for life as we know it to exist. This is thanks to the high thermal retention properties of greenhouse gases.

However, the large-scale increase of greenhouse gases in the atmosphere is contributing to climate change, which has environmental impacts ranging from frequent and severe heatwaves to droughts and storms to rising sea levels. Using GWP, efforts to target the reduction of greenhouse gases can more successfully evaluate the different potencies of greenhouse gases and inform regulatory policies for human-sourced emissions. 

🎥 Watch: AP Environmental Science Streams