‍What Are Greenhouse Gas Emissions?

In this lesson, we’ll explore the fundamentals of greenhouse gas emissions—what they are, what causes them, and why they contribute to global warming. 
Blair Spowart
Co-founder

In this lesson, we’ll explore the fundamentals of greenhouse gas emissions—what they are, what causes them, and why they contribute to global warming. 

Understanding how greenhouse gases (GHGs) interact with the Earth's atmosphere is essential for grasping the science behind climate change and the urgent need to reduce emissions.

What Are Greenhouse Gas Emissions?

Greenhouse gases (GHGs) are gases in the Earth's atmosphere that trap heat. They are responsible for the greenhouse effect, a natural process that warms the Earth’s surface and keeps our planet’s temperature stable. Without GHGs, the Earth would be too cold for life as we know it. However, human activities have dramatically increased GHG concentrations, intensifying the greenhouse effect and leading to global warming.

Major Greenhouse Gases

The most common greenhouse gases include:

  • Carbon dioxide (CO2): The most well-known GHG, primarily produced by burning fossil fuels, deforestation, and certain industrial processes.
  • Methane (CH4): A highly potent GHG released from agriculture (especially livestock), landfills, and the extraction of fossil fuels.
  • Nitrous oxide (N2O): Emitted from agricultural activities (such as fertiliser use), industrial processes, and fossil fuel combustion.

Minor Greenhouse Gases

Along with the major ones, here are some of the more minor GHGs:

  • Water Vapor (H₂O): Although not considered a direct emission by human activities, water vapor is the most abundant GHG in the atmosphere. Its presence is largely influenced by climate feedback mechanisms rather than direct human emissions. Increased temperatures lead to more water vapor, amplifying the greenhouse effect.
  • Ozone (O₃): Ozone in the lower atmosphere (tropospheric ozone) acts as a greenhouse gas. It is formed by reactions between pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. While not emitted directly in large quantities, tropospheric ozone contributes to the greenhouse effect and is considered a minor GHG.
  • Chlorofluorocarbons (CFCs): Although their production and use have been heavily regulated under the Montreal Protocol due to their role in depleting the ozone layer, CFCs are potent GHGs. They have been mostly phased out, but some CFCs still remain in older equipment or as residual atmospheric concentrations.
  • Hydrochlorofluorocarbons (HCFCs): These are transitional gases that replaced CFCs and were meant to be less harmful to the ozone layer. However, HCFCs still contribute to the greenhouse effect. Although they are also being phased out, they continue to be released from older equipment.
  • Perfluorocarbons (PFCs): PFCs are a group of fluorinated gases with long atmospheric lifetimes. They are primarily emitted from aluminum production and semiconductor manufacturing. Although emitted in smaller amounts, PFCs are highly effective at trapping heat in the atmosphere.
  • Sulfur Hexafluoride (SF₆): SF₆ is used in electrical insulation and other industrial processes. It has an extremely high global warming potential (GWP) and a long atmospheric lifetime. Emissions are relatively low, but its potency makes it a significant concern.
  • Nitrogen Trifluoride (NF₃): Used primarily in the electronics industry for the manufacture of semiconductors and LCD displays, NF₃ is another synthetic gas with a high GWP. Its emissions have been increasing due to the growth of these industries.

While these minor greenhouse gases are not as prevalent as the major GHGs like carbon dioxide and methane, their global warming potential (GWP) can be substantially higher. Even in small quantities, these gases can significantly contribute to the greenhouse effect due to their high heat-trapping abilities and long atmospheric lifetimes.

Pie chart- Major Greenhouse Gases from People's Activities
Reference: https://archive.epa.gov/climatechange/kids/basics/today/greenhouse-gases.html

What Causes Greenhouse Gas Emissions?

Human activities are responsible for the sharp increase in GHG emissions, which has accelerated since the Industrial Revolution.

Here are the key sources of GHG emissions:

1. Burning Fossil Fuels Through Energy Production and Use

The largest source of GHG emissions comes from burning fossil fuels for electricity, heat, and transportation. The energy sector alone is responsible for approximately 75% of global emissions. But why does burning these fuels release carbon?

What Exactly Are Fossil Fuels?

Fossil fuels—namely coal, oil, and natural gas—are energy-rich substances that have formed over millions of years from the remains of ancient plants, animals, and microorganisms. These organic materials were buried under layers of sediment and rock, where they were exposed to intense heat and pressure. Over vast geological timescales, these processes transformed decayed plant and animal matter into the fossil fuels we use today.

  • Coal: Formed primarily from ancient, decayed plant matter such as ferns, trees, and other vegetation that accumulated in prehistoric swamps. Over millions of years, the buried plant material underwent a process called carbonisation under high pressure and heat, turning it into carbon-rich coal deposits.
  • Oil: Often referred to as crude oil, this fuel was formed from the remains of tiny marine organisms like plankton and algae that settled on the ocean floor. As these microorganisms died and decomposed, their organic matter mixed with sediments. Over time, heat and pressure transformed this mixture into oil.
  • Natural Gas: Primarily composed of methane (CH₄), natural gas formed in a similar manner to oil, but under different temperature and pressure conditions. It is often found in the same reservoirs as oil and is a byproduct of the decomposition of marine microorganisms.

Why These Fossil Fuels Release Carbon When Burned?

The reason fossil fuels release carbon dioxide (CO₂) and other greenhouse gases when burned is because of their chemical composition.

Fossil fuels are hydrocarbons, meaning they are made up primarily of carbon (C) and hydrogen (H) atoms. When fossil fuels are burned, they react with oxygen in the air in a process called combustion. This reaction releases energy in the form of heat, along with CO₂ and other gases as byproducts.

Fossil Fuels and the Carbon Cycle

Fossil fuels represent carbon that was once part of the Earth’s natural carbon cycle. Millions of years ago, carbon was absorbed by plants and microorganisms through photosynthesis. These plants and organisms eventually died, and their remains became buried and locked away deep in the Earth. Over time, the carbon in these remains was converted into coal, oil, and natural gas deposits.

However, when humans extract and burn these fuels, the long-stored carbon is released back into the atmosphere as CO₂. This sudden reintroduction of carbon, which had been sequestered underground for millennia, disrupts the natural carbon balance and leads to increased concentrations of GHGs in the atmosphere.

The Role of Transportation

Transportation accounts for a significant portion of global GHG emissions due to the widespread use of fossil fuels. Cars, trucks, ships, and planes primarily run on gasoline, diesel, and jet fuel—derived from crude oil. Here’s how each contributes:

  • Road Transport: Vehicles burn gasoline and diesel, releasing CO₂ and small amounts of methane (CH₄) and nitrous oxide (N₂O) from their exhausts. Additionally, incomplete combustion of fuel or leakage can release methane, and nitrogen compounds in the fuel can react to form nitrous oxide.
  • Shipping and Aviation: Ships and planes use heavy fuel oils and jet fuels, which are also hydrocarbon-based. The combustion of these fuels releases CO₂ in large quantities due to the scale of fuel consumption in global trade and travel.
  • Industrial Transportation and Machinery: Diesel-powered heavy machinery and equipment used in mining, construction, and agriculture also contribute to GHG emissions. These machines release not only CO₂ but also pollutants like black carbon, which contributes to warming.

2. Agriculture and Livestock

Agriculture significantly contributes to methane and nitrous oxide emissions.

  • Livestock: Ruminants like cows produce methane during digestion. Livestock farming is one of the largest sources of methane emissions.
  • Rice cultivation: Flooded rice paddies release methane due to the anaerobic conditions that develop in waterlogged soils.
  • Fertilisers: Synthetic fertilisers used in agriculture emit nitrous oxide.

3. Deforestation and Land-Use Change

Forests absorb CO2, but when trees are cut down, the stored carbon is released back into the atmosphere. Deforestation, especially in tropical rainforests, is a major contributor to rising CO2 levels.

4. Industrial Processes

Some industrial processes produce GHGs directly:

  • Cement production: Cement manufacturing releases CO2 as a byproduct of heating limestone.
  • Fluorinated gases: These gases are produced during refrigeration and air conditioning processes and have extremely high global warming potentials.

5. Waste Management

Decomposing waste in landfills produces methane as organic material breaks down without oxygen. Waste incineration also releases CO2 and other pollutants into the atmosphere.

Natural Processes That Produce GHGs

While human activities are a major driver of increased GHG emissions, natural processes also contribute to the presence of greenhouse gases in the atmosphere. These natural sources have existed for millions of years, maintaining a delicate balance in the Earth's climate system.

Here are key natural sources of GHG emissions:

1. Volcanic Eruptions

Volcanic activity releases CO₂, water vapor, and other GHGs into the atmosphere. While volcanic eruptions are sporadic, large-scale eruptions can contribute significant amounts of GHGs over short periods, affecting global temperatures.

Key GHGs Released: Carbon dioxide (CO₂), water vapor, sulfur dioxide (which can lead to temporary cooling through aerosols).

2. Respiration and Decomposition

All living organisms, including humans, exhale carbon dioxide during respiration. Additionally, when plants and animals die, their decomposition by microbes releases CO₂ and methane into the atmosphere.

Key GHGs Released: Carbon dioxide (CO₂) during respiration, methane (CH₄) from anaerobic decomposition in waterlogged conditions like wetlands.

3. Wetlands and Peatlands

Wetlands, such as swamps and marshes, are a major natural source of methane. In these waterlogged environments, organic matter decomposes anaerobically (without oxygen), leading to methane production.

Key GHGs Released: Methane (CH₄) from microbial activity in anaerobic conditions.

4. Oceanic Processes

Oceans are both a source and sink for CO₂. Marine organisms like phytoplankton absorb CO₂ during photosynthesis. However, oceans release CO₂ back into the atmosphere through the natural cycling of water and marine life processes.

Key GHGs Released: Carbon dioxide (CO₂) through the exchange between ocean waters and the atmosphere.

5. Permafrost Thawing

In Arctic and sub-Arctic regions, permafrost (frozen soil) stores large amounts of organic carbon. As global temperatures rise, the permafrost thaws, leading to the release of methane and CO₂ from the decomposing organic matter.

Key GHGs Released: Methane (CH₄) and carbon dioxide (CO₂) from thawing and decomposing organic material.

6. Forest Fires

Natural forest fires, often sparked by lightning, release stored carbon in trees and vegetation. While these events can contribute to short-term spikes in CO₂ levels, they are part of natural cycles that have historically been balanced by forest regrowth.

Key GHGs Released: Carbon dioxide (CO₂) and smaller quantities of methane (CH₄).

Why Do Greenhouse Gases Heat Things Up?

The Greenhouse Effect

The greenhouse effect is the process that makes the Earth warm enough to support life. GHGs play a critical role in this process by trapping heat.

Here’s how the greenhouse effect works:

  1. Solar radiation (sunlight) enters the Earth’s atmosphere. Some of it is reflected back into space by clouds, snow, and ice, while the rest reaches the Earth’s surface.
  2. The Earth's surface absorbs the sunlight, warms up, and radiates heat (infrared radiation) back toward space.
  3. Greenhouse gases in the atmosphere absorb some of this heat and prevent it from escaping into space. They then re-radiate the heat back to the Earth’s surface, warming the planet.

Why More GHGs Cause More Warming

The problem arises when the concentration of GHGs increases due to human activities. As more GHGs accumulate in the atmosphere, they trap more heat, causing the Earth's average temperature to rise.

  • Carbon dioxide (CO2), the most prevalent GHG from human activities, absorbs specific wavelengths of infrared radiation that would otherwise escape into space. This additional heat is trapped, leading to global warming.
  • Methane (CH4) is 25 times more effective at trapping heat than CO2 over a 100-year period, making it a significant contributor to short-term warming.
  • Nitrous oxide (N2O) is 300 times more powerful than CO2 at trapping heat, even though it’s released in smaller quantities.
  • Fluorinated gases are synthetic gases with very high global warming potential, some being thousands of times more potent than CO2.

As GHG levels rise, they disturb the Earth’s energy balance, causing more heat to be trapped in the atmosphere. This leads to an increase in global temperatures—a process known as global warming.

Positive Feedback Loops

Global warming also triggers positive feedback loops that accelerate the process. For example:

  • Melting ice: As polar ice melts due to rising temperatures, it exposes darker land or ocean surfaces that absorb more heat, accelerating warming.
  • Permafrost thawing: Thawing permafrost releases stored methane and CO2, further increasing GHG concentrations and heating the planet even more.

These feedback loops make climate change faster and more difficult to control.

Conclusion

Understanding the causes and impacts of GHG emissions is essential for developing effective climate solutions and reducing our carbon footprint. The urgency to reduce GHG emissions and transition to renewable energy sources cannot be overstated if we want to mitigate the worst effects of climate change.

Blair Spowart
Co-founder
Seedling Climate School
Carbon Action 101

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