Earth's Atmosphere
3. Greenhouse Gases
Learning outcomes
-
I can identify major greenhouse gases.
- I can explain how greenhouse gases trap heat in the atmosphere.
- I can distinguish between the greenhouse effect and global warming.
- I can describe natural and human sources of greenhouse gases.
- I can evaluate the role of greenhouse gases in climate regulation.
Introduction
Earth receives enormous amounts of energy from the Sun every day. Some of this energy is reflected back into space, while the rest warms Earth's surface. As the warm surface cools, it releases energy in the form of infrared radiation (heat).
If all of this heat escaped directly into space, Earth would be an icy planet with an average temperature of about –18°C. Fortunately, certain gases in the atmosphere absorb and re-radiate some of this heat, keeping Earth's average surface temperature close to 15°C. These gases are called greenhouse gases, and together they produce the natural greenhouse effect, which makes life on Earth possible.
What Are Greenhouse Gases?
Greenhouse gases (GHGs) are gases in the atmosphere that absorb and re-emit infrared radiation, trapping some heat near Earth's surface.
Unlike nitrogen and oxygen, which make up most of the atmosphere, greenhouse gases interact strongly with infrared radiation.
Definition:
Greenhouse gases are atmospheric gases that absorb and re-radiate infrared radiation, helping to warm Earth's lower atmosphere.
Major Greenhouse Gases
The main greenhouse gases are:
| Greenhouse Gas | Chemical Formula. | Main Role |
|---|---|---|
| Water vapour | H₂O | Largest natural greenhouse gas |
| Carbon dioxide | CO₂ | Climate regulation and photosynthesis |
| Methane | CH₄ | Very effective heat-trapping gas |
| Nitrous oxide | N₂O | Long-lasting greenhouse gas |
| Ozone | O₃ | Protects from UV radiation and traps heat |
| Fluorinated gases. | Various | Human-made gases with very high warming potential |
Although some greenhouse gases exist in very small concentrations, they can have a significant effect on Earth's temperature.
How Greenhouse Gases Trap Heat
The Sun emits mostly visible light, which passes easily through the atmosphere.
After Earth's surface absorbs this energy, it warms up and emits infrared radiation (heat).
Greenhouse gases:
- absorb some of this outgoing infrared radiation,
- re-radiate it in all directions,
- send some heat back toward Earth's surface,
- reduce the amount of heat escaping into space.
This process keeps Earth much warmer than it would otherwise be.
The Greenhouse Effect vs Global Warming
These two terms are often confused, but they are not the same.
| Greenhouse Effect | Global Warming |
|---|---|
| A natural process | A long-term increase in Earth's average temperature |
| Essential for life | Largely driven by increased greenhouse gases from human activities |
| Keeps Earth warm enough for life. | Can disrupt climate systems if excessive |
The Natural Greenhouse Effect
The natural greenhouse effect:
- has operated for hundreds of millions of years,
- maintains a suitable climate,
- supports liquid water,
- allows ecosystems to thrive.
Without it, most life on Earth would not exist.
Enhanced Greenhouse Effect
Human activities have increased greenhouse gas concentrations.
More greenhouse gases trap more heat, strengthening the greenhouse effect and contributing to global warming.
Natural Sources of Greenhouse Gases
Greenhouse gases are produced naturally by many processes.
| Source | Greenhouse Gas |
|---|---|
| Respiration | Carbon dioxide |
| Decomposition. | Carbon dioxide and methane |
| Oceans | Water vapour and carbon dioxide |
| Wetlands | Methane |
| Volcanoes | Carbon dioxide and water vapour |
| Wildfires | Carbon dioxide |
These natural sources have existed for millions of years and are balanced by natural sinks such as forests and oceans.
Human Sources of Greenhouse Gases
Human activities have greatly increased greenhouse gas emissions.
Major sources include:
Burning Fossil Fuels
Produces large amounts of:
- carbon dioxide
Examples:
- power stations
- vehicles
- factories
- aircraft
Deforestation
Removing forests:
- reduces carbon dioxide absorption,
- releases stored carbon when trees are burned or decay.
Agriculture
Agriculture produces:
- methane from cattle and sheep,
- methane from rice paddies,
- nitrous oxide from fertilisers.
Industry
Industrial activities produce:
- carbon dioxide,
- fluorinated gases,
- nitrous oxide.
Why Different Greenhouse Gases Matter
Not all greenhouse gases trap heat equally.
For example:
- Water vapour is the most abundant natural greenhouse gas and responds quickly to changes in temperature.
- Carbon dioxide is less effective molecule-for-molecule than methane but remains in the atmosphere for much longer and is emitted in much larger quantities.
- Methane traps much more heat per molecule than carbon dioxide over a 100-year period, but it has a shorter atmospheric lifetime.
- Nitrous oxide is both a powerful greenhouse gas and contributes to ozone depletion in the upper atmosphere.
- Fluorinated gases are present in very small amounts but can remain in the atmosphere for centuries or even thousands of years.
Climate Regulation
Greenhouse gases help regulate Earth's climate by maintaining a balance between incoming solar energy and outgoing heat.
A balanced greenhouse effect helps maintain:
- liquid water
- stable ecosystems
- agriculture
- biodiversity
- habitable temperatures
However, if greenhouse gas concentrations rise too much, more heat is retained, increasing the risk of:
- rising global temperatures,
- melting glaciers,
- sea-level rise,
- more frequent extreme weather events,
- changes to ecosystems.
Worked Example
A student says:
"The greenhouse effect is bad because it causes global warming."
Question
Is this statement completely correct?
Solution
No.
The natural greenhouse effect is essential because it keeps Earth warm enough for life.
Global warming occurs when additional greenhouse gases, mainly from human activities, strengthen this natural effect and trap more heat than usual.
Real-World Connection
Many countries are working to reduce greenhouse gas emissions by increasing the use of renewable energy sources such as solar, wind, and hydroelectric power. Improvements in energy efficiency, electric vehicles, forest conservation, and carbon capture technologies are all designed to reduce the amount of carbon dioxide entering the atmosphere while maintaining reliable energy supplies.
Did You Know?
Although water vapour is the most abundant greenhouse gas, scientists focus heavily on carbon dioxide because human activities directly increase its concentration. As carbon dioxide warms the atmosphere, warmer air can hold more water vapour, creating a positive feedback loop that can amplify warming.
Key Terms
- Greenhouse gas (GHG) — a gas that absorbs and re-radiates infrared radiation.
- Greenhouse effect — the natural warming of Earth caused by greenhouse gases trapping some outgoing heat.
- Global warming — the long-term increase in Earth's average surface temperature, largely driven by enhanced greenhouse gas concentrations.
- Infrared radiation — heat energy emitted by Earth's surface.
- Climate regulation — the processes that help maintain Earth's long-term temperature and weather patterns.
- Enhanced greenhouse effect — the strengthening of the natural greenhouse effect due to increased greenhouse gas emissions from human activities.
Key Takeaways
- Greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, ozone, and fluorinated gases.
- These gases absorb and re-radiate infrared radiation, helping to keep Earth warm enough for life.
- The natural greenhouse effect is essential for maintaining a habitable planet.
- Global warming refers to the recent increase in Earth's average temperature caused primarily by higher concentrations of greenhouse gases from human activities.
- Burning fossil fuels, deforestation, agriculture, and industrial processes are major human sources of greenhouse gases.
- Maintaining a balanced greenhouse effect is essential for regulating Earth's climate and supporting life.