4.3.4Additional lessons
Carbon Dioxide Levels and the Greenhouse Effect — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 56 of 98
Part 1 of 5The Natural Greenhouse Effect
The greenhouse effect is natural, but human activity is strengthening it.
Incoming radiation
- The Sun emits mostly visible and ultraviolet radiation, much of which passes through the atmosphere and warms the Earth's surface.
Re-emission
- The warmed surface emits infrared radiation, which has a longer wavelength than the incoming radiation.
Absorption
- Greenhouse gases in the atmosphere absorb some of this infrared radiation and re-emit it in all directions,
- including back towards the Earth.
Result
- This keeps the average temperature of the Earth higher than it would otherwise be, and without it life as we know it could not exist.
Part 2 of 5Why Some Gases Absorb Infrared
Why Some Gases Absorb Infrared
Bond vibrations
- Infrared radiation has the right energy to make covalent bonds stretch and bend,
- so molecules absorb it only if the bond vibration is allowed.
Polarity change
- A vibration absorbs infrared only if it changes the dipole moment of the molecule, which is why N₂ and O₂ do not absorb it.
Greenhouse gases
- CO₂, CH₄ and H₂O have vibrations which change the dipole, so they absorb infrared and act as greenhouse gases.
Comparison
- Gases differ in their global warming effect, depending on how strongly they absorb and how long they stay in the atmosphere.
| Gas | Absorbs infrared? | Reason |
|---|---|---|
| N₂ | No | Diatomic and symmetrical, so stretching does not change polarity |
| CO₂ | Yes | Bending and asymmetric stretching change the dipole |
| H₂O | Yes | Polar bonds whose vibrations change the dipole |
Part 3 of 5The Enhanced Greenhouse Effect
1
Burning fossil fuels releases carbon that was locked in the ground for millions of years into the atmosphere as carbon dioxide.
2
The atmospheric concentration of CO₂ has risen from about 280 ppm before industrialisation to over 400 ppm today.
3
More CO₂ means more infrared radiation absorbed and re-emitted towards the surface, raising average global temperatures.
4
Warming leads to melting ice, rising sea levels and changes in weather patterns and ecosystems.
5
Carbon dioxide also dissolves in the oceans, forming carbonic acid and lowering their pH.
Part 4 of 5Reducing the Problem
Reducing the Problem
Energy efficiency
- Using less energy for the same task lowers the amount of fuel burnt and the CO₂ released.
Renewable sources
- Wind, solar and hydroelectric power generate electricity with no CO₂ emissions at the point of use.
Carbon capture
- CO₂ can be captured from flue gases and stored underground, although the technology adds cost.
Evidence
- Scientists use data on atmospheric composition, temperature records and models to evaluate the link between CO₂ and warming.
Part 5 of 5Exam Checklist & Tips
4.3.4 – Key Points for the Exam
- Greenhouse effect The warming of the Earth because gases in the atmosphere absorb and re-emit infrared radiation.
- Greenhouse gas A gas whose molecules absorb infrared radiation, such as CO₂, CH₄ and H₂O.
- Infrared radiation Electromagnetic radiation of longer wavelength than visible light, emitted by warm surfaces.
- ppm Parts per million, a unit of concentration for gases in the atmosphere.
Common Mistakes
- Confusing the greenhouse effect with the depletion of the ozone layer, which is a separate issue.
- Saying that CO₂ traps visible light, when it is infrared radiation that is absorbed.
- Claiming that all gases in the atmosphere are greenhouse gases, including nitrogen and oxygen.
- Treating the greenhouse effect itself as harmful, when it is the enhanced effect that causes concern.
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