Greenhouse effect

IB Physics SL· Topic 2.3, 2025 IB Physics SL Guide· 12 min read

1. Core Mechanism of the Natural Greenhouse Effect★★☆☆☆⏱ 3 min

The natural greenhouse effect is a critical process that keeps Earth's surface warm enough to support liquid water and life. Without it, the average surface temperature would be well below the freezing point of water.

📘 Definition

Natural Greenhouse Effect

The process where atmospheric greenhouse gases absorb outgoing longwave infrared radiation emitted by the Earth's surface, re-radiating a portion of that energy back towards the surface rather than letting it escape directly to space.

📐 Worked Example

Identify which of the following radiation types is absorbed by greenhouse gases: 400 nm visible light, 10 μm infrared radiation, 200 nm ultraviolet radiation.

1
First, recall the peak wavelength of radiation emitted by the Earth's ~290 K surface using Wien's displacement law.
2
$\lambda_{\text{peak, Earth}} = \frac{2.9 \times 10^{-3} \, \text{m K}}{290 \, \text{K}} = 10 \, \mu\text{m}$
3
Greenhouse gases are transparent to visible and most UV radiation, so they only absorb the 10 μm infrared radiation.

✓ Quick check

Test your understanding of the basic mechanism:

What type of radiation do greenhouse gases primarily absorb?
- Incoming shortwave UV
- Outgoing longwave infrared
- Incoming visible light
- Outgoing gamma radiation
Reveal answer
Outgoing longwave infrared —
Greenhouse gases do not absorb most incoming solar radiation, only the longer wavelength radiation re-emitted from the warm Earth surface.

2. Key Greenhouse Gases and Absorption Properties★★☆☆☆⏱ 3 min

The four most important greenhouse gases for Earth's climate are water vapour, carbon dioxide, methane, and nitrous oxide. Each has distinct absorption bands in the infrared spectrum and different atmospheric lifetimes.

Greenhouse Gas	Primary Natural Source	Primary Anthropogenic Source	Atmospheric Lifetime
Water vapour (H₂O)	Evaporation from oceans	Combustion of fossil fuels	~10 days
Carbon dioxide (CO₂)	Volcanic eruptions, respiration	Fossil fuel combustion, deforestation	~100 years
Methane (CH₄)	Wetland decomposition	Livestock, landfill, natural gas leaks	~12 years
Nitrous oxide (N₂O)	Soil bacterial activity	Agricultural fertilisers	~120 years

3. Equilibrium Temperature Calculations★★★☆☆⏱ 4 min

✓ Calculator OK

🔬 Derivation

Goal:

Derive the no-atmosphere equilibrium Earth temperature

Starting from:

Total incoming solar power equals total power radiated by Earth as a black body

1
Total solar power incident on Earth's cross-sectional area: $P_{in} = S \times π R^{2} \times (1 - α)$
2
Total power radiated from Earth's full spherical surface via Stefan-Boltzmann law: $P_{out} = σ T^{4} \times 4 π R^{2}$
3
Cancel common terms and rearrange to solve for T:
4
$T = \left( \frac{S(1-\alpha)}{4 \sigma} \right)^{1/4}$

Result:

For S=1360 W m⁻², α=0.3, this gives T ≈ 255 K (-18 °C), far below the actual average surface temperature of 288 K (15 °C). The 33 K difference is the natural greenhouse effect warming.

📐 Worked Example

Calculate the new equilibrium temperature if the average albedo of Earth decreases from 0.3 to 0.25 due to melting sea ice.

1
Substitute the new albedo value into the equilibrium temperature formula:
2
$T = \left( \frac{1360 \times (1-0.25)}{4 \times 5.67 \times 10^{-8}} \right)^{1/4}$
3
Simplify the numerator and denominator inside the brackets:
4
$T = \left( \frac{1020}{2.268 \times 10^{-7}} \right)^{1/4} = (4.497 \times 10^9)^{1/4} \approx 259 \, \text{K}$
5
Convert to Celsius for the final answer: 259 K - 273 = -14 °C, a 4 K increase from the original no-atmosphere value.

4. Enhanced Greenhouse Effect and Exam Phrasing★★★☆☆⏱ 2 min

The enhanced greenhouse effect describes the additional warming caused by human activities increasing atmospheric greenhouse gas concentrations beyond pre-industrial levels, leading to a rise in average global surface temperatures.

5. Common Pitfalls

Wrong move:

Stating that greenhouse gases absorb incoming shortwave solar radiation

Why:

Almost all incoming solar radiation is shortwave visible light, which passes unabsorbed through the atmosphere to reach the surface

Correct move:

Explicitly state that greenhouse gases only absorb outgoing longwave infrared radiation re-emitted by the Earth's warm surface

Wrong move:

Claiming the natural greenhouse effect is a harmful, human-caused process

Why:

The natural greenhouse effect is a necessary, life-supporting process that has existed in Earth's atmosphere for billions of years

Correct move:

Reserve the term 'enhanced greenhouse effect' exclusively for the anthropogenic additional warming from human activity

Wrong move:

Using the full surface area of the Earth when calculating incoming solar power

Why:

Solar radiation only hits the cross-sectional circular area of the Earth, not the full 4πR² spherical surface

Correct move:

Use πR² for incoming power, and 4πR² for total outgoing radiated power in equilibrium calculations

Wrong move:

Including nitrogen (N₂) as a major greenhouse gas

Why:

Symmetric diatomic molecules with no dipole moment change during vibration cannot absorb infrared photons

Correct move:

Only list polyatomic molecules like CO₂, H₂O, CH₄ and N₂O as primary greenhouse gases

Wrong move:

Forgetting to convert temperature from Kelvin to Celsius in final answers

Why:

IB mark schemes almost always expect final surface temperature values to be stated in degrees Celsius for climate-related questions

Correct move:

Always subtract 273 from your Kelvin result to give the temperature in °C unless explicitly told otherwise

6. Quick Reference Cheatsheet

Quantity	Formula	Units	Key Value
Solar constant S	Measured at top of atmosphere	W m⁻²	1360
Earth average albedo α	Dimensionless reflectivity	Unitless	0.3
No-greenhouse equilibrium temp	$T = (\frac{S ( 1 - α )}{4 σ})^{1/4}$	K	255 K (-18 °C)
Actual average surface temp	Measured global average	°C	15 °C (288 K)
Natural greenhouse warming	Difference between actual and no-greenhouse temp	K	33 K

7. Frequently Asked

Why is oxygen not a greenhouse gas?

Oxygen (O₂) is a diatomic molecule with no net change in dipole moment when it vibrates, so it cannot absorb infrared radiation. Only polyatomic molecules like CO₂, H₂O and CH₄ have vibrational modes that interact with longwave infrared photons.

Does the greenhouse effect violate the law of conservation of energy?

No. The effect simply traps more outgoing energy near the surface, raising the surface temperature until the total energy radiated to space once again balances the total incoming solar energy.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2024 · Paper 2
Short answer on greenhouse gas sources
2022 · Paper 1
MCQ on absorbed radiation wavelength
2021 · Paper 2
Equilibrium temperature calculation

What's Next

Mastering the greenhouse effect gives you a foundational understanding of atmospheric energy balance that is tested in almost every IB Physics SL exam session. This concept directly connects to real-world climate physics questions that frequently appear in Paper 2 data analysis sections. You will next apply these calculation skills to albedo variation scenarios, before moving on to explore the physics of climate mitigation strategies including carbon capture and renewable energy thermal systems. This knowledge also overlaps with content from your IB Environmental Systems and Societies course, helping you cross-reference concepts across your subject portfolio.