2.3.3 Radiation – Earth’s Temperature and Energy Balance
Learning Objective
Explain how the temperature of the Earth is affected by the factors that control the balance between incoming solar radiation and radiation emitted from the Earth’s surface.
Definition Box (Core wording)
Thermal radiation is electromagnetic radiation in the infrared region. It does not require a material medium and is emitted by every object that has a temperature above absolute zero.
Key Concepts (Core content)
- Radiation transfers energy through space – unlike conduction and convection, it can travel through the vacuum between the Sun and the Earth.
- All objects emit thermal radiation – the amount depends mainly on temperature and on the surface’s emissivity (how efficiently it radiates).
- Energy‑balance definition – the Earth’s average temperature remains steady when the amount of solar energy absorbed equals the amount of infrared energy emitted.
- Greenhouse effect (qualitative) – certain gases in the atmosphere absorb infrared radiation and re‑emit it in all directions, reducing the net loss of heat to space and keeping the surface warmer.
- Surface colour/texture (albedo) – light‑coloured or shiny surfaces reflect a large fraction of incoming sunlight (high albedo), while dark, matte surfaces absorb more (low albedo). This influences how much solar energy is retained.
Recall: Earth’s Rotation, Orbit and Seasons (Link‑in paragraph)
In the wider IGCSE syllabus the Earth’s rotation causes day and night, while its tilted orbit around the Sun produces the seasons. The tilt changes the angle of incidence (θ) of sunlight on a given location, altering the effective solar flux (S cos θ). This seasonal variation is one of the factors that appears in the energy‑balance discussion.
Factors Controlling Incoming Solar Radiation
- Solar constant (S) – average solar energy received at the top of the atmosphere per unit area perpendicular to the Sun’s rays (≈ 1361 W m⁻²).
- Earth–Sun distance (r) – varies slightly because the orbit is elliptical; solar flux changes as 1⁄r².
- Angle of incidence (θ) – the effective flux on a horizontal surface is S cos θ. A larger θ (sun lower in the sky) means less energy per unit area.
- Albedo (α) – fraction of incident radiation reflected back to space. Global average ≈ 0.30.
Factors Controlling Outgoing Terrestrial Radiation
- Surface temperature (T) – warmer surfaces emit more infrared radiation (the relationship is steep, roughly proportional to T⁴).
- Emissivity (ε) – a number between 0 and 1 that describes how effectively a surface radiates. Most natural surfaces have ε ≈ 0.9–1.
- Atmospheric absorption (greenhouse gases) – gases such as CO₂, H₂O, CH₄ and N₂O absorb infrared radiation and re‑emit it both upwards and downwards, reducing the net loss to space.
- Cloud cover – clouds reflect part of the incoming solar radiation (increasing the planetary albedo) and also absorb and emit infrared radiation.
Conceptual Energy‑Balance Idea
At the Earth’s average temperature:
Absorbed solar power ≈ Emitted infrared power
In words, the fraction of the solar constant that is not reflected ((1 – α) S) must equal the infrared energy that eventually leaves the Earth system. This simple proportional idea is sufficient for the Core syllabus; the detailed Stefan‑Boltzmann formula is part of the Supplement.
Why the Surface Is Warmer Than the “Effective Radiating Temperature”
- The “effective radiating temperature” (~255 K) is the temperature a black‑body would need to emit the same amount of energy to space as the Earth actually does.
- Greenhouse gases absorb some of the infrared radiation from the surface and re‑emit it. The upward‑going part escapes from higher, colder layers of the atmosphere, so a lower temperature (≈ 255 K) is enough to balance the incoming solar energy.
- The surface receives the downward‑re‑emitted infrared radiation, keeping it warmer (~288 K). This temperature difference is the quantitative expression of the greenhouse effect.
How Changes in Each Factor Influence Earth’s Temperature
| Factor | Typical Change | Effect on Energy Balance | Resulting Temperature Trend |
|---|
| Solar constant (S) | Increase (e.g., solar activity) | More solar energy absorbed | Warmer |
| Albedo (α) | Increase (more ice, clouds, volcanic aerosols) | More reflected, less absorbed | Cooler |
| Greenhouse‑gas concentration | Increase (CO₂, CH₄, etc.) | Less infrared escapes to space | Warmer |
| Surface emissivity (ε) | Decrease (e.g., more water vapour, low‑emissivity surfaces) | Reduced efficiency of infrared emission | Warmer |
| Earth–Sun distance (r) | Decrease (Earth nearer the Sun) | Solar flux rises as 1⁄r² | Warmer |
Illustrative Example – Effect of a Change in Albedo
Assume the planetary albedo rises from 0.30 to 0.35 (e.g., after a large volcanic eruption). Using the simple proportional balance:
New absorbed solar fraction = (1 – α) S
Because the absorbed fraction falls, the Earth must cool until the emitted infrared matches the reduced input. Roughly, a 5 % increase in albedo leads to a temperature drop of about 4 °C, which matches observations of short‑term volcanic cooling.
Classroom Activities (Core‑level)
- Albedo measurement – Place coloured paper (black, white, metallic) on a light table, use a light meter to record reflected vs. incident light, and calculate reflectivity. Relate the results to planetary albedo.
- Infrared cooling demonstration – Heat a black matte metal plate and a polished aluminium plate with the same heat source. After switching off the source, record the temperature drop with an infrared thermometer. Discuss how emissivity influences cooling.
- Simple energy‑balance model – Provide a spreadsheet that contains the proportional balance (1 – α) S = emitted infrared. Let students vary S, α and a qualitative “emissivity” factor to see the effect on the resulting temperature.
Suggested Diagram (to be drawn by teacher)
Energy‑flow sketch showing: Sun → incoming solar radiation → (a) reflected by albedo, (b) absorbed by the surface, (c) absorbed/re‑emitted by greenhouse gases and clouds → outgoing infrared radiation to space.
Practice Questions
- Explain how an increase in cloud cover can both cool and warm the Earth.
- The solar constant increases by 2 % while albedo (0.30) and emissivity remain unchanged. Using the proportional balance, indicate whether the Earth’s temperature will rise or fall and why.
- Why is the effective radiating temperature of the Earth (~255 K) lower than the observed average surface temperature (~288 K)? Include the role of greenhouse gases in your answer.
- Describe how the colour and texture of a surface affect its absorption, emission and, consequently, the Earth’s energy balance.
Key Points to Remember
- Radiation is the only way energy is transferred between the Sun and the Earth because it does not need a material medium.
- All objects emit infrared radiation; hotter objects emit more.
- Albedo, greenhouse‑gas concentration, emissivity and Earth–Sun distance are the main factors that shift the Earth’s energy balance.
- Even small changes in any of these factors can produce noticeable climate variations over time.
Summary
Understanding the balance between incoming solar radiation and outgoing terrestrial radiation is central to the IGCSE Physics syllabus. By mastering the concepts of albedo, emissivity, the greenhouse effect and the effect of Earth’s orbital geometry, students can explain why the Earth’s average temperature is what it is, why it differs from the effective radiating temperature, and how natural or human‑induced changes can alter the climate.