The TEMPERATURE of any object, including Earth, is determined by the BALANCE between incoming and outgoing radiation.
EQUILIBRIUM CONDITION:
Temperature stays CONSTANT when rate of energy absorbed = rate of energy emitted.
If absorbed > emitted → temperature RISES.
If emitted > absorbed → temperature FALLS.
EARTH'S RADIATION BALANCE:
INPUT: solar radiation (mostly visible light and UV from the Sun).
Earth absorbs solar radiation → warms up → emits infrared (longer wavelength — Earth is much cooler than Sun).
OUTPUT: infrared radiation emitted by Earth to space.
At equilibrium: incoming solar power = outgoing infrared power.
This determines Earth's average surface temperature (~15°C currently).
The Greenhouse Effect
GREENHOUSE GASES (CO₂, H₂O, CH₄) in the atmosphere absorb outgoing infrared radiation.
NATURAL GREENHOUSE EFFECT:
Sun emits visible light → passes through atmosphere (not absorbed) → absorbed by Earth's surface → Earth warms → emits infrared.
Infrared partially absorbed by greenhouse gases → atmosphere warms → re-emits radiation in all directions (including back to Earth).
This RAISES Earth's temperature above what it would be without the atmosphere.
Without any greenhouse effect: Earth's average temperature would be about −18°C.
ENHANCED GREENHOUSE EFFECT:
Human activities increase concentrations of greenhouse gases (burning fossil fuels → CO₂; livestock → CH₄).
More infrared absorbed → more re-emitted back to Earth → warming → GLOBAL WARMING.
CLIMATE CHANGE EFFECTS:
Melting ice caps → rising sea levels.
Extreme weather events more frequent.
Shifting ecosystems and agricultural zones.
Earth's Temperature and Radiation
FACTORS AFFECTING EARTH'S TEMPERATURE:
1. SOLAR INTENSITY: distance from Sun → amount of solar radiation reaching Earth.
2. ALBEDO: reflectivity of Earth's surface (ice and snow reflect more; oceans and forests absorb more).
3. GREENHOUSE GAS CONCENTRATION: more gases → more insulation.
4. VOLCANIC ACTIVITY: releases CO₂ and sulfur dioxide (short-term cooling from aerosols).
PLANET TEMPERATURES:
MERCURY: close to Sun but no atmosphere → extreme temperature swings, no greenhouse effect.
VENUS: similar distance to Earth but thick CO₂ atmosphere → 465°C (extreme greenhouse effect).
MARS: thin atmosphere → −60°C average.
A BODY RADIATES MOST AT WAVELENGTH determined by its temperature:
Hotter → peak at shorter wavelength.
Sun (~5500°C) → visible light.
Earth (~15°C) → infrared.
This wavelength difference is why greenhouse gases (which absorb IR but not visible) are key to climate.
⚠️ Common Mistake
Earth emits INFRARED radiation — not the same as the visible light from the Sun. Greenhouse gases absorb INFRARED (outgoing from Earth) not visible light (incoming from Sun). The natural greenhouse effect is beneficial — without it, Earth would be too cold for life. The ENHANCED greenhouse effect from human emissions is the problem.
📌 Key Note
Radiation balance: absorbed energy = emitted energy → constant temperature. Earth absorbs solar (visible), emits IR. Greenhouse gases absorb outgoing IR → re-emit → warming. Natural greenhouse: necessary (without it, −18°C). Enhanced: human emissions increase warming. Factors: solar intensity, albedo, greenhouse gas concentration.
🎯 Matching Activity — Radiation Balance
Match each statement to the correct radiation balance concept. — drag the symbols on the right to match the component names on the left.
Absorbed > emitted
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Absorbed = emitted
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Natural greenhouse effect
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Enhanced greenhouse effect
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Greenhouse gases absorb outgoing IR and re-emit — raises Earth's temperature to ~15°C
Object at constant temperature — thermal equilibrium
Object temperature rises — radiation imbalance causing warming
Increased CO₂ from fossil fuels → more IR absorbed → global warming
⭐ Higher Tier Only
HT only — explain how radiation balance determines temperature. Explain the greenhouse effect mechanistically — why greenhouse gases warm Earth. Evaluate the effects of increasing greenhouse gas concentrations on Earth's radiation balance and temperature.
🔬 Triple Science Only
Radiation balance and Earth's temperature (HT only, physics only) — not in Combined Science as a quantitative topic.
🎯 Test Yourself
Question 1 of 2
1. Why do greenhouse gases absorb outgoing infrared from Earth but not incoming solar visible light?
2. Without any greenhouse effect, what would Earth's average temperature be?
⭐ How Well Do You Understand This Topic?
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