Earth's Temperature and Radiation Balance Revision Notes | GCSE AQA Higher Physics

1. Why Earth Doesn't Freeze or Fry

Earth sits 150 million kilometres from a nuclear furnace that could vaporise our planet in seconds. Yet here we are, with liquid water and life thriving. What keeps Earth at just the right temperature?

The answer lies in a delicate balance. Earth receives energy from the Sun and loses energy to space. When these two processes are in equilibrium, Earth's temperature stays stable. This is called radiation balance.

Think of it like your bank account. Money comes in (your salary), money goes out (your spending). If they balance, your account stays steady. Earth works the same way with energy - radiation comes in from the Sun, radiation goes out to space.

2. The Three Key Processes

When radiation hits any object, including Earth, three things can happen. The radiation can be absorbed, reflected, or transmitted through the object. For Earth as a whole, transmission is negligible, so we focus on absorption and reflection.

Absorption converts the radiation into thermal energy, heating up the material. A black car gets hot in sunlight because it absorbs most of the radiation. Reflection bounces the radiation away without heating the material - like a mirror in sunlight staying relatively cool.

Earth also emits radiation because of its temperature. All objects above absolute zero emit electromagnetic radiation. The hotter the object, the more radiation it emits. This is how Earth loses energy to space.

3. Everyday Examples of Radiation Balance

You experience radiation balance every day. When you step from shade into sunlight, you feel warmer because you're absorbing more solar radiation. Your body temperature rises until you emit enough extra radiation (and lose heat through other processes) to balance the input.

A car parked in the sun demonstrates this perfectly. Initially, the car absorbs more radiation than it emits, so it heats up. Eventually, it reaches a temperature where emission balances absorption. The car stops getting hotter - it's reached radiation balance.

Different materials reach different balance temperatures. A black car gets much hotter than a white car because black absorbs more solar radiation. The white car reflects more radiation away, so less energy is available to heat it up.

At night, objects cool down because they continue emitting radiation but receive very little input. They lose more energy than they gain, so their temperature drops until a new balance is reached with the cooler surroundings.

4. Earth's Radiation Balance System

Earth receives about 1,360 watts per square metre of solar radiation at the top of the atmosphere. However, not all of this reaches the surface. Clouds, atmospheric gases, and particles reflect or absorb some radiation before it reaches the ground.

The radiation that does reach Earth's surface gets absorbed by land, oceans, and vegetation. This absorbed energy heats the surface, which then emits infrared radiation back towards space. Some of this infrared radiation escapes directly to space, but some gets absorbed by greenhouse gases in the atmosphere.

The atmosphere itself also emits radiation - some towards space and some back towards Earth's surface. This creates a complex system where radiation bounces back and forth between the surface and atmosphere before eventually escaping to space.

For Earth to maintain a stable temperature, the total energy emitted to space must equal the total solar energy absorbed. If more energy comes in than goes out, Earth warms up. If more goes out than comes in, Earth cools down.

5. Factors That Tip the Balance

Several factors can change Earth's radiation balance and alter its temperature. Changes in solar output affect how much energy Earth receives. The Sun's brightness varies slightly over time, influencing Earth's energy input.

Earth's surface properties matter enormously. Ice and snow reflect up to 90% of incoming radiation, while forests and oceans absorb most of it. When ice melts and exposes darker surfaces underneath, more radiation gets absorbed and Earth warms further.

Atmospheric composition plays a crucial role. Greenhouse gases like carbon dioxide and water vapour absorb infrared radiation emitted by Earth's surface. This reduces the amount of energy escaping directly to space, causing the surface to warm up.

Clouds have complex effects. They reflect incoming solar radiation (cooling effect) but also absorb and re-emit infrared radiation (warming effect). The net effect depends on cloud type, altitude, and thickness.

6. Exam Success with Radiation Balance

Exam questions often ask you to explain temperature changes using radiation balance. The key is to identify what's changing - is it absorption, emission, or reflection? Then explain how this affects the energy balance and therefore temperature.

When describing radiation processes, use precise language. Say 'absorbs radiation' not 'takes in heat'. Say 'emits infrared radiation' not 'gives off energy'. Examiners look for correct scientific terminology.

For diagram questions, identify the radiation arrows carefully. Incoming solar radiation is usually shown as yellow arrows. Emitted infrared radiation is typically red arrows. Reflected radiation bounces back in the same direction it came from.

Worked example: Worked Method: Earth's Temperature and Radiation Balance

Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.

1. Write down the correct physics formula. — Write down the correct physics formula.
2. Substitute known values with units. — Substitute known values with units.
3. Rearrange if needed and calculate carefully. — Rearrange if needed and calculate carefully.
4. Give the final answer with correct unit and sensible precision. — Give the final answer with correct unit and sensible precision.

Answer: Final answer should include unit and appropriate significant figures.