Geophysics: The Physics of the Earth and Its Climate

What is geophysics?

Geophysics is physics used on the Earth. It studies heat, waves, gravity, magnetism and flows inside and around our planet.

Earth is a system. Energy comes in from the Sun, moves through air, water, rock and ice, and goes back to space. Physics lets us count this energy.

Climate processes: energy in equals energy out

The Sun gives about 1361 W on every square metre facing it (the solar constant, S). The Earth is a sphere, so on average each square metre of the whole surface gets S ÷ 4.

About 30 percent is reflected by clouds, ice and sea (this fraction is the albedo, a = 0.3). So the absorbed power is S × (1 − a) ÷ 4 ≈ 238 W/m².

A warm body sends out heat. The hotter it is, the more it sends: P = σT⁴ (σ = 5.67 × 10⁻⁸ W/m²K⁴). When P out = 238 W/m², T ≈ 255 K = −18 °C.

The greenhouse effect

Greenhouse gases (water vapour, CO₂, methane) let sunlight in but catch outgoing infrared. They send part of it back down. The surface must warm until enough heat still escapes. This lifts the average to about 288 K = 15 °C. It is a natural effect that makes life possible. Adding more gas makes it stronger.

Inside the Earth: seismic waves

Nobody can dig to the centre, so we use waves from earthquakes. P-waves (push-pull) are faster and go through solids and liquids. S-waves (sideways shake) are slower and cannot go through liquid.

S-waves do not arrive on the far side of the Earth. This showed that the outer core is liquid. Waves bend and change speed at layer boundaries, so a recorder (seismometer) can map the crust, mantle and core.

The lag between the P and S arrival tells the distance to the quake: d = Δt ÷ (1/v_S − 1/v_P).

Analysing geophysical data

Geophysical data are numbers over time or place: temperature, sea level, quake times, magnetic field. To analyse them:

1. Plot the data with clear axes and units.
2. Look for a trend (slow change) and for noise (small random ups and downs).
3. Average over a long time (30 years for climate) so one hot year does not fool you.
4. Check the size of the change against the error of the instrument.
5. Do not mix up correlation with cause. Use physics to explain why.

Try it

Put a thermometer in a closed clear jar and another in an open jar, both in sunlight. After 20 minutes, compare. Predict first, then check. Which is warmer and why?

Key formulas and definitions

• Absorbed power = S × (1 − a) ÷ 4
• Heat radiated: P = σT⁴
• No-greenhouse temperature ≈ 255 K = −18 °C
• Quake distance: d = Δt ÷ (1/vS − 1/vP)
• K = °C + 273

Worked examples

Common mistakes

• Thinking the greenhouse effect is bad in itself. Without it Earth would be about −18 °C. The problem is making it stronger.
• Mixing weather and climate. Climate is the average over about 30 years.
• Using °C inside P = σT⁴. You must use kelvin.
• Saying S-waves pass through the liquid core. They cannot.