VCE Environmental Science (Year 12) · Unit 4 — Sustainable Energy · 16m read

Earth's Energy Balance and the Enhanced Greenhouse Effect

Without the greenhouse effect, Earth would be frozen; with too much of it, the climate shifts in ways that affect every ecosystem and economy. Unit 4 starts with the physics of energy in and energy out.

Learning objective: Explain Earth’s energy balance, the natural and enhanced greenhouse effect, and feedbacks in the climate system.

By the end of this lesson you can:

• I can describe incoming and outgoing radiation and albedo.
• I can distinguish the natural and enhanced greenhouse effect.
• I can explain positive and negative climate feedbacks.

The enhanced greenhouse effect is an extra blanket. Natural greenhouse gases keep Earth habitable; extra CO₂ and methane thicken the blanket and trap more heat.

Energy in, energy out

ENERGY BALANCE:
• The Sun emits mostly shortwave radiation (visible, UV).
• About 30% of incoming radiation is reflected (Earth’s ALBEDO is about 0.3).
• The rest is absorbed, warming the surface, which emits LONGWAVE (infrared) radiation.
• Stable climate when energy in ≈ energy out.

NATURAL GREENHOUSE EFFECT:
• Greenhouse gases (water vapour, CO2, methane, nitrous oxide) absorb and re-emit infrared radiation, warming the lower atmosphere.
• Keeps Earth’s average surface temperature around 15 °C instead of well below freezing.

ENHANCED GREENHOUSE EFFECT:
• Human activities (burning fossil fuels, land clearing, agriculture) add greenhouse gases.
• Atmospheric CO2 has risen from about 280 ppm before industrialisation to over 420 ppm today.
• The extra energy retained has warmed the planet by about 1.1 °C since pre-industrial times (IPCC).

FEEDBACKS:
• POSITIVE (amplifying): ice–albedo feedback, water vapour feedback, permafrost methane release.
• NEGATIVE (dampening): increased plant growth absorbing CO2, some cloud effects.

Quick reference

| Concept | Meaning |
|---|---|
| Energy balance | Incoming solar = outgoing IR |
| Greenhouse gases | CO₂, CH₄, N₂O, water vapour |
| Albedo | Reflectivity (ice high) |
| Radiative forcing | Change in energy balance |
| Feedbacks | Ice-albedo, water vapour |

Key ideas at a glance

• Natural vs enhanced: Distinguish.
• GWP: Methane much stronger than CO₂ short-term.
• Positive feedbacks amplify: Melting ice.
• Evidence-based: Data trends.

Worked example (Medium)

Explain the ice–albedo feedback.

1. Start. Warming melts sea ice and snow.
2. Albedo. Ice reflects most sunlight; exposed ocean and land are darker and absorb more.
3. Loop. More absorption causes more warming and more melting — a positive feedback.

Answer: Melting reduces albedo, increasing absorption and further warming — a self-reinforcing loop.

Common mistakes

• Saying greenhouse gases "trap sunlight" They absorb outgoing infrared, not incoming shortwave. Fix: Specify longwave (infrared) radiation from Earth’s surface.
• Confusing the greenhouse effect with ozone depletion They are different processes. Fix: Ozone depletion concerns UV in the stratosphere.

The Keeling Curve

Since 1958, CO₂ has been measured at Mauna Loa, Hawaii. The Keeling Curve shows a steady rise (from ~315 ppm to over 420 ppm) with seasonal wiggles as northern forests breathe.

It's one of the clearest records of human impact on the atmosphere.

Practice questions

1. Which is a NEGATIVE climate feedback?

   1. Melting permafrost releasing methane
   2. Increased plant growth absorbing more CO2
   3. Loss of sea ice
   4. More water vapour in a warmer atmosphere

   (1 marks)

Summary

• Energy balance: shortwave in, longwave out, albedo ~0.3.
• Natural greenhouse effect keeps Earth habitable.
• Human emissions enhance it; CO2 > 420 ppm.
• Feedbacks can amplify or dampen change.