Sunlight in, infrared out
Every warm object gives off radiation. The hotter it is, the shorter the wavelength. The Sun (about 5 500 °C) gives mostly visible light. Earth's surface (about 15 °C) gives mostly infrared (IR), which we cannot see but can feel as heat.
On average about 340 W of sunlight reaches each square metre at the top of the atmosphere (a quarter of the 1 361 W/m² solar constant, because Earth is a sphere that spins).
Energy balance: if Earth takes in more energy than it gives out, it warms up until it gives out enough to match. Energy in = energy out sets the average temperature.
Albedo: the share that bounces back
Albedo is the fraction of sunlight a surface reflects. 0 means it absorbs everything; 1 means it reflects everything.
- Fresh snow: about 0.8–0.9
- Clouds: about 0.4–0.8
- Desert sand: about 0.3–0.4
- Forest: about 0.1–0.2
- Ocean: about 0.06
Earth's average albedo is about 0.30. When ice melts, dark sea replaces white ice, albedo falls, more light is absorbed and more ice melts. This is a feedback loop.
Why some gases trap infrared (selective absorption)
Nitrogen (N₂) and oxygen (O₂) make up 99% of air, but they do not absorb infrared. Molecules with three or more atoms, or two different atoms, can bend and stretch in ways that soak up IR of certain wavelengths. These are the greenhouse gases:
- Water vapour (H₂O) – the biggest share, but it follows temperature;
- Carbon dioxide (CO₂) – from burning coal, oil and gas, cutting forests;
- Methane (CH₄) – from cattle, rice paddies, landfills, gas leaks; about 80 times stronger than CO₂ over 20 years;
- Nitrous oxide (N₂O) – from fertilisers.
This is selective absorption: the gases are almost transparent to visible light but absorb IR. After absorbing, a molecule re-emits IR in all directions, so about half goes back down. The ground gets extra warming.
Natural vs enhanced effect
The natural greenhouse effect keeps Earth liveable (about +15 °C instead of −18 °C). The enhanced effect is the extra warming from gases added by people: CO₂ has risen from about 280 ppm before 1800 to over 420 ppm today.
Carbon oxides, methane and the carbon cycle
Carbon dioxide (CO₂) is not poisonous in normal amounts and plants need it. It warms the climate and makes the oceans more acidic. Carbon monoxide (CO) is different: it forms when fuel burns with too little oxygen, has no smell and is poisonous because it stops blood carrying oxygen. Never run a stove or generator in a closed room.
The carbon cycle
Carbon moves round: plants take in CO₂ by photosynthesis → animals eat plants → respiration and decay return CO₂ → oceans dissolve CO₂ → some carbon is buried for millions of years as coal, oil and gas. Burning fossil fuels puts that old carbon back into the air much faster than plants and oceans can remove it.
Methane: fuel and greenhouse gas
Methane (CH₄) is the main part of natural gas and biogas. Burnt as fuel it gives CO₂ and water (CH₄ + 2O₂ → CO₂ + 2H₂O). Leaking unburnt, it is a strong greenhouse gas. Capturing methane from dung and waste in a biogas plant (common in Indian villages) gives cooking fuel and stops it escaping. Decarbonisation means cutting carbon emissions: renewables, efficiency, electric transport, forests.
Try it: two jars in the sun
Put two thermometers in two glass jars in the sun. Cover one jar with cling film. Read both every 5 minutes for 30 minutes. The covered jar warms more because warm air cannot escape. (The real atmosphere traps infrared rather than air, but the idea of slowing heat loss is the same.) Then in the 3D, raise CO₂ to 840 ppm and see the balance temperature go up.
Key formulas and definitions
- Energy balance: energy absorbed = energy radiated
- Absorbed sunlight = (1 − albedo) × incoming sunlight
- Albedo = reflected ÷ incoming
- Methane burning: CH₄ + 2O₂ → CO₂ + 2H₂O
- No greenhouse gases ≈ −18 °C; today ≈ +15 °C (greenhouse warming ≈ 33 °C)
Worked examples
1. Of 340 W/m² of sunlight, 102 W/m² is reflected. Find the albedo.
Albedo = 102 ÷ 340 = 0.30.
2. With albedo 0.30, how much of 340 W/m² is absorbed?
Absorbed = (1 − 0.30) × 340 = 0.70 × 340 = 238 W/m².
3. A patch of sea ice (albedo 0.8) melts into ocean (albedo 0.06). How much more of 200 W/m² sunlight is absorbed?
Ice absorbs 0.2 × 200 = 40 W/m². Ocean absorbs 0.94 × 200 = 188 W/m². Extra = 148 W/m², so melting speeds up warming.
4. Why does O₂ not act as a greenhouse gas while CO₂ does?
O₂ has two identical atoms and cannot bend or stretch in a way that absorbs infrared. CO₂ has three atoms and can bend, so it absorbs IR.
Common mistakes
- Thinking the greenhouse effect is bad in itself. The natural effect keeps Earth warm enough for life; the problem is the extra (enhanced) effect.
- Saying greenhouse gases block sunlight. They let visible sunlight in; they absorb the infrared going out.
- Mixing up the ozone hole and the greenhouse effect. Ozone loss lets in more UV; it is a different problem.
- Thinking CO and CO₂ are the same. CO is poisonous to breathe; CO₂ mainly causes warming.