The climate system and feedbacks
The climate system is made of the air, the oceans, ice, land and living things. They swap heat, water and gases all the time.
A feedback is a loop. A change causes an effect, and that effect changes the first thing again. If it makes the change bigger, it is a positive feedback (an amplifier). If it makes the change smaller, it is a negative feedback (a brake).
Albedo: how shiny is the ground?
Albedo is the fraction of sunlight a surface reflects. 0 means it takes in everything; 1 means it reflects everything.
- Fresh snow and ice: about 0.8 to 0.9
- Clouds: about 0.4 to 0.8
- Desert sand: about 0.3 to 0.4
- Forest: about 0.1 to 0.15
- Open sea: about 0.06 to 0.07
The part that is not reflected is absorbed and turns into heat: absorbed = (1 − albedo) × incoming sunlight.
The ice-albedo feedback loop
1. The air gets a little warmer. 2. Some ice melts. 3. Dark sea or land is uncovered, so albedo falls. 4. More sunlight is absorbed. 5. The place gets warmer still. 6. More ice melts. And round again.
It is a positive feedback. It is one reason the Arctic is warming faster than the rest of the world (polar amplification). The same idea works on land: melting snow uncovers dark soil and forest.
Other feedbacks and what stops runaway change
Other positive feedbacks: warmer air holds more water vapour (a greenhouse gas), and thawing frozen ground (permafrost) can release methane. Negative feedbacks: a warmer Earth gives out more heat to space, and more CO₂ helps some plants grow faster.
The real climate is the sum of many feedbacks. Scientists put them into climate models to see how much the world will warm.
Try it: white versus black
Put two ice cubes of the same size in the sun, one on white paper and one on black paper. Note which melts first and by how much after 10 minutes. The black paper has low albedo, so it absorbs more heat. Then use the 3D slider to see the same idea on a polar sea.
Key formulas and definitions
- Albedo = reflected sunlight ÷ incoming sunlight
- Absorbed = (1 − albedo) × incoming
- Mean albedo of a mixed area = (share of ice × 0.85) + (share of sea × 0.07)
- Positive feedback: effect strengthens the cause. Negative feedback: effect weakens the cause.
Worked examples
1. Ice with albedo 0.85 receives 200 W/m² of sunlight. How much is reflected and how much absorbed?
Reflected = 0.85 × 200 = 170 W/m². Absorbed = 200 − 170 = 30 W/m².
2. Open sea (albedo 0.07) receives the same 200 W/m². How much does it absorb, and how many times more than the ice?
Absorbed = (1 − 0.07) × 200 = 186 W/m². 186 ÷ 30 ≈ 6.2 times more than the ice.
3. An area is half ice (0.85) and half sea (0.07). Find the mean albedo and the heat absorbed from 200 W/m².
Mean albedo = 0.5 × 0.85 + 0.5 × 0.07 = 0.46. Absorbed = (1 − 0.46) × 200 = 108 W/m².
4. The ice shrinks to 25% of the area (75% sea). Find the new absorbed heat and the increase over the 50% case.
Mean albedo = 0.25 × 0.85 + 0.75 × 0.07 = 0.265. Absorbed = 0.735 × 200 = 147 W/m². Increase = 147 − 108 = 39 W/m².
Common mistakes
- Thinking a "positive" feedback is good. Positive here means it strengthens the change, so it can make warming worse.
- Mixing up albedo and absorption. High albedo means more reflected and less absorbed.
- Saying dark surfaces reflect sunlight. Dark surfaces absorb it.
- Believing one feedback decides the climate. The result comes from many feedbacks acting together.