The albedo effect

Albedo is an astronomer’s term for the depth of colour of a planet, i.e. its lightness or darkness, and hence its reflectivity. A black planet, reflecting no light, has an albedo of 0.0; a grey planet, reflecting half the light incident on it. has albedo 0.5; and a white one, reflecting all incident light, has albedo 1.0.

Put most simply, the surface temperature of a planet depends on the balance between the heat it receives from its sun, and the heat it returns to space. On Earth this equation is complicated by the greenhouse effect. It is also strongly influenced by albedo, which varies with the planet’s topography (see diagram). Light areas such as polar caps, snow, or clouds can reflect 70 or 80 per cent of incoming sunlight back to space. They have high albedo. Dark areas such as belts of forest, or oceans, have low albedo; they absorb the Sun’s heat, and then radiate it back in the infrared, in which case it can be retained by the greenhouse gases. The Earth’s albedo changes seasonally with ice and cloud cover and the growth of vegetation. (see variation 30)