Albedo

Albedo is the fraction of light that is diffusely reflected by a surface. If all light is reflected, $A = 1$. For example $A_{ice} = 0.6$. $A = \frac{\Phi_{reflected}}{\Phi_{reflected}+\Phi_{absorbed}}$ And considering that $c_x$ is fractional surface coverage, $A_{ave} = \sum c_x A_x, \text{where } \sum c_x = 1.$

Why it matters: the fraction that is absorbed is $(1-A)$. The lower the albedo, the more irradiance is absorbed, and the warmer the surface gets. e.g. ice absorbs $1 - 0.6 = 40\%$, ocean absorbs $1 - 0.06 = 94\%$.

Ice-albedo feedback: ice melts → albedo decreases → more absorption → more warming → more ice melts. A decrease in albedo makes $\Phi_{in} > \Phi_{out}$ (until the Earth warms up and emits more infrared), which gives a positive Radiative forcing.

Two-layer structure: $A_{Earth} = c_{ocean}A_{ocean} + c_{land}A_{land}$ $A_{land} = c_{ice}A_{ice} + c_{other}A_{other}$ Within each layer, $\sum c_x = 1$.