Energy balance of the earth: 0-dimensional, steady-state view

Steady-state: d(energy storage)/dt = inflow per time - outflow per time = 0
0-dimensional: represented by one T, ignoring spatial inhomogeneity

absorbed solar radiation = emitted terrestrial radiation
π r^2 × (1 - α_p) × S_0 = 4π r^2 × σ T^4
Absorbed solar radiation:  assumed to be constant (average of observations)
S_0("solar constant"): Energy per unit area per unit time arriving to a perpendicular surface at the average sun-earth distance. 1370 W/m^2.
α_p(planetary albedo): the fraction of reflection. 0.30 .
Emitted terrestrial radiation:  assumed to be blackbody radiation.
Energy per unit area per unit time σ T^4 (σ:Stefan-Boltzmann constant)
r: radius of earth (eliminated when divided by the surface area of the earth)