Final answer:
To maintain equilibrium, Earth must emit as much energy as it receives from the sun, which was historically 240 W/m². Energy balance is described by the Stefan-Boltzmann law.
Step-by-step explanation:
Equilibrium and Earth's Energy Balance
To maintain equilibrium, the energy leaving Earth must equal the energy the Earth receives from the sun. Under pre-industrial conditions, this was 240 W/m². When Earth is at equilibrium, incoming solar power is balanced by outgoing infrared radiation. The Stefan-Boltzmann law describes this balance with the equation P = σA(T4 - Tenv4), where σ (sigma) is the Stefan-Boltzmann constant and T represents temperature in Kelvin.
Considering that the planet absorbs energy at a rate of 240 W/m² on average, this means that it must also radiate 240 W/m² back to space to maintain equilibrium. If there is any change, such as an increase in CO2 leading to radiative forcing, Earth's temperature would adjust until the outgoing infrared radiation matches the excess energy input.