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Section 3.4 Energy budget of the Earth

As discussed in a previous section, the solar flux is about 1368 W/m\(^2\) at Earth’s orbit (incoming flux at top of atmosphere). Because the Earth is rotating, this energy is spread over the entire surface area of the Earth facing the Sun at any given time. So, we have (area of disk)/(area of sphere) = 1/4 of this value incident, on average, at the top of the Earth’s atmosphere: 342 W/m\(^2\text{.}\)

The radiation transfers are via shortwave (visible) or longwave (infrared) radiation. Why these two? Because of their temperatures, the Sun is emitting in the visible and Earth materials are emitting in the infrared. As we will see in the next section, this is the basis for the greenhouse effect.

Energy Budget Diagram

All fluxes eventually reach equilibrium - if an imbalance, then temperature will change in response (to restore equilibrium):

  • incoming energy = outgoing energy, \(\Delta T=0\)

  • incoming energy > outgoing energy, \(\Delta T>0\)

  • incoming energy < outgoing energy, \(\Delta T<0\)

We see this even during the course of a day, as the temperature changes in response to incoming and outgoing radiation. For example, temperatures cool at night because solar energy is no longer being absorbed at the surface while the Earth's surface continues to emit radiation to space (in the infrared). So at night, the outgoing energy exceeds the incoming energy, leading to cooling.