Earth's Energy Budget

Figure: Absorption Spectrum of a Blackbody

Absorption Characteristics

The rate at which something radiates and absorbs energy depends strongly on its surface characteristics: color, texture, moisture content, temperature
If it absorbs as much as it emits, we call it a blackbody
- Perfect absorber
- In other words, the object absorbs all the radiation that hits it and emits the maximum radiation possible at its given temperature
- Blackbodies do not have to be colored black
From a radiative standpoint, the Sun and Earth are both nearly blackbodies. However, the atmosphere is not...

All substances (including the Earth) continuously emit and absorb radiation
If something emits more than it absorbs, it will cool
If it absorbs more than it emits, it will warm
Objects that emit and absorb energy at equal rates hold a constant temperature
- Radiative Equilibrium: Incoming = outgoing

Since the Earth itself is a perfect blackbody, it will emit as much as it absorbs
Energy in = energy out
However...

Selective absorbers (CO2, H20, CH4, etc.) absorb Earth's outgoing longwave radiation, and re-emit back down to earth
So, at Earth's surface, energy in > energy out
- Leads to warming of our planet
- We call this the greenhouse effect.

Reflection is like scattering, but the light is mainly sent backwards
An objects albedo is the percent of radiation returning from a given surface compared to the amount of radiation initially striking the surface.
On average, about 30% of the incoming solar radiation is scattered or reflected back to space. This is called the Earth’s albedo.
Each surface has a different albedo – snow and clouds are very reflective, water and dark ground are not

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