So now I want to do a thought experiment.

What if the Earth had no atmosphere?

What would its temperature be, in that situation?

So, I've constructed here a graph of the input from the Sun.

That's 1362 watts per meter squared, at the top of the Earth's atmosphere.

This energy gets absorbed unevenly, around the Earth.

Half of it is in sunlight, remember, but half of it is in darkness.

But this energy is absorbed, and for the Earth to be in thermal equilibrium.

In other words, the temperature of the Earth is neither rising or

decreasing, and is just reached at a steady state.

If the Earth is in thermal equilibrium,

then the amount of energy emitted in the infrared part of the spectrum,

must equal the incoming solar radiation.

And that's what we see here.

The incoming solar radiation equals the outgoing

terrestrial radiation in order for the Earth to be in thermal equilibrium.

Well, it turns out that we can use those equations, the radiation laws,

to calculate what the temperature of the Earth would be in this situation.

So when we do this, we find that the temperature of the Earth, if it had

no atmosphere, would be -18 degree Celsius, globally average temperature.

But we know that the Earth has a much higher temperature than that,

from the observations.

We can calculate what the average temperature on the Earth is, and

we find that it has an average temperature of 15 degrees Celsius.

So there's a 33 degree Celsius difference between our simple calculation of

the Earth without an atmosphere, and the temperature that the Earth actually has.

So we can see that the atmosphere is adding 33 degrees of temperature,

to the average surface temperature of the Earth.