So we constructed, at the very beginning of the class,
a very simple model of the climate of the earth
and since then we've been improving it and talking about ways in which
it should be more realistic in order to approximate the real world.
Here, we started with just original bare rock model of energy
coming into and out of a planet and that determines the planet's temperature.
Then we added the greenhouse effect,
with this plate of glass that absorbs
all the infrared light coming from the ground.
And then we talked about how the greenhouse gases
in the atmosphere don't absorb all of the light.
They absorb only certain parts of the light energy.
And those other, other frequencies, such as the atmospheric
window here, which greenhouse gases don't absorb at all.
And then in the last unit, we talked about another mechanism
of carrying heat around in the atmosphere, in addition to
the heat fluxes from light, which is convection.
And in particular, we found out that moist convection
is what drives the thermal structure of our atmosphere.
You have air rising and it cools and condenses and that's where it rains,
but it dumps latent heat up here and also carries heat around.
That's where we are.
But there's one more thing that the real world has
that our conceptual models do not have yet.
And that is that the whole world is not a single place.
It's not all the same temperature, and the sunlight intensity is
not always in balance with the energy going out by infrared locally.
So the real geometry is that you have
sunlight all coming from one direction from the sun,
and it's coming in most strongly at the equator.
Infrared light is leaving everywhere around the earth,
And the rate at which it leaves depends on
the temperature of the ground according to Epsilon's sigma T to the fourth.
And it turns out that if you look at a given
location on the earth, the incoming and outgoing energy budgets do not balance.
They only balance for the earth as a whole on a long enough time average.
Here is plot of the sunlight intensity in watts per
square meter as a function of latitude and
its very strong at the equator.
Here is a plot of the infrared energy
leaving the earth as a function of latitude.
And its also stronger near the equator because it is warmer at the equator than
it is at the poles, but there is a deficit.
It's not hot enough at the Equator to drive the
energy out as quickly as the sun is bringing it in.
And it's not cold enough at the Equator,
at the high latitudes, for the infrared to be as low as the sunlight coming in.
And the difference is what we call heat transport.
It's heat being carried by the atmosphere and by the
oceans from the low latitudes to the high latitudes.