A Satellite's View of the Climate Energy Budget
The diagram we have just been considering (repeated above), presents a good overview of how energy flows through the Earth’s climate system, but it does not give us a sense of how that energy is distributed across the surface of the globe and there are some important things to be learned from looking at this spatial pattern. For many years now, satellites have been monitoring these energy flows using spectrometers that measure the intensity of energy at different wavelengths flowing to the Earth and from the Earth. So, let’s see what can be learned from a quick study of these satellite views. First, we consider the insolation at the top of the atmosphere averaged for the month of March.
Of course, not all of this insolation strikes the surface — remember that just 49% of it reaches the ground. If we then look at the insolation reaching the ground, we see the following:
Notice that the highest flux is about 190 W/m2; far less than the maximum of almost 440 W/m2 that reaches the top of the atmosphere. The difference is due to reflection from clouds, reflection from the surface, and absorption by atmospheric gases.
Now, let’s look at what comes back from the Earth, in the form of long wavelength energy, for the same time period.
At the simplest level, we see that the tropics emit much more energy than the poles. This makes sense since we know they are warmer, and the Stefan-Boltzmann law tells us that the amount of energy emitted varies as the fourth power of temperature, and the tropics are warmer because they receive much more insolation (see figures above). Looking closely at the nearest above image, we see some interesting variations near the tropics — look at South America, central Africa, and Indonesia, where the emitted energy is far less than we see elsewhere at these same latitudes. Why is this? Is it colder there? No, it is not colder there, which leads to another question — is the atmosphere above these regions absorbing more of the infrared energy emitted by the surface? Recall that one of the main heat-absorbing gases is water, and where you have a lot of water, you have a lot of clouds. So, let’s have a look at the typical average cloud cover for this time of year.
So, it is indeed the case that the amount of energy leaving the Earth varies according not only to the temperature but also to the concentration of heat-absorbing gases such as water.
Recall that we are focused on the energy budget here and whenever you do a budget, at the end, you look at the balance between what is coming in and what is going out. So, let’s do that now with the energy as measured by the satellites.
As can be seen in the figure above, the tropics receive more energy than they emit, while the poles emit more than they receive. This picture can also be seen in a somewhat simpler diagram in which we average the net energy flow at each latitude.