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A Satellite's View of the Climate Energy Budget

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A Satellite's View of the Climate Energy Budget

Graphic model of the energy budget for Earth's climate system. See text description below
Energy Flows in the Climate System
Click here for a text description

The energy budget of the climate system, based largely on satellite data; the numbers inside the circles are globally averaged, and annually averaged flows in units that reflect the percentage of solar energy Earth receives in a year. The numbers in the boxes are the amounts of thermal energy stored in the atmosphere and surface reservoirs (the surface, in this case, is mainly the surface water of the oceans). The two large flows on the right represent a kind of energy recycling program that constitutes the greenhouse effect; heat emitted from Earth’s surface is absorbed by gases in the atmosphere and then re-radiated back to the surface.

Here 100 energy units = 5.56e24J/year, the total annual solar energy received averages 342 W/m^@ over the surface of the Earth

Incoming solar radiation: 100

Insolation Reflected by Clouds and Aerosols: 23

Insolation Reflected off Land Surface: 9

Insolation Absorbed by Surface: 49

Atmosphere Reservoir: 16.5

Surface Reservoir (30% Land, 70% Water): 271.2

Heat Transfer to Atmosphere: 133

Heat Lost to Space: 11

Heat returned to Surface (Greenhouse Effect): 95

Heat Radiated into Space from top of Atmosphere: 57

Credit: © Kiehl and Trenberth, 1997 Used with permission

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.

Graphic map of global insolation at the top of the atmosphere for March 2003, trends described in caption
The insolation averaged over March 2003 from NASA’s CERES satellite. In March, the insolation is at its greatest right at the equator and drops off to nearly zero at the poles. Note that this is the insolation before it interacts with the atmosphere, so it is a fairly simple pattern.
Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

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:

Map of insolation reaching surface March, 1960. Red over s. america, china, middle east & africa, blue over most ocean dark blue @ poles
The difference between energy coming in and energy leaving the Earth for March 1985-1989, from NASA’s ERBE experiment. The red areas are places where more energy is coming in than is going out, while the blue areas are places where more energy is leaving than is coming in. So the red areas have a surplus of insolation, while the blue areas are running a deficit relative to insolation. 
Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

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.

Global map of the emitted long wavelength energy March, 1985-1989, more energy emitted over tropics, a lot over the water, see text below
The emitted long wavelength energy averaged over March 1985-1989, from NASA’s ERBE experiment. This is the emitted infrared energy after it interacts with the atmosphere (the satellite is way above the atmosphere, looking down on Earth). The complex pattern reflects variability in surface temperature and concentrations of greenhouse gases.
Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

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.

Global map of the percent cloud cover for March. Matched spots of less radiation from image above. Lots of clouds between poles and tropics
The cloud fraction averaged over March 2005, from NASA’s CERES satellite. White is 100% cloud cover, while dark blue is 0% cloud cover. Note that the white areas of high cloud cover correspond to the regions where there is less long wavelength energy emitted.
Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

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.

Global map of net radiation; the incoming SW minus the outgoing LW. Red @ equator yellow @ tropics, then light blue & dark blue at poles
The difference between energy coming in and energy leaving the Earth for March 1985-1989, from NASA’s ERBE experiment. The red areas are places where more energy is coming in than is going out, while the blue areas are places where more energy is leaving than is coming in. So the red areas have a surplus of insolation, while the blue areas are running a deficit relative to insolation. 
Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

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.

Energy surplus between -45 and 45 degrees latitude, deficiency between (+-) 45 and (+-) 90 degrees
The insolation reaching the surface averaged over March 1960, from NASA’s ERBE experiment.
Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

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