The Earth's Energy Budget II -- Radiation Emitted by the Earth, the Greenhouse Effect, and the Overall Energy Balance

On the previous page we looked at the absorption of solar radiation by the Earth. This is the energy into the planet Earth. Because the average temperture of the Earth is nearly constant from year to year, using the principle of energy balance, we know that the radiation energy emitted by the Earth must equal the radiation energy absorbed from the Sun. Using the radiation laws, we could compute the average temperature for the planet Earth, which we called the radiative equilibrium temperature. As mentioned on the previous page, the radiative equilibrium temperature of the Earth is 0°F. This is actually the average temperature at which radiation is emitted from the Planet Earth. If there were no atmosphere (and no change in the amount of solar radiation energy absorbed by the planet), this would be the average temperature at the Earth's surface.

The Greenhouse Effect

The radiative equilibrium temperature of the planet Earth (based on the amount of radiation energy that the planet emits to space) is quite a bit colder than the average temperature of the Earth's surface. The reason this is possible is because the atmosphere plays a large roll in the emission of infrared radiation out to space. In effect, it slows down the net rate at which the ground surface cools by radiation. This is known as the greenhouse effect. This WORD document describing the fundamental of the greenhouse effect offers a simplified explanation for how it operates.

I will try to make a simple analogy with using a blanket to keep warm that may help. Let's assume that the human body generates internal heat at a constant rate. (Of course this is not true because the body uses thermoregulation to maintain a near constant temperature). The internal heat generated will cause the surface temperature of your body to heat up to the point where the energy out equals the internal energy generated. This is like the concept of radiative equilibrium temperature for the Earth. Now suppose a blanket is placed over the body. This will slow down the net rate of heat loss from the surface of the body as some of the heat given off by the body will be absorbed by the blanket and returned to the body. As a result the surface temperature of the body will start to increase since the net rate of heat loss (at the body surface) is now less than the heat input or internal heat generation. The surface of the body will continue to warm until the net rate of heat loss again equals the rate of internal heat generation. However, this will be at a higher temperture than before the blanket was put on. So even though the energy input did not change, the surface temperature of the body became warmer because the blanket slowed down the net rate of heat loss from the body. Somewhere on the outside surface of the blanket the temperature would be such that radiatve equilibrium is satisfied (energy lost by the system equals the energy input), but it is warmer underneath the blanket at the body surface. This is very much like having a greenhouse atmosphere. Greenhouse gases slow down the net rate at which the Earth's surface loses radiation energy, while still allowing the energy input to mainly heat the surface. So even with no change in the energy input from the sun, adding greenhouse gases to the atmosphere will make the surface warmer than if there were no greenhouse gases or greenhouse effect. Adding more greenhouse gases, i.e., strengthening the greenhose effect should make the Earth's surface warmer just like adding more blankets would act to make the surface of the body warmer.

Basics of the greenhouse effect:

• The atmosphere allows the majority of the Sun's radiation (visible radiation) to pass through to the surface where much of it is absorbed and goes into heating the surface.
• The atmosphere absorbs the majority of radiation emitted from the surface of the Earth (infrared radiation). This energy is not lost to space, so it does not cool the planet.
• Each type of gas molecule in the atmosphere interacts differently with radiation, however, in the atmosphere of Earth, when it is clear (not cloudy) it is mainly water vapor and carbon dioxide that determine the the strength of the greenhouse effect. These are the most important greenhouse gases. Keep in mind that water vapor and carbon dioxide are trace gases in the atmosphere of Earth. You should also realize that of the two dominant greenhouse gases on Earth, water vapor is by far the most important in the overall greenhouse effect.
• The most abundant gas molecules, nitrogen and oxygen, do not interact much with infrared radiation and are not greenhouse gases.
• Clouds absorb infrared photons very efficiently, and in essense contribute to the greenhouse effect. In fact the cloud effect is a huge contributor to the overall greenhouse effect on Earth.
• The infrared radiation energy absorbed by the greenhouse gases and clouds heats the atmosphere. The energy is shared by all gas molecules by conduction, i.e., collisions between the greenhouse gases and non-greenhouse gases.
• The atmosphere emits infrared radiation in all directions. The part of the radiation that is emitted downward is absorbed by the Earth's surface further warming it, that is, the Earth's surface receives radiation energy from both the Sun and the atmosphere, and therefore is warmer than if there were no atmosphere. Another way to think of it is that not all of the radiation energy emitted by the surface is lost to space. A good portion of that energy is absorbed by the atmosphere and then returned to the surface, slowing the overall rate of energy loss from the surface, thus keeping it warmer. The part of the radiation that is emitted upward goes off to space and cools the planet. However, since the gases in the atmosphere are at a colder temperature than the surface of the Earth, the radiating temperature of the planet is colder than the surface temperature. The amount of radiation energy that is emitted from the planet Earth to space is equal to what you would calculate for an object at a temperature of 0°F.
• Again it is only the greenhouse gases and clouds that are capable of emitting infrared radiation. (NOTE: gases that absorb infrared radiation also emit infrared radiation. However, these are two separate processes. Once a photon is absorbed, the energy is transferred to the absorbing gas, and the photon no longer exists. In a separate process, the greenhouse gases emit different infrared photons.)

The precise details of the greenhouse effect are quite complicated. For example if we were to look more closely at how individual gas molecules interact with different types of radiation, it gets very complicated. The simplified explanation provided above is sufficient to understand the basic operation of the greenhouse effect.

The greenhouse effect and global warming

You should understand that the natural greenhouse effect on Earth is not a bad thing. In fact it is necessary for life as we know it to exist. If there were no greenhouse effect, the temperature of the Earth's surface would be 0°F, and most water would be frozen. The concern with anthropogenic global warming is that of an enhanced greenhouse effect whereby the surface temperature of the Earth will increase above the present value of 59°F. One way this could happen is by increasing the concentrations of greenhouse gases in the atmosphere. It is a fact that human activities are adding greenhouse gases to the atmosphere and that their concentrations in the atmosphere are increasing.

Let me try to simplify how additional greenhouse gases may act to warm the surface temperature:

• The Earth's surface (Land and Oceans) is constantly emitting (or giving off) infrared radiation. This process acts to cool the surface because energy is being removed from the surface when it emits radiation.
• The atmosphere (the greenhouse gases) absorbs some of this radiation energy from the surface and then returns a portion of it back to the Earth's surface when it (greenhouse gases) emits radiation downward, which gets absorbed by the surface.
• Therefore, the Earth's surface does not cool as rapidly as it would if there were no atmosphere because a portion of the radiation energy it emits is returned back to the surface.
• Adding more greenhouse gases to the atmosphere makes the atmosphere more opaque to infrared radiation, i.e., a larger fraction of the radiation energy emitted by the Earth's surface is absorbed by the atmosphere.
• Since the atmosphere absorbs a larger portion of the radiation energy emitted by the surface, it follows that it will also return more of this energy back to the surface of the Earth, further slowing the rate of surface cooling. Or in other words, warming the surface by strengthening the atmospheric greenhouse effect. We have come to call this effect anthropogenic global warming. Considering changes in radiation only, there is no doubt that an increase in greenhouse gases due to human activities will act to push the surface temperature upward.
• The above sequence of events assumes that every other process in Earth's climate system does not change in response to the changes in the radiation forced by adding greenhouse gases. As we will see, because radiation is coupled to other processes, like weather patterns, cloud formation, etc., other processes will change. The overall effect of all of these coupled changes (on surface temperatures, rainfall, or whatever else) is very difficult to determine because the Earth's climate system is so complex and not fully understood.

Clouds have a large influence both on solar radiation input and infrared radiation out.

• Clouds absorb and emit infrared radiation like a solid surface, i.e., clouds are not selective absorbers/emitters of infrared radiation. Cloud bottoms emit a continuous spectrum of infrared radiation downward and cloud tops emit a continuous spectrum of infrared radiation upward. Because of this, clouds contribute to the greenhouse effect by emitting infrared radiation that warms the surface. This is very noticeable at night. If all other factors are equal, cloudy nights are warmer than clear nights.
• Clouds reflect much of the Sun's radiation back to space, which keeps the surface of the Earth cooler. This is very noticeable during the day. If all other factors are equal, cloudy days are cooler than sunny days.
• Thus, clouds have both strong cooling (mainly by reflecting visible radiation from the Sun) and strong warming (mainly by contributing to the greenhouse effect) effects on climate. Therefore, any changes in cloud amount or in the radiation properties of clouds will result in climate changes for the planet. And it is very likely that any change in greenhouse gas concentrations will influence cloud formation and cloud properties.

Greenhouse effect on Venus

The greenhouse effect also occurs on other planets. Depending upon the composition of the atmosphere, the greenhouse effect can be quite strong. For example, lets look at Venus:

The Planet Venus

• Covered by thick clouds, the planet absorbs 22% of the incoming solar radiation energy and reflects 78% of the incoming solar energy.
• Even though it is closer to the Sun than the Earth, because of the high reflection, it actually absorbs less radiation energy from the Sun than the Earth does. The average temperature of the planet Venus (or its radiative equilibrium temperature) is -31°F.
• But because the atmosphere is very dense and largely composed of carbon dioxide, it has a strong greenhouse effect, and the average temperature of the surface is 860°F.

Completing the Energy Budget Diagrams for the Earth

First let's add the outgoing Earth radiation on the same diagram as the solar energy input. Upward longwave radiation from Earth's surface is largely absorbed by clouds and atmospheric greenhouse gases. Some, however, escapes directly into space. In normalized units, the Earth's surface radiates 105 units of infrared energy upward. Atmospheric gases (water vapor, carbon dioxide, and other greenhouse gases) and clouds absorb 99 of the 105 units of infrared radiation energy emitted by the Earth's surface. Greenhouse gases and clouds also emit radiation both upward (64 units) to space, which cools the planet, and downward (85 units) to warm the Earth's surface. This behavior is commonly referred as the Greenhouse Effect. The figure below shows all of the radiational energy exchanges into, out of, and within the Earth-Atmosphere system. For the entire planet, the radiation energy in equals the radiation energy out, which determines the radiative equilibrium temperature. However, note that the Earth's surface absorbs more radiation energy than it emits, while the atmosphere emits more radiation energy than it absorbs.

Without some other types of energy exchange beside radiation between the Earth's surface and the atmosphere, we would expect that the Earth's surface would be warming (energy in > energy out) and the Earth's atmosphere would be cooling (energy out > energy in). This does not happen because there is a transfer of energy from the surface to atmosphere through latent heat (moist convection) and sensible heat (dry convection and conduction) transfers that balances energy in with energy out, so that both the surface and atmosphere remain at a nearly constant average temperature when averaged over yearly time periods (see figure below).

Summarizing the energy balances:

• The planet Earth is in radiation balance (at least it was before the recent increase in greenhouse gases). The energy in (absorbed radiation from the Sun = 70 units) is equal to the energy out (radiation emitted out to space = 70 units).
• The Earth's surface is in balance. Energy in (absorption of solar radiation plus absorption of radiation from the atmosphere = 135 units) equals energy out (emission of radiation plus latent heat flux plus sensible heat flux = 135 units).
• An interesting point here is that the Earth's surface actually receives more radiation energy from the atmosphere than from the Sun. How is this possible?
• The Earth's atmosphere is in balance. Energy in (absorption of radiation from the surface plus moist convection plus absorption of solar radiation plus dry convection and conduction = 149 units) equals energy out (emission of radiation = 149 units).

A nice diagram from another source

The Earth's annual and global mean energy balance. Of the incoming solar radiation, 49% (168 Wm-2) is absorbed by the surface. That heat is returned to the atmosphere as sensible heat, as evapotranspiration (latent heat) and as thermal infrared radiation. Most of this radiation is absorbed by the atmosphere, which in turn emits radiation both up and down. The radiation lost to space comes from cloud tops and atmospheric regions much colder than the surface. This causes a greenhouse effect.

Consequences of the radiation imbalances

If you consider only the radiation terms, the Earth's surface absorbs more radiation energy than it emits and the atmosphere emits more radiation energy than it absorbs. The radiation imbalances are made up for by convection and conduction. These radiation imbalances drive the overturning circulations of the atmosphere, which for the most part is manifested in the formation of clouds and storms. Another way to look at it is the bottom of the atmosphere (where it touches the surface) is heated by the radiation imbalance that takes place at the Earth's surface (radiation energy in > radiation energy out), while higher up, the atmosphere cools by radiation (radiation energy out > radiation energy in). This generates the instability (warming air below cooling air) that sets that stage for rising vertical motion, cloud formation, and storms. In fact radiation drives the system toward being unstable and when it gets too unstable, overturning happens, often resulting in clouds and precipitation, which transfers energy from the surface upward into the atmosphere.

Radiation imbalances also drive horizontal weather and ocean circulations that transport energy from the tropics where there is a surplus of radiation energy (solar radiation absorbed > radiation emitted to space) to the polar regions where there is a deficit of radiation energy (solar radiation absorbed < radiation emitted to space). This is shown in the figure below. You should be able to understand what is shown in the picture. The red line represents the amount of solar radiation absorbed at different latitudes (this is the radiation energy input), while the blue line represents the radiation energy emitted to space at different latitudes. "Surplus" shows the latitudes where the radiation energy in from the sun is greater than radiation energy emitted to space while "Deficit" means the opposite. Again without some energy exchange between different latitudes, the surplus areas would be getting warmer with time and the deficit areas would be getting colder. However, energy is transported across latitudes by the large-scale atmospheric and ocean circulations. The heat transfer labeled in the figure below is accomplished by atmospheric circulations (about 60% of the transfer) and ocean currents (about 40% of the transfer). Another way to to look at it is that atmospheric and oceanic circulations moderate the temperature differences between the tropics and the polar regions. If these circulations did not occur, the tropics would be much hotter and the polar regions would be much colder.