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 of 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 average 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 restricts the flow of infrared radiation out to space. This is known as the greenhouse effect. A simplified diagram to help you understand the basics of the greenhouse effect will be drawn in class.

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, 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.
• The most abundant gas molecules, nitrogen and oxygen, do not interact much with infrared radiation.
• Clouds absorb infrared photons very efficiently, and in essense contribute to the greenhouse effect.
• The infrared radiation energy absorbed by the greenhouse gases 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 that are capable of emitting infrared radiation. On Earth, it is mainly water vapor and carbon dioxide. (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.)
• Clouds emit infrared radiation very efficiently, in essense contributing to the greenhouse effect.

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 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 that atmosphere and that their concentrations in the atmosphere are increasing.

Let me try to simplify this:

• 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 global warming.

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.

The details of the greenhouse effect are quite complicated. I may mention a few of these complications in class, but I will not expect you to understand them on an Exam.

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.

Completing the Energy Budget Diagrams for the Earth

Addition of infrared radiation to the energy budget diagrams for the Earth

The Earth's energy budget including all solar fluxes plus upward longwave radiation from the Earth's surface. Atmospheric gases (water vapor, carbon dioxide, etc.) and clouds absorb 99 of the 105 units emitted by the Earth's surface. Note that for what we've included so far the net flux at the Earth's surface is negative (50-105 = -55) while the atmosphere shows a large surplus.

Greenhouse gases (water vapor, carbon dioxide, methane, ozone, CFCs, nitrous oxides) and clouds emit radiation upward (64 units) to space and downward (85 units) to warm the Earth's surface. This behavior is commonly referred as the Greenhouse Effect. The above figure 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 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). Transfer of energy from surface to atmosphere through latent heat transfer balances energy in with energy out, so that both the surface and atmosphere remain at a nearly constant average temperature.

Addition of Energy Transfers via Conduction and Convection between Earth's Surface and Atmosphere

The Earth's energy budget including the sensible heat flux, transfered through the processes of conduction and dry convection, and the latent heat flux , transfered through the process of moist convection (phase changes of water, i.e., evaporation from the surface and condensation in clouds) In a balanced budget, the energy storage is neither increasing or decreasing, that is: Energy Input - Energy output = 0 The sum of the inputs equals the sum of the outputs; the budget balances, and the entire system is neither warming nor cooling.

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 that sets that stage for rising vertical motion, cloud formation, and storms.

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). 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.