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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 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 distributed as an in-class handout.

Basics of 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 how additional greenhouse gases may act to warm the surface temperature:

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

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.

NOTE: Due to time constraints, we are not going to cover the material below here. You will not be tested on the material; it is there if you are interested.

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:

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.

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