Objectives:

• Understand and apply the idea of a balanced heat budget to the long-term, global average budgets of the earth's atmosphere and surface separately.
• Identify the sources and sinks of heat, and their relative (long-term, global average) magnitudes, for the atmosphere and for the earth's surface separately.
• Begin to understand how the budget of one affects the budget of the other, and how they relate to the long-term, global average budget of energy for the earth as a whole (that is, the combined atmosphere and surface).

Materials:

• Figures 3.9, 3.10, 3.23, and 3.24, 3.26, and 3.31, provided as handouts.
• An atmospheric absorption spectrum (another handout figure).

Background. In Lab Activity #4: Introduction to the Earth's Heat Budget, you applied the Stefan-Boltzmann Law and global, monthly-average surface temperature data from 1987, to calculate (see results here) that the annual, global average emission flux of terrestrial (longwave IR) radiation from the earth's surface is about 396 W/m². The effective radiative temperature of the earth's surface, based on this global average emission flux, is about 16°C (about 61°F). However, you also found that the annual, global average flux of terrestrial radiation leaving the top of the earth's atmosphere (as observed by satellites) is only about 246 W/m², implying an effective radiative temperature of about –17°C (about 1°F). How can we resolve this large discrepancy? For one thing, it appears that we will have to consider the heat budget for the earth's surface separate from the budget for the planet as a whole (which consists of both the earth's surface and the atmosphere). That in turn will require that we consider the heat budget for the earth's atmosphere.

In this exercise you will investigate the separate, long-term, global average heat budgets for the atmosphere and the earth's surface. Like the budget for the earth as a whole, each of these separate budgets, averaged over a sufficiently long time (one year at a minimum), is very nearly balanced.

In the questions below, we consider the atmosphere to consist of (a) a mixture of gases, together with (b) clouds and other aerosols (that is, tiny liquid or solid particles suspended in the air).

(1) Refer to handout Figure 3.9, which shows a normalized budget of solar radiation reaching the earth. ("Normalized" in this case means that units of energy are rescaled so that 100 of these units of energy arrive at the top of the earth's atmosphere each year, and all other energy amounts in the budgets are scaled accordingly.) Also refer to the atmospheric absorption spectrum figure.

1. Sixteen units of solar energy are absorbed globally each year by gases in the atmosphere. What wavelengths (UV, visible, or near-infrared) are being absorbed, mostly, and by what gases?
2. Four units of solar energy are absorbed by clouds. Do you have enough information to tell which wavelength(s) are being absorbed, mostly?
3. Six units of solar energy are scattered back to space by air molecules. Based on the color of the sky, what wavelength(s) do you think are likely being scattered, mostly?
4. How many units of solar energy are absorbed by the atmosphere before reaching the surface? What percentage of incoming solar radiative energy is absorbed by the atmosphere?
5. What percentage of incoming solar radiative energy actually reaches the earth's surface? Is this likely to vary much from place to place and time to time? Why or why not?
   
   

(2) Refer to Figures 3.9 and 3.10.

1. How many units of solar energy are reflected or scattered back to space without ever being absorbed somewhere by the earth? Based on your answer, what is the global average albedo of the planet?
2. What component of the earth contributes the most to the atmosphere's albedo?
3. What is the average albedo of the earth's surface?
4. In a budget of planetary heat content, does the solar radiative energy that strikes the planet but scatters/reflects back to space represent heat either gained or lost by the earth? Why or why not?
5. What proportion of solar energy is actually absorbed by the earth's surface? Of what wavelength(s) does this energy consist, mostly?
   
   

(3) Refer to Figure 3.23. How does the amount of radiative energy emitted by the earth's surface compare to the amount of solar energy arriving at the top of the earth's atmosphere? How does it compare to the amount of solar energy absorbed by the surface? Is the surface heat budget implied by this figure balanced?

(4) Refer to Figure 3.24.

1. What proportion of the terrestrial (longwave IR) radiation emitted by the earth's surface escapes directly to space?
2. In this figure, are the amounts of energy entering and leaving the top of the atmosphere (in effect, the planetary energy budget) balanced?
   
   

(5) Refer to Figure 3.26.

1. What is the total amount of terrestrial (longwave IR) radiation emitted by the atmosphere?
2. What proportion of the total amount of terrestrial radiation emitted by the atmosphere escapes directly to space? What happens to the rest of it?
3. What proportion of the terrestrial radiation emitted directly to space by the atmosphere is emitted by gases? Which gases? What proportion emitted to space is emitted by clouds?
4. Is the surface heat budget balanced, as shown in this figure? What about the atmosphere's heat budget? (Do you see any possible connection between your answers?) Is the heat budget for the planet as a whole balanced? (Remember, solar energy that strikes the planet but reflects back to space does not participate in the heat budget.)
   
   

(6) Refer to Figure 3.31.

1. What non-radiative fluxes of heat have been added to the budget diagram? Which one(s), if any, involve transformations of energy? (Which of the radiative energy fluxes don't involve a transformation of energy?)
2. How are the non-radiative fluxes of energy likely to vary from place to place and time to time?
3. Does the surface heat budget balance? If so, what is it's primary source of energy (on the average)? What is it's primary means of getting rid of heat (on the average)?
4. Does the atmosphere's heat budget balance? If so, what is it's primary source of energy (on the average)? What is it's primary means of getting rid of heat (on the average)?
5. Do the non-radiative fluxes affect the heat budget for the planet as a whole? Why or why not?
6. From what part of the earth system pictured in these budget diagrams (that is, the earth's surface, air, or clouds) does the most energy lost to space come, and how (by what mechanism) is it lost?
   
   

(7) Based on (a) the balanced heat budgets that you've just analyzed in some detail, and (b) the Stefan-Boltzmann relation, why is the earth's surface necessarily so much warmer than the effective radiative temperature of the planet as a whole?

(8) Suppose that all gases that absorb terrestrial radiation (notably water vapor and carbon dioxide, but other, less important ones, too) were removed from the atmosphere. Note that removing water vapor would also mean that no clouds could be present, either, which would reduce the atmosphere's (and hence the earth's) albedo. There would be less absorption of solar radiation in the atmosphere as well. How would these changes affect the heat budget if it were to be balanced? (You should be able to estimate quantitative changes in the budget.) What implications would this have for the surface temperature (invoking the Stefan-Boltzmann relation)?

(9) Suppose that more carbon dioxide (or water vapor, say) were added to today's atmosphere.

1. What instantaneous effect would this have on the various fluxes of energy to and from the atmosphere and surface? (Remember that finite temperature changes require imbalances in the heat budget acting over at least a short period of time.) Which budget(s), if any, would be out of balance as a result of the initial effects?
2. Assuming that the composition of the atmosphere doesn't change any further, how would the temperature of the atmosphere and surface change over time in response to the imbalance(s) that you've identified? How would the fluxes in the budget change in response to the changes in temperature? Once a new balance (also called equilibrium) is achieved (if in fact it is achieved!), how will the temperatures and fluxes differ from their pre-disturbance state?