METR 104:
Our Dynamic Weather
(Lecture w/Lab)
Thought Questions on
the Earth's Long-Term,
Global Average Energy Budget

Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Fall 2012


Objectives:

Materials:

Background Information. Some relevant facts:

Two relevant physical principles (besides two others embedded in the facts above):

Introduction:

In this exercise you will investigate the long-term, global average heat budgets for the atmosphere and for 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. Small imbalances, sustained over time (a number of years), lead to climate change.

We will refer to the accompanying diagram showing the earth's long-term, global-average rates of absorption and reflection of solar radiation, absorption and emission of longwave infrared (LWIR) radiation, and transfer of heat between the earth's surface and the atmosphere by conduction and evaporation of water and condensation of water vapor in the atmosphere (to make clouds).

The left-hand part of the diagram shows what happens to solar radiation arriving at the top of the atmosphere, while the right-hand part shows what happens to longwave infrared (LWIR) radiation emitted by the surface and by the atmosphere. The center of the diagram shows the average transfer of heat by conduction from the surface into the (on average cooler) atmosphere and transfer of heat from the surface into the atmosphere by evaporation of water from the surface followed subsequently by condensation of water vapor to form clouds.

The numbers represent energy intensity (also called energy flux). They tell us the rate at which energy passes through, into, or out of each unit of (horizontal) area of the surface or the atmosphere. (In the diagram, energy is represented in units of Joules, and rates of energy transfer are expressed in Joules per second, or Watts. Horizontal surface area is represented in square meters, or m2. Hence, the fluxes are expressed in Watts/m2.)

The diagram shows that the flux of solar radiative energy (that is, insolation) arriving on a horizontal surface at the top of the earth's atmosphere, averaged over the whole globe for a long time (24 hours a day for many years), is 342 Watts/m2. The flux of solar radiation that is scattered and reflected back to space by clouds, aerosols, air, and the earth's surface is a total of 107 Watts/m2, which is (107/342) x 100% = 31% of the solar radiation arriving—hence, the long-term, global average albedo of the earth is about 31%. (Two thirds of this scattering and reflection is due to clouds, which reflect very well.)

The observations on which these numbers and the others on the diagram are based come from satellites (which have been making observations of solar and longwave infrared (LWIR) radiation since the early 1970s) and from surface-based instruments.

Questions:

  1. 67 Watts/m2 solar energy are absorbed each year by the atmosphere. What wavelengths of solar radiation (UV, visible, or short wavelengths of infrared) are being absorbed, and by what, exactly?


  2. What is the flux of solar radiation actually reaching the earth's surface? How much is it, expressed as a percentage of the solar radiation arriving at the top of the atmosphere?


  3. What is the average albedo of the earth's surface? [Note: To answer this, you must have a clear understanding of how albedo is defined.]


  4. What percentage of the solar radiation arriving at the top of the atmosphere is actually absorbed by the earth's surface? Of what wavelength(s) does this energy consist, mostly?


  5. 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?


  6. What percentage of the longwave infrared (LWIR) radiation emitted by the earth's surface escapes directly to space? What happens to the rest of it?


  7. In this figure, are the amounts of energy entering and leaving the top of the atmosphere (in effect, the planetary energy budget) balanced?


  8. Does the surface heat budget balance? If so, what is the primary mechanism by which the surface gains heat (on the average)? What is its primary mechanism by which the surface loses heat (on the average)?


  9. Does the atmosphere's heat budget balance? If so, what is the primary mechanism by which the atmosphere gains heat (on the average)? What is the primary mechanism by which the atmosphere loses heat (on the average)?


  10. How (that is, by what means) does the planet lose heat to space? From what part of the earth system pictured in this budget diagrams (the surface, air, or clouds) does the the earth lose the most heat to space?


  11. How does the radiative energy emitted by the earth's surface compare to the radiative energy that the earth as a whole emits to space?


  12. Based on (a) the balanced heat budgets that you've just analyzed in some detail, and (b) the basic law of radiation that says that the warmer an object is, the more radiative energy it emits, why is the earth's surface necessarily so much warmer than the temperature of the planet treated as a whole, as seen from space?


  13. Suppose that all gases that absorb longwave infrared (LWIR) 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 earth's albedo. For the sake of simplicity, suppose that the albedo doesn't change—the answer will be qualitatively the same.) What implications would this have for the surface temperature, if the heat budget were balanced?


  14. Suppose that more carbon dioxide were added to today's atmosphere. (Humans are doing this by burning coal, oil, and natural gas and by cutting down forests.)

    What effect would this have on the absorption of longwave IR radiation by the atmosphere and the longwave IR emitted by the surface that escapes directly to space? How would the atmosphere's temperature respond? What would happen to the amount of radiation it emits? What would happen to the temperature of the surface as a result?


Physical relationships for your reference:

Conservation of energy relation:
Rate at which
an object's heat content
(and hence temperature)
changes
= Sum of rates
at which the object gains heat
by various mechanisms
Sum of rates
at which an object loses heat
by various mechanisms
One of the Basic Laws of Radiation:
Objects emit radiative energy faster
(and hence lose heat by this means faster) when they are warmer than when they are colder

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