1. The Earth’s Global Energy Balance
Chapter 2
The Earth’s Global
Energy Balance

2. Energy Balance What is energy balance?
Energy Balance- equilibrium between flow of energy or radiation reaching Earth and flow of energy or radiation leaving Earth
69% absorbed, 31% reflected
Importance: controls daily and seasonal variations in temperature, currents, and life

3. Global Radiation Balance
Absorption of shortwave (solar) radiation must equal emission of longwave (terrestrial) radiation
Globally, some areas have surplus of solar radiation while others have deficit  circulation in atmosphere and oceans
Areas receive different amounts of solar radiation at different times of year  circulation patterns vary through year
Annually, lower latitudes (~35°N to ~35°S) tend to have surplus, while higher latitudes (~35°N to 90°N and ~35°S to 90°S) tend to have deficit

4. Why does the Earth not receive all of the Sun’s energy?
Absorption-when molecules and particles in the atmosphere intercept and absorb radiation at particular wavelengths (Solar radiation)
Gasses?
Scattering-when incoming solar radiation is turned in a different direction, often deflected back into space
Received vs. Reflected

5. Electromagnetic Spectrum
Electromagnetic Radiation-Wave-like form of energy radiated by any object either as light (visible) or heat (invisible)
Emitted at different wavelengths
Crest and Troughs
Measured in micrometers (one-millionth of a meter)
Short wavelengths=Higher heat
Longer wavelengths=Lower heat

6. Wien’s Law
Inverse relationship between object’s temp and maximum wavelength of emitted radiation
Hotter objects  shorter maximum wavelengths
Example: Sun (~5800K) vs. Earth (~288K)
Example: The surface temperature of Venus is 773K. At what wavelength does Venus emit radiation?
*3.75μm (Infrared)

7. Stephan-Boltzmann Law
Describes amount of energy emitted by a black body
A black body is an object that absorbs 100% of the radiation striking it and radiates heat at a max. rate (blacktop)
Earth not perfect black body, but can approximate
Amount energy emitted directly related to temp
What does this mean?
Hotter objects emit more energy
T4= E/8.17 x 10-11
Example: What is the average temperature of a body with an intensity of incoming radiation of 7.29L?
T=547K
CAVEAT-If you are calculating the Temperature of a Planet, you must divide the solar constant by 4 before using it in the Equation

8. Example
You have crash landed on Mercury and your instruments tell you that the solar constant is 13.64L. What is the average temperature of Mercury?
/4=3.41
2. T4=3.41/8.17 x 10-11
3. T=452K

9. Net Radiation Net Radiation=Incoming – Outgoing Radiation
Rn=I (1-a) + R↓ - σT4e
Example: What would the net radiation be if insolation were 1200 W/m2; the albedo of the surface was 0.33; emissivity was 0.90, temperature was 325K, and downward longwave radiation was 313 W/m2?
Rn=1200(1-.33)+313-(5.67 x 10-8) x 3254 x .90
Rn= W/m2

10. Global Radiation Balance
Scattering of shortwave vs. longwave radiation
Insolation-interception of solar energy (shortwave radiation) by an exposed surface
W/m²

11. Insolation
Insolation (incoming solar radiation) is dependent on 4 things:
1. Angle of Incidence- the angle between the sun’s rays and a line perpendicular to the Earth’s surface (also called the Zenith angle)
2. Length of daylight
3. Transparency of atmosphere (dust, smog)
4. Variation in Solar Irradiance- insolation received at the outer edge of the atmosophere

12. Insolation
Angle of the Sun (SA)-the angle between the Earth’s surface and the sun’s rays
SA + ZA = 90°
Solar Constant- the maximum potential intensity of insolation available to the Earth at the outer edge of the atmosphere
Solar Constant = 1.94 Langleys
Sine Law
Summarizes the relationship between the SA and radiation intensity
Ib=(Ia) x (Sin SA)
Example: What is the maximum potential intensity of radiation if the SA = 60°?
1.68L

14. Net Radiation, Latitude, and Energy Balance
Net radiation: difference between incoming and outgoing radiation
Varies strongly by latitude, but balance on global scale due to heat transfer
Transfer occurs by methods discussed in both atmospheric and oceanic circulation

15. Terrestrial Radiation
Energy emitted by earth
Earth absorbs certain wavelengths of solar radiation, then is converted to another form and emitted at different wavelength
Wavelength much longer than that of solar radiation (Wien’s Law)

16. World Latitude Zones
Equatorial (10°N to 10°S): intense annual insolation
Tropical (10°N to 25°N; 10°S to 25°S): some seasonal variation, but still high annual insolation
Subtropical (25°N to 35°N; 25°S to 35°S): more seasonal variation; fairly high annual insolation
Mid-latitude (35°N to 55°N; 35°S to 55°S): strong seasonal contrasts in insolation
Sub-Arctic (55°N to 60°N), Sub-Antarctic (55°S to 60°S): very strong seasonal contrasts in insolation
Arctic (60°N to 75°N), Antarctic (60°S to 75°S): intense seasonal contrasts in insolation
Polar (75°N to 90°N; 75°S to 90°S): greatest seasonal contrasts in insolation... Range is ~500W/m2

17. Other Types of Heat Transfer
2 main ways in addition to longwave and shortwave radiation...
1. Sensible heat
Sensible heat transfer
Conduction OR convection
2. Latent heat
Latent heat transfer
Occurs during change of state

18. Composition of the Atmosphere
Important to energy flow and energy balance
Different gases impact heat absorption in air
Nitrogen, oxygen, and argon – three most abundant; low importance to climate
Carbon dioxide, water vapor, ozone, and methane – low content in
atmosphere, but very
important to climate
- “Greenhouse Gasses”

19. Diffuse Radiation
Diffuse Radiation-when incoming radiation is scattered by dust and other atmospheric particles causing radiation to bounce in various directions
Clouds can reflect a lot of radiation
Direct Absorption
15% incoming radiation absorbed by CO² and water vapor
17% absorbed by diffuse radiation
80% reaches ground

20. Miscellaneous Topics
Albedo-the proportion of shortwave radiation that is reflected and scattered by an object’s surface
Different surfaces and colors
Counterradiation-LW radiation of the atmosphere directed downward toward the Earth’s surface
CO² and Water vapor

21. Greenhouse Effect
Greenhouse Effect- the ability of the Earth’s atmosphere to admit most of the insolation and prevent its re-radiation from the Earth
Although the natural “greenhouse effect” enables the Earth to remain livable…increased pollutants have caused an adverse reaction creating an “enhanced effect”
The “enhanced effect” is when human activities increase the ability for the Earth’s atmosphere to absorb radiation causing a net warming of the atmosphere over time
Greenhouse Gasses?
Water Vapor, Ozone, Carbon Dioxide, and Methane