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

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

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

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

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

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)

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

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? 1. 13.64/4=3.41 2. T4=3.41/8.17 x 10-11 3. T=452K

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=547.68 W/m2

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

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

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

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

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)

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

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

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”

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

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

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