chapter_03 - Global Energy Balance Global The Greenhouse...

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Unformatted text preview: Global Energy Balance Global The Greenhouse Effect Chapter 3 Dr. Emily Berndt Fundamentals of the Climate System EAS 253 I. Electromagnetic Radiation I. a. Properties of EM Radiation I. Electromagnetic Radiation I. b. Photons and Photon Energy i. Also behaves as a stream of particles ii. Photons- single pulse of electromagnetic radiation 1. smallest discrete amount of energy 2. E=hυ E=hc/λ 2. h=6.63x10^-34 J/s h=6.63x10^-34 3. higher υ (short λ) -photons have high energy -photons -can break molecules apart -can -cause chemical reactions -cause 4. low υ (long λ) - photons have low energy photons -cause molecules to rotate faster or vibrate -cause I. Electromagnetic Radiation I. c. The EM Spectrum – the full range of EM radiation by wavelength i. visible radiation (measured in nm) 1. 400-700 nm (.4-.7 Mm) 2. color of light depends on λ a. longest is red b. shortest is blue ii. Infrared radiation (longer λ) 1. measured in μm 2. range from .07-100 Mm iii. Ultraviolet radiation (shorter λ) 1. 10 nm- 400 nm 2. absorbed by stratosphere O3 iv. Sun’s radiation 1. 50% in visible spectrum 2. 40% in IR spectrum 3. 10% in UV spectrum 3. I. Electromagnetic Radiation I. d. Flux (F) – the amount of energy that d. passes through a given area per unit time. time. i. Units are W/m2 iii. Measured perpendicular to the i. direction the wave is traveling direction I. Electromagnetic Radiation I. e. The Inverse-Square Law – the rate at which e. the solar flux decreases with increasing distance distance r0 S = S0 r 2 Consider Planet X located twice as far Consider from the Sun as Earth from r0 is the average Earth-Sun distance (149,600,000 km or 1 AU) (149,600,000 S0 is the Earth’s solar flux (1366 W/m2) is If r = 2 AU find the solar flux at Planet X W 1AU S = 1366 2 m 2 AU W S = 341.5 2 m 2 II. Temperature a. Temperature – the average kinetic energy of the molecules in a substance energy b. Heat – the transfer of energy from hot to b. cold objects cold III. Blackbody Radiation III. a. Blackbody – emits (or absorbs) EM a. radiation with 100% efficiency at all wavelengths wavelengths III. Blackbody Radiation III. c. Wien’s Law i. Flux of radiation emitted by a Flux blackbody has peak λ of radiation base on its temperature temperature ii. Hotter bodies shorter λ Hotter iii. Colder bodies longer λ λmax 2898mmK = T 3000 ( µm K ) Sun : λmax = = 0.5µm 6000( K ) Earth : λmax 3000 ( µm K ) = = 10 µm 300( K ) III. Blackbody Radiation III. d. Stefan-Boltzmann Law ii. The energy flux . emitted by a blackbody is related to the 4th power is of the body’s temperature temperature iii. Total energy flux is i. proportional to the area under the curve (w/m^2) under F =s T 4 IV. Planetary Energy Balance IV. A. The amount of energy emitted by Earth The must equal the amount absorbed must B. Not exactly true! If so the average Not surface temperature would never change. change. C. Is Earth’s energy budget out of balance Is due to increased greenhouse gases or natural causes? natural IV. Planetary Energy Balance IV. D. Earth’s surface temperature depends D. on: on: ii. The solar flux available at the distance . of earth’s orbit (13.66 w/m^2) of ii. Earth’s albedo (30%) iii. Greenhouse warming IV. Planetary Energy Balance IV. E. Magnitude of the Greenhouse Effect i. Treat Earth as a Blackbody ii. Effective Radiating Temperature (Te) given the amount of radiation Earth radiates this is the temperature a blackbody would this need to be to radiate the same amount of energy as Earth radiates energy IV. Planetary Energy Balance IV. iiii. Use Stefan-Boltzmann to calculate the ii. energy emitted by Earth and balance it with the Incoming Solar Energy with Outgoing IR Energy S σT = (1 - A) 4 4 e Planetary Energy Balance Equation! Incoming Solar Energy IV. Planetary Energy Balance IV. iiv. Now lets derive the planetary energy v. balance equation balance 1. assume energy emitted by Earth = 1. energy absorbed by Earth energy 2. Treat Earth as a blackbody with a Te 3. The energy emitted per unit area must 3. σTe4 equal 4. Earth radiated over the entire surface 2 area 4p Re IV. Planetary Energy Balance IV. v. The significance of Te 1. The temperature at the height in 1. the atmosphere from which most of the outgoing radiation derives outgoing 2. The average temperature Earth’s 2. surface would reach if the planet had no atmosphere and albedo Greenhouse gases remit or trap longwave radiation from earth’s surface. This effect = 15oC-(-18oC)=33oC or 59°F IV. Planetary Energy Balance IV. vi. Let’s calculate Te for Earth vi. 1. 2. 2. 3. 4. 5. σT(e)^4=S/4(1-A) σT(e)^4=S/4(1-A) T(e)=[(2/4σ)(1-A)]^1/4 T(e)=[(1366w/m^2/4x5.67x10^-8w/m^2K^4)(1-0.3)]^1/4 T(e)=254.81 K or T(e)=-18°C Green house effect ΔT(g)=T(s)-T(e) 15°C-(-18°C) Green r N Ð ΔT(g)= 33°C IV. Planetary Energy Balance IV. vii. How does the Greenhouse Effect Work? Work? V. Atmospheric Composition and Structure and V. Atmospheric Composition and Structure and V. Atmospheric Composition and Structure and VI. Physical Causes of the Greenhouse Effect Greenhouse A. B. Why do some gases contribute to the GH effect and others don’t Gas Molecules absorb/emit IR radiation in two ways: A. Change the rate of rotation of the molecule A. Change the number of revolutions per second B. If incident radiation is at the right frequency, molecule will absorb radiation If and rotate faster and C. Emission leads to slower rotation D. The frequency or wavelength absorbed or emitted depends on the The molecule’s structure molecule’s A. -H2O absorbs 12μm and longer B. -H2O rotation band B. Changing the amplitude of vibration of the molecule A. Frequency of incident radiation matches the frequency of molecule vibration B. Molecule will absorbs radiation and vibrates C. CO2 has a bending mode of vibration A. -Absorbs IR radiation at 15μm B. -CO2 band C. -Band near peak of earth’s outgoing IR Selective Absorbers Selective c. Other Greenhouse Gases Click here to animate this figure VI. Physical Causes of the Greenhouse Effect Greenhouse d. Why are N2 and O2 poor absorbers of IR and radiation? radiation? VII. Effect of Clouds VII. a. Opposing climatic effects of Clouds ii. Cloudy days are cool; cloudy nights . are warm are iii. Climatic impact depends on cloud i. height and thickness height 1. 1. 1. Stratus clouds: reflect incoming radiation cooling effect Stratus Higher temperature and radiate energy at a shorter λ 2. 1. 2. 3. Cirrus clouds: allow more solar radiation to reach the surface Cirrus warming effect warming Contribute more to greenhouse warming (won’t stop radiation from passing Contribute through but absorb outgoing radiation) through Radiation less energy to space colder Radiation Radiate energy at a longer λ Higher radiation flux and contributes to GH effect lower radiation flux and contributes less to GH effect VII. Effect of Clouds VII. b. Earth’s Global Energy Balance VIII. Climate Feedbacks VIII. Water Vapor Feedback (consider warming) Snow and Ice Albedo Feedback (consider cooling) VIII. Climate Feedbacks VIII. The IR/ Flux Temperature Feedback Stabilizes climate on short time scales Can fail if there are large amounts of Can water vapor in the atmosphere water ...
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