Lec19-Mar29 - 3/29/11 Quiz QuesJon #1 What is the...

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Unformatted text preview: 3/29/11 Quiz QuesJon #1 What is the primary reason for the greenhouse effect? Astro 109 Lecture 19: Atmospheres – The Greenhouse Effect March 29 A.  Plant life makes a planet’s surface look green from a distance. B.  Plant life affects the composiJon of a planet’s atmosphere. C.  Energy radiated by a planet’s surface is trapped by the atmosphere. D.  The surface of the Sun is hoOer than the surface of a planet. E.  A planet’s surface warms up during the day and cools off at night. Mar. 29 Quiz QuesJon #2 Greenhouse gases A.  B.  C.  D.  E.  absorb both visible and infrared light. are transparent to visible light but absorb infrared light. are transparent to infrared light but absorb visible light. are transparent to visible light but absorb UV light. are found only inside greenhouses. Key Concepts •  Earth reflects and emits light •  albedo •  greenhouse gases, greenhouse effect •  climate change Mar. 29 Mar. 29 Earth from space •  Sunlight hits the planet •  Some is reflected … what we see! •  Some is absorbed … heats the planet Earth glows •  To avoid overheaJng, the planet must radiate •  Wien’s law  ­ ­> emission spectrum peaks at infrared wavelengths •  Image: infrared emission from water vapor hOp://antwrp.gsfc.nasa.gov/apod/ap020323.html Mar. 29 Mar. 29 1 3/29/11 Discussion QuesJon Albedo In general, how does albedo relate to a planet’s surface temperature? •  The fracJon of incoming light that is reflected: –  1.00, all light reflected –  0.00, no light reflected Mar. high 29 A.  Higher albedo leads to a higher temperature. B.  Higher albedo leads to a lower temperature. C.  Albedo does not affect temperature. low Mar. 29 Wikipedia Earth’s albedo ≈ 0.31 Mar. 29 Discussion QuesJon Mar. 29 Energy Balance incident from Sun: 1.7×1017 W Which inner planet has the highest albedo? A.  B.  C.  D.  Mar. 29 reflected: 0.5×1017 W emiOed: 1.2×1017 W Mercury Venus Earth Mars Mar. 29 2 3/29/11 Average temperature of Earth •  Stefan ­Boltzmann law for thermal radiaJon (Lec. 9): P = 4π R 2 σ T 4 σ = 5.67 × 10−8 W m−2 K−4 •  In order to maintain energy balance, the effecJve average temperature of the Earth needs to be: T4 = = ⇒ T P 4π R2 σ 1.20 × 1017 W 4π (6.378 × 106 m)2 (5.67 × 10−8 W m−2 K−4 ) Discussion QuesJon The predicted average surface temperature of a planet depends mainly on the planet’s A.  B.  C.  D.  E.  density and distance from the Sun rotaJon period and Jlt angle rotaJon period and albedo albedo and mass albedo and distance from the Sun = 4.14 × 109 K4 = 254 K = −19◦ C = −2◦ F Brrrr! Mar. 29 Mar. 29 The Greenhouse Effect Temperature of Earth •  We predict average T = 254 K =  ­19° C =  ­2° F •  Actual average temperature is 287 K = 14° C = 57° F •  Why the difference? atmosphere absorbs and reemits infrared light  ­ ­> heats surface! Mar. 29 Mar. 29 Infrared absorpJon light incident on Earth Atmospheric absorpJon light that reaches surface light emiOed by surface light that actually escapes Mar. 29 hOp://www.atmos.umd.edu/~owen/CHPI/IMAGES/transir.html Mar. 29 hOp://physicsweb.org/arJcles/world/16/5/7/1 3 3/29/11 Greenhouse gases Atmospheric carbon dioxide Molecule Abundance Contribu1on to Greenhouse effect Concern? N2 78.1% – no O2 20.1% – no Ar 0.9% – no H2O <1% 60% indirect CO2 0.035% 26% yes CH4, N2O, O3 2 ppm 8% yes CH3Cl 0.6 ppb yes CCl2F2 0.3 ppb yes CCl4 0.1 ppb yes Mar. 29 Mar. 29 Global temperature Mar. 29 Mar. 29 North Pole Climate change Greenhouse effect? Definite Rising CO2? Yes Global warming? Apparently Caused by humans? Many think so Future climate change? Climate models say yes hOp://www.telegraph.co.uk/news/worldnews/3226747/Climate ­change ­is ­faster ­and ­more ­extreme ­than ­feared.html Mar. 29 Mar. 29 4 3/29/11 Feedback Feedback •  NegaJve feedback: temperature rises  ­ ­> more water evaporates  ­ ­> more clouds form  ­ ­> reflect more sunlight  ­ ­> moderate the warming •  House gets cold  ­ ­> heat kicks on •  House gets hot  ­ ­> cooling kicks on •  PosiJve feedback: temperature rises  ­ ­> more water evaporates  ­ ­> more greenhouse gas  ­ ­> more warming •  “NegaJve feedback” •  What if when the house gets hot, you turn up the heat? “Posi@ve feedback” Mar. 29 •  PosiJve feedback: temperature rises  ­ ­> snow/ice melt  ­ ­> reflect less sunlight  ­ ­> more warming Mar. 29 Discussion QuesJon Comparing atmospheres Venus is said to have a very strong greenhouse effect. This means that Venus Earth Mars Nitrogen (N2) 3.5% 78.08% 2.7% Oxygen (O2) almost zero 20.95% almost zero Carbon dioxide (CO2) 96.5% 0.035% 95.3% Water vapor (H2O) 0.003% 1% 0.03% almost zero almost zero 2% Other gases Mar. 29 A.  Venus is covered with glass. B.  sunlight is strongly absorbed because the planet’s surface is dark green. C.  sunlight cannot reach the surface because it is blocked by thick clouds. D.  most carbon dioxide (CO2) molecules have been split apart by ultraviolet light from the Sun. E.  CO2 molecules in the atmosphere keep Venus’s surface heat from radiaJng into space. Mar. 29 Climate change Venus: Runaway greenhouse effect •  Young Venus probably had liquid water and an atmosphere like young Earth’s. •  But Venus was warmer  ­ ­> more water evaporated  ­ ­> enhanced greenhouse effect  ­ ­> more evaporaJon… •  Lost ability to trap CO2 in rocks. •  H2O molecules split by UV light, and H evaporated. Mar. 29 •  Hear about “global warming” •  Maybe say “global weirding ” – more extreme weather •  Thomas Friedman, NY Times, Feb. 17, 2010: “The fact that it has snowed like crazy in Washington – while it has rained at the Winter Olympics in Canada, while Australia is having a record 13 ­ year drought – is right in line with what every major study on climate change predicts: The weather will get weird; some areas will get more precipita@on than ever; others will become drier than ever.” Mar. 29 5 ...
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This note was uploaded on 01/19/2012 for the course ASTRO 109750 taught by Professor Keeting during the Fall '11 term at Rutgers.

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