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Lec21-Apr7 - Discussion QuesKon Without the...

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Unformatted text preview: 4/7/10 Discussion QuesKon Without the greenhouse effect, Astro 109 Lecture 21: Atmospheres – The Greenhouse Effect April 7 A.  B.  C.  D.  Earth’s polar ice would have melted long ago. we would not have to worry about ozone depleKon. we could grow tropical orchids outdoors in New Jersey. the average temperature on Earth’s surface would be about  ­20° C. E.  None of the above. There is no greenhouse effect. Apr. 7 Discussion QuesKon Venus is said to have a very strong greenhouse effect. This means that 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 radiaKng into space. Apr. 7 Key Concepts •  Earth reflects and emits light •  albedo •  greenhouse gases, greenhouse effect •  climate change Apr. 7 Earth from space •  Sunlight hits the planet •  Some is reflected … what we see! •  Some is absorbed … heats the planet Earth glows •  To avoid overheaKng, the planet must radiate •  Wien’s law  ­ ­> emission spectrum peaks at infrared wavelengths •  Image: infrared emission from water vapor http://antwrp.gsfc.nasa.gov/apod/ap020323.html" Apr. 7 Apr. 7 1 4/7/10 Albedo •  The fracKon of incoming light that is reflected: –  1.00, all light reflected –  0.00, no light reflected Earth’s albedo ≈ 0.31 Apr. high 7 low Apr. 7 Discussion QuesKon Venus has the highest albedo (0.59) of all planets in the solar system because its surface is covered by Wikipedia Apr. 7 A.  B.  C.  D.  E.  light ­colored rocks snow and ice clouds dust volcanic ash Apr. 7 Energy balance incident from Sun: 1.7×1017 W reflected: 0.5×1017 W emided: 1.2×1017 W Temperature of Earth •  Stefan ­Boltzmann law for thermal radiaKon (Lec. 10): P = 4π R 2 σ T 4 σ = 5.67 × 10−8 W m−2 K−4 •  In order to maintain energy balance, the effecKve average temperature of the Earth needs to be: T4 = = ⇒ Apr. 7 T P 4π R2 σ 1.20 × 1017 W 4π (6.378 × 106 m)2 (5.67 × 10−8 W m−2 K−4 ) = 4.14 × 109 K4 = 254 K = −19◦ C = −2◦ F Brrrr! Apr. 7 2 4/7/10 Discussion QuesKon 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 rotaKon period and Klt angle rotaKon period and albedo albedo and mass albedo and distance from the Sun 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? Apr. 7 Apr. 7 The Greenhouse Effect Discussion QuesKon Greenhouse gases are those that atmosphere absorbs and reemits IR light  ­ ­> heats surface! Apr. 7 A.  B.  C.  D.  E.  are transparent to visible light but absorb ultraviolet light are transparent to visible light but absorb infrared light are transparent to infrared light but absorb visible light absorb both visible and infrared light are found only inside greenhouses Apr. 7 Infrared absorpKon light incident on Earth Atmospheric absorpKon light that reaches surface light emided by surface light that actually escapes Apr. 7 Apr. 7 http://www.atmos.umd.edu/~owen/CHPI/IMAGES/transir.html" http://physicsweb.org/articles/world/16/5/7/1" 3 4/7/10 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 Apr. 7 Apr. 7 CO2 and global temperature Apr. 7 Apr. 7 Climate change Feedback Greenhouse effect? Definite Rising CO2? Yes Global warming? Many think so Future climate change? Apr. 7 Apparently Caused by humans? Climate models say yes •  House gets cold  ­ ­> heat kicks on •  House gets hot  ­ ­> cooling kicks on •  “Nega;ve feedback” Apr. 7 4 4/7/10 Feedback •  PosiKve feedback: temperature rises  ­ ­> more water evaporates  ­ ­> more greenhouse gas  ­ ­> more warming •  NegaKve feedback: temperature rises  ­ ­> more water evaporates  ­ ­> more clouds form  ­ ­> reflect more sunlight  ­ ­> moderate the warming •  PosiKve feedback: temperature rises  ­ ­> snow/ice melt  ­ ­> reflect less sunlight  ­ ­> more warming Apr. 7 Apr. 7 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 evaporaKon… •  Lost ability to trap CO2 (and SO2). •  H2O molecules split by UV light, and H evaporated. Apr. 7 Climate change •  Hear about “global warming” •  Beder to discuss “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.” Apr. 7 HW quesKons Chapter 9 8.  How does the greenhouse effect influence the temperature of the atmosphere? Which properKes of greenhouse gases in the atmosphere cause this effect? 26. Carbon dioxide and ozone each make up only a fracKon of a percent of our atmosphere. Why, then, should we be concerned about small increases or decreases in the atmospheric abundance of these gases? 27. How are scienKsts able to measure what the atmospheric CO2 concentraKon and average surface temperature were in the distant past? 28. Does global warming increase the surface temperature of all parts of Earth by equal amounts or by different amounts? What consequence does this have? Apr. 7 HW quesKons 29.  The total power in sunlight that reaches the top of our atmosphere is 1.75×1017 W. a)  How many wads of power are reflected back into space due to the Earth’s albedo? b)  If the Earth had no atmosphere, all of the solar power that was not reflected would be absorbed by the Earth’s surface. In equilibrium, the heated surface would act as a blackbody that radiates as much power as it absorbs form the Sun. How much power would the enKre Earth radiate? c)  How much power would one square meter of the surface radiate? d)  What would be the average temperature of the surface? Give your answer in both the Kelvin and Celsius scales. e)  Why is the Earth’s actual average temperature higher than the value you calculated in (d)? Apr. 7 5 ...
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