Thursday, November 1st.

Transcription

1 Thursday, November 1st. Announcements. Homework 7 - due Tuesday, Nov. 6 Homework 8 - paper 2 topics, questions and sources due Tuesday, Nov. 13 Midterm Paper 2 - due Tuesday, Nov. 20 I will hand out a list of topics next tuesday Lecture #16-1

2 In the news.greenhouse gases Lecture #16-2

3 Localities promoting renewables. Lecture #16-3

4 Nuclear industry watch. Lecture #16-4

5 Oil price watch Lecture #16-5

6 We re talking about the greenhouse effect. Some points of review.. Raises the temperature of Earth s surface by about 60F. Greenhouse gases allow visible light from the Sun to get in, but block thermal radiation from the surface from getting out. Nitrogen and oxygen, the major components of our atmosphere are not greenhouse gases. Greenhouse gases can have big effects even though they may be very small components of the atmosphere. Water vapor and CO 2 are the most abundant greenhouse gases. Lecture #16-6

7 We left off last time talking about quantum mechanics, with the goal of understanding why greenhouse gas molecules absorb and emit infrared radiation. The basic idea (from quantum mechanics) is that atoms and molecules can make transitions between different quantized energy levels by emitting and absorbing packets of light, called photons, with the correct energies. Hydrogen energy levels Lecture #16-7

8 How do photons work? In classical physics, light is a wave and its energy depends both on its wavelength and the amplitude of the wave. The energy in a light wave increases with increasing amplitude. In quantum mechanics, the picture is a bit different. Light comes in quantized packets of energy called photons. The energy of a photon depends only on its wavelength. For light beams with many, many photons, the classical picture of a wave with an amplitude becomes accurate. But for processes involving atoms and molecules we need the quantum description of light in terms of photons. Lecture #16-8

9 Einstein received the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect which established that light does come in quantum packets - photons - with energy Planck s constant h=6.6 x J s E = hc! Speed of light c=3 x 10 8 m/s Wavelength of photon Lecture #16-9

10 Another in class assignment. Planck s constant h=6.6 x J s E = hc! Speed of light c=3 x 10 8 m/s Wavelength of photon The maximum sensitivity of a light adapted eye is to photons of wavelength around Part 1! = 555nm = 555 " 10 #9 m This is in the green range What is the energy of a photon with this wavelength? 380 nm 780 nm Lecture #16-10

11 In class assignment. Planck s constant h=6.6 x J s E = hc! Speed of light c=3 x 10 8 m/s! = 555nm = 555 " 10 #9 m Wavelength of photon What is the energy of a photon with this wavelength? E = (6.6! 10"34 Js)(3! 10 8 m / s) 555! 10 "9 m = 3.6! 10 "19 J The photoreceptor molecules in our eyes are sensitive to such tiny amounts of energy Lecture #16-11

12 Part 2 The sunlight absorbed by Earth s surface is on average p = 168 W/m 2 E photon = 3.6! 10 "19 J / photon If all this was in green light, how many photons per second hit a square meter of ground? Lecture #16-12

13 Part 2 The sunlight absorbed by Earth s surface is on average p = 168 W/m 2 E photon = 3.6! 10 "19 J / photon If all this was in green light, how many photons per second hit a square meter of ground? N = E total E photon = (168W / m2 )(1m 2 ) 3.6! 10 "19 J / photon = 4.7! 1020 photon / s Lecture #16-13

14 Greenhouse gas molecules like CO 2 have energy levels with transitions in the infrared range. These correspond to bending and stretching modes of the molecule. In classical physics you can stretch a spring by any amount. The energy goes up as you stretch it more. In quantum physics, these energy levels too are quantized. Lecture #16-14

15 How do greenhouse gases block infrared radiation going up from Earth s surface? If a greenhouse gas molecule absorbs an infrared photon coming up from Earth s surface, it will eventually re-emit one. However, when this new photon is equally likely to be going back down instead of up. If it goes down, it takes with it some of the energy radiated by the surface. If the new photon goes up, it too can be absorbed by another molecule and then re-emitted either up or down. This makes it hard for infrared photons to escape the atmosphere into space.. It also means that a good deal of the energy emitted by Earth s surface gets re-directed back at it. Lecture #16-15

16 These figures show the absorption of thermal (IR) radiation from the Earth by greenhouse gases in the atmosphere. Ozone shields us from UV radiation from the Sun Scattering makes the sky appear blue. This infrared window with little greenhouse gas absorption is quite important. Lecture #16-16

17 The complete picture of what happens to all the energy in sunlight and how it gets re-emitted back into space, is somewhat complicated. Lecture #16-17

18 Key features 342W/m 2 Incoming Solar Radiation is simply the solar constant divided by 4. Earth s surface temperature can be calculated from the 390W/m 2 of Surface Radiation. 324W/m 2 Back Reaction is the energy redirected back to the surface by greenhouse gases 24W/m 2 Thermals represents direct warming of air by Earth s surface. The warm air rises. 78W/m 2 Evapo-transpiration is energy carried upward by water evaporating from lakes, rivers and oceans. Down arrows always add up to up arrows, representing energy conservation and thermal equilibrium. Lecture #16-18

19 Earth s surface temperature can be calculated from the 390W/m 2 of Surface Radiation. T = ( p! )1/4 = ( 390W / m " 10 #8 W / m 2 K 4 )1/4 = 288K 288K = 15C = 59F is indeed Earth s average surface temperature Lecture #16-19

20 Now onto global warming. How is Earth s temperature changing in response to greenhouse gas emissions and other factors? Earth s temperature is rising. The 2007 IPCC report notes that. 11 of the last 12 years rank among the 12 warmest years since records have been kept (1850). Over the past century the Earth has warmed by approximately 0.74C. The warming is accelerating. Over the past 50 years, average temperatures have risen by 0.13C per decade, nearly twice the rate for the whole 100 year period. Global mean surface temperature anomaly 1850 to 2006 relative to The IPCC report lists many other aspects of climate change associated with this long term warming as well. Lecture #16-20

21 In its cautious language, the IPCC report states that "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. The previous (2001) IPCC report used the term likely. Anthropogenic effects, processes, objects, or materials are those that are derived from human activities, as opposed to those occurring in natural environments without human influences. (wikipedia) How greenhouse gas emissions affect Earth s average temperature and other aspects of its climate is a very complex set of questions. The IPCC summarizes the effects of both anthropogenic and natural causes of global warming in terms of radiative forcing. Radiative forcing indicates how a given phenomena effects the energy balance between incoming and outgoing radiation. A positive radiative forcing tends to increase global temperatures, while a negative forcing tends to lower temperatures. Lecture #16-21

22 There are many contributions to consider In the upper atmosphere, Ozone absorbs UV radiation from the Sun providing a negative radiative forcing. In the lower atmosphere it acts as a greenhouse gas, absorbing outgoing IR radiation. Various anthropogenic effects on Earth s albedo yield both positive and negative radiative forcings. Changes in the Sun s radiance The net effects of individual forcings from different sources do not add directly, because of redundancies. Lecture #16-22