Wednesday, September 8, 2010 Infrared Trapping the Greenhouse Effect

Transcription

1 Wednesday, September 8, 2010 Infrared Trapping the Greenhouse Effect Goals to look at the properties of materials that make them interact with thermal (i.e., infrared, or IR) radiation (absorbing and reemitting that radiation). Gases like H 2 O, CO 2, and CH 4 trap some of the thermal radiation from the surface of the earth before it can escape to space, thereby warming the surface above the effective radiating temperature. Some frequencies trapped by these greenhouse gases and why do other frequencies pass through the atmosphere and escape to space in spectral regions we call windows

2 Material covered in this lecture Atmospheric composition (p 44-46) Atmospheric Structure (p 46-48) Heat transport in fluids (p 47, Figure 3-10) Molecular motions and greenhouse gases (p 48-49) Selective absorption of gases (p 49, Figure 3-13) Atmospheric layers and the greenhouse effect (p 45)

3 Last Friday - Greenhouse effect demo Selective absorption. Greenhouse gases transmit visible light, but absorb infrared light. This increases the flux of light (or energy) that hits the surface, because the atmosphere will reradiate some of this energy back to Earth s surface. Let s look at atmospheric composition What we know: Materials aren t all blackbodies some will selectively absorb light at different frequencies. Some gases in Earth s atmosphere will allow solar (visible) radiation to pass through, thereby warming the surface, but they will trap heat (infrared) radiation from the surface. Let s see how this impacts our calculation of earth s temperature.

4 Atmospheric Composition The Main Constituents in Earth s atmosphere

5 Atmospheric Composition The Greenhouse Gases

6 This is parts of chemical X per 1 million parts of total air that is, if something represented 1% of air, it would be 10,000 parts per million (or ppm ). Note: 10,000/1,000,000 = 0.01, or 1%

7 How molecules interact with infrared light Fig 3-12 By absorbing infrared light, molecules can change their states meaning that they can shake, rattle and roll (vibrate, rotate, bend) faster. By emitting IR light, they slow down. Different molecules absorb and emit IR radiation at different frequencies depending on their atoms and bonds. Fig 3-14 Fig. 3.12

8 Looking in detail at how infrared light is affected by atmospheric molecules. Note that the most important absorbers (i.e., greenhouse gases) in the atmosphere are H 2 O (naturally occuring gas) and CO 2 (natural and man-made sources). Detailed studies show that H 2 O provides most of the thermal warming of the planet, followed by CO 2. Note that ozone (O 3 ) is also a greenhouse gas, but not a major one. Fig. 3.13

9 Why does a layer of gas above the surface make the surface warmer? Treat the layer of infrared-active gas as a blackbody, so that we can use the simple equation that relates the emission (or flux ) to the temperature of the layer First, note surface balance with no atmosphere: st s4 = S/4 (1 A) Box Figure 3-2

10 Add an atmosphere Incoming: S/4(1-A) + st e 4 now, two terms! Outgoing: st s 4 Box Fig. 3.2

11 New surface balance with atmosphere: st s4 = S/4 (1 A) + st e 4 The extra term comes from insulating properties of the atmosphere - the greenhouse effect Box Figure 3-2

12 Question For an atmosphere that is in radiative balance (i.e., incoming radiation equals outgoing radiation for all layers), how does temperature change with height? Hint assume that the atmosphere is made up of a large number of layers, stacked like pancakes.

13 Add more layers (i.e., thicken the atmosphere with more GH gases), and the surface gets even warmer because the layer above it is warmed by the layer above it, and so on.! st 4 st 4 Surface

14 Thus, the surface is warmest, and temperature decreases with height up to an altitude of about km. The decrease in temperature with height is called the temperature lapse rate or, often, just the lapse rate. Note that temperature increases again in the stratosphere. This is due to absorption of ultraviolet light by ozone Fig. 3.9

15 So we now have a fairly detailed picture of how heat is distributed vertically in the atmosphere. Sunlight that hits the surface heats the surface, which then radiates some of that heat back up in the infrared. Greenhouse gases like H 2 O, CO 2, and a few others absorb some of that radiation, heating the atmosphere. There are also vertical motions (e.g., convection) and evaporation and condensation of water that redistribute heat upward. Air cools when it rises (we ll talk about this Friday), so this leads to a decrease in temperature with altitude in the lower atmosphere. sun Fig. 3.9

16 Note that pressure always decreases in the atmosphere with every 16 km of altitude, pressure drops by a factor of 10 (that is, pressure is 1000 millibars at the surface, it is 100 millibars at 16 km, it is 10 mbar at 32 km, etc.). Fig. 3.9

17 The sun emits a little bit of light in the ultraviolet (only a few percent of its total output of energy). This light is absorbed by ozone in the upper atmosphere, which creates a warm layer that we call the stratosphere (see Page Fig. 3.9

18 Other than radiation, what forms of heat transfer are important in the atmosphere Fig. 3.10

19 A schematic of Earth s energy budget a bit more complex model! Fig. 3.19

20 Friday We ll take a closer look at the details of energy balance and water vapor and clouds