Lecture 21 Wednesday, Oct 19, 2000
The Greenhouse effect
characteristic wavelength of thermal radiation:
hot objects (sun) 5000K --> visible light
warm "room temperature" 300K (60 F) --> infrared
hot sun emits energy as visible light
visible light absorbed by planetary surface
surface heats up
warm surface re-emits energy as infrared
(NOT reflected)
A fundamental question will be the source of these molecules in the atmosphere.
Question for later: source/sink of sulfur, water
High albedo(high reflection)lowers energy input from sun
Don't worry about the details. The important point is that every temperature has a characteristic wavelength, and the lower the temperature the longer the wavelength. The surface of the sun is hot and emits in the visible. But surface of terrestrial planets are much cooler than sun and so emit in infrared. This is the key point you must remember.
Sun - hot --> high flux & visible light (short wavelngth
planets receive low flux --> cool --> infrared(long wvl)
Recall the transparency window from telescopes. Notice that it is not completely opaque in the infrared--Earth's atmosphere is "leaky." Most of the Sun's radiation is emitted as visible light, and so it falls on the surface. This heats up the surface to a certain temperature which then emits energy at a certain wavelength.
GREENHOUSE EFFECT:
atmosphere transparent to visible light (from sun)
mostly opaque to infrared light (from planet surface) --> trapped!
--> HEAT RESERVOIR--HIGH TEMP
Greenhouse effect: atmosphere is transparent to visible but opaque in infrared, so energy gets "trapped." The energy leaks out eventually because there must be a balance, but the opaqueness to infrared raises a barrier so that there is a heat reservoir. Eventually the excess energy leaks out over the top. The heights of the barriers are greaters, successively, for Mars, Earth, and Venus, and it has to do with how much greenhouse gases are in the atmosphere.
Not all gases cause the greenhouse effect.
Greenhouse gases: CO2, H2O, and methane (CH4)
non-Greenhouse gases: N2, O2, H2, He
So if you had an atmosphere of pure oxygen it would not be noticeably warmer.
Is greenhouse effect bad? No, because it keeps us 35° C warmer than we would be.
If no Greenhouse effect, earth would be frozen.
But you can have too much--"Global warming."
Effects of some greenhouse gases complicated: particularly, water on Earth. Water is a greenhouse gas, so water vapor in atmosphere raises temperature; on the other hand, clouds (from water vapor) reflect light--increase albedo--so less sunlight falls on surface and this lowers the temperature. This is why it is so difficult to precisely calculate global warming!
Stop and ask if any questions at this point on the Greenhouse effect:
ATTENTION: Ozone has NO EFFECT on Greenhouse
It is an entirely DIFFERENT problem.
Ozone NOT a greenhouse gas
UV light irrelevant to GREENHOUSE (but not skin cancer!)
Greenhouse effect depends on greenhouse gases
composition of atmosphere, including greenhouse gases depend on
--initial (primordial) composition
--escape of gases
we have covered these two already and you should understand them; third is
--interaction with crust of planet
Our next topic:
ATMOSPHERE-SURFACE INTERACTIONS
sources: volcanic outgassing
sinks (losses): precipitation (rain)
Most important: the CARBON (CO2) CYCLE
Mediated by water H2O
water removes CO2
volcanoes replenish CO2
SHORT ANSWER TO GOLDILOCKS PROBLEM:
Venus lacks water--too much CO2 in atmosphere
Mars lacks active volcanoes--not enough CO2 in atmosphere
Earth has both water & volcanoes and negative feedbacks (thermostat) maintains CO2 balance & THUS CLIMATE
Water vapor: a major Greenhouse gas
Most people talk about carbon dioxide, but water vapor is as important if not dominant. However the action of water is very complicated. It is an example of a positive feedback loop, that is something that grows. The higher the temp, the more water vapor is produced through evaporation, increasing the greenhouse effect...except that clouds increase the albedo and therefore turn down the greenhouse effect.
Carbon dioxide-- a greenhouse gas
The CARBON CYCLE - a negative feedback loop
(example of negative feedback loop--thermostat)
Talk about how thermostats work; tell how a positive feedback loop for thermostat would work but would be disastrous.
part I: water carries CO2 into crust
main constituents of primordial atmosphere: N2 and CO2
what about oxygen?
N2 does not dissolve in water
CO2 dissolves in water--rain "washes" CO2 out of atmosphere
H2O + CO2 = H2CO3 (carbonic acid)
If left standing, the CO2 will return to the atmosphere, just like the fizz in your soda;
BUT interaction with rocks changes this
"acid rain" weathers (erodes) calcium-silicate rocks
releasing Ca++ and HCO3- into water --> oceans
action of
organic life (plankton etc.)
non-organic precipitation
produce CaCO3 calcium carbonate
settle on sea floor --> carbonate sediments
CO2 NOW IN CRUST
takes roughly 1-10 million years to remove all CO2
Ask students for examples of carbonate rocks: limestone, marble
Earth & Venus have roughly same amount of CO2,
Earth's in crust in carbonate rocks
Venus in atmosphere
It is thought that Mars may have considerable CO2 also locked up in crust, but this is not known. Scientists have found meteorites that they think were chipped off Mars and they do have some carbonate in them so this supports this idea. Science fiction writers suggest that if you could just jump start the greenhouse effect, warming Mars would heat some of the CO2 out of the crust, although I am not sure this would really work.
Similarly, Earth & Venus have roughly the same total amounts of nitrogen, but they are in different relative proportions in the atmosphere because Venus is dominated by CO2.
Part II: Volcanic action carries CO2 into atmosphere
On Earth, plate tectonics subduct carbonate sediments deep into crust where heat and pressure release CO2
Volcanoes outgas CO2, other gases