Download Greenhouse Effect

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 H2O, CO2, and CH4 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”
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)
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.
Atmospheric Composition
The Main Constituents in Earth‟s atmosphere
Atmospheric Composition
The Greenhouse Gases
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%
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
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 H2O (naturally occuring gas) and CO2 (natural and man-made
sources). Detailed studies show that H2O provides most of the thermal warming
of the planet, followed by CO2. Note that ozone (O3) is also a greenhouse gas,
but not a major one.
Fig. 3.13
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:
sTs4 = S/4 (1 – A)
Box Figure 3-2
Add an atmosphere
Incoming: S/4(1-A) + sTe4
Outgoing:
sTs4
now, two terms!
Box Fig. 3.2
New surface balance with atmosphere:
sTs4 = S/4 (1 – A) + sTe4
The extra term comes from insulating properties of the atmosphere the “greenhouse effect”
Box Figure 3-2
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.
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….!
sT4
sT4
Surface
Thus, the surface is warmest, and temperature decreases with
height up to an altitude of about 10-15 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
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 H2O, CO2, 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
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
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
Other than radiation, what forms of heat transfer are
important in the atmosphere
Fig. 3.10
A schematic of Earth’s energy budget
a bit more complex model!
Fig. 3.19
Friday – We‟ll take a closer look at the details of energy
balance and water vapor and clouds