This figure is
pretty complicated. It
would be
difficult to start with this figure and find the greenhouse effect in
it. That's why we started with a simplied version.
I've
kept
the same color scheme so that you identify the different
parts of the picture. Once you
understand the upper figure, you should be
able to find and understand the corresponding parts in the lower figure.
Two or three things to note in the bottom figure
(i) First
the ground receives more energy from the atmosphere (96 units) than it
gets from the sun (51 units). Part of the reason for this is
that the sun just shines for part of the day. We receive energy
from the atmosphere 24 hours per day.
(ii) The ground emits more energy (117
units) than it gets from the sun (51 units). It is able to
achieve energy balance because it also gets energy from the
atmosphere (96 units).
(iii) The atmosphere emits 64 units upward and 96 units
downward. This might be explained by the lower atmosphere being
warmer than higher up in the atmosphere. Part of the explanation
is also that there is more air in the bottom of the atmosphere (the air
is denser) than
near the top of
the atmosphere.
(iv) Note also the minor but necessary roles played by conduction
& convection (7
units), and latent heat (23 units) energy transport at the left side of
the bottom figure. 80% of the energy transported from the ground
is done by electromagnetic radiation. But the other transport
processes are needed to bring the figure into
energy balance.
Next we used our simplified representation of the greenhouse
effect to understand the effects of clouds on daytime high and
nighttime low temperatures.
Here's the simplified picture of
radiative equilibrium again (you're probably getting pretty tired of
seeing this). The
two pictures below show what happens at night
when you remove
the
two green rays of incoming sunlight.
The picture on the left shows a
clear night. The ground is losing
3
arrows of energy and getting one back from the atmosphere. That's
a
net loss of 2 arrows. The ground cools rapidly and gets cold
during
the night.
A cloudy night is shown at right. Notice the effect of the
clouds.
Clouds are good absorbers
of infrared
radiation. If we could see IR light,
clouds would appear black, very different from what we are used
to (because clouds also emit IR light, if we could see IR light the
clouds might also
glow). Now none of
the IR radiation emitted by the ground passes through the atmosphere
into space. It is all absorbed either by greenhouse gases or by
the
clouds. Because the clouds and atmosphere are now absorbing 3
units of
radiation they must emit 3 units: 1 goes upward into space, the other 2
downward to the ground. There is now a net loss at the ground of
only
1 arrow.
The ground won't cool as quickly and won't get as cold on a cloudy
night as it does on a clear night. That makes for nice early
morning bicycle rides this time of the year.
The next two figures compare clear and cloudy days.
Clouds are good reflectors
of visible
light (we see visible light and clouds appear white). The effect
of this is to
reduce the amount of sunlight energy reaching the ground in the right
picture. With less sunlight being absorbed at the ground, the
ground
doesn't need to get as warm to be in energy balance.
It is generally cooler during the day on a cloudy day than on a
clear
day.
Clouds raise the nighttime minimum temperature and lower the
daytime
maximum temperature. Here are some typical daytime high and
nighttime
low temperature values on clear and cloudy days for this time of the
year.
We'll use
our simplified representation of radiative equilibrium to understand
enhancement of the greenhouse effect and global warming.
The figure (p. 72c in the
photocopied Class Notes) on the
left
shows
energy balance on the earth
without
an atmosphere (or with an atmosphere that doesn't contain greenhouse
gases). The ground achieves energy balance by emitting only 2
units of energy to balance out what it is getting from the sun.
The ground wouldn't need to be
very warm to do this.
If you add an atmosphere and greenhouse gases, the atmosphere will
begin to absorb some of the outgoing IR radiation. The atmosphere
will also begin to emit IR radiation, upward into space and downard
toward the ground. After a period of adjustment you end up with a
new energy balance. The ground is warmer and is now emitting 3
units of energy even though it is only getting 2 units from the
sun. It can do this because it gets a unit of energy from the
atmosphere.
In the right figure the concentration of greenhouse gases has
increased
even more (due to human activities). The earth would find a new
energy balance. In this case the ground would be warmer and would
be emitting 4 units of energy, but still only getting 2 units from the
sun. With more greenhouse gases, the atmosphere is now able to
absorb 3
units of the IR emitted by the ground. The atmosphere sends 2
back to the ground and 1 up into space.
The next figure shows a common misconception about the cause of
global
warming.
Many people know that sunlight
contains UV light and that
the ozone
absorbs much of this dangerous type of high energy radiation.
People also know that release of chemicals such as CFCs are destroying
stratospheric ozone and letting some of this UV light reach the
ground. That is all
correct.
They then conclude that it is
this additional UV energy reaching the ground that is causing the globe
to warm. This
is not correct. There isn't much UV light in sunlight in
the
first place and the small amount of additional UV light reaching the
ground won't be enough to cause global warming. It will cause
cataracts and skin cancer and those kinds of problems but not global
warming.
