Happy Holidays! Today’s post is a little longer but you have the whole holiday to read it! It also condenses a lot of complex science into what I believe is a very digestible chunk – if I am wrong please let me know!
The Greenhouse effect is something politicians and
scientists bandy about however never try to explain. Heck they do not even mention the most common
Greenhouse Gas – Water Vapor. Today we
are going to describe what makes a greenhouse gas, how they heat the planet,
and why more of them may not be the best thing!
Everything in the Universe is constantly emitting
energy. Some things, like say the desk
I’m typing at, do not have much, if any energy to emit. Other things, like the Sun, have a ton of
energy. Since there is nothing touching
the Sun, and there is no air for it to immediately heat up, the only way it has
to get rid of that energy is radiation.
Radiation is typically measured by two numbers: Amplitude, and Wave Length.
Since all waves are described the same way, we’ll use our friendly
analog of water to describe Amplitude
and Wave Length.
Amplitude
measures how “high” the wave is. If two
waves are exactly identical, but one has a greater Amplitude than the other, it is carrying more energy. Let’s think about two sets of beaches with
waves crashing every 30 seconds. The
waves on the first beach are 10 ft high and the waves on the second beach are
30ft high. The waves on the second beach
are going to be moving much more water than the first beach.
Wavelength
measures how often the waves occur. It
is the distance between two peaks of the wave.
If all things between two waves are the same, the one with the smaller wave length will have more
energy. Going back to our beaches assume the exact same waves except
looking out from beach 1 you see a wave crest every 50 ft and at beach 2 you
see a wave crest every 25 ft. It’s easy to see that the waves at beach 2 are
going to be moving more water as they crash twice as often.
Radiation
Sun’s Radiation at Sea Level
This is a graph of the Sun’s radiation at sea level. The horizontal axis shows the wavelength and the vertical axis shows
the energy. We can see most of the suns energy is
radiated at between 400 and 800
nanometers (a nanometer – nm – is very small, it is .000000001 meters or
.00000004 inches). The important message from this graph is that the sun releases energy
at all types of wavelengths along the scale.
The earth, as an object in space, also radiates
heat. Some of this heat is generated
internally – by the very hot core - and some is external caused by the sun
beating down on the earth. The earth is
much less hot than the sun so we expect it to have less energy to radiate away.
Thus the wavelengths of its
radiation are expected to be larger and their amplitudes to be less
(remember the beach). This is what the
radiation from the earth, out to space looks like (The blue part!).
Notice earths radiation has a wavelength
that is greater than 10 times that of the sun
Where Does
the Energy Go?
A fundamental law of the universe is that Energy Is Conserved. What this boils down to is when two things
exert energy on each other the energy MUST go somewhere. When you push a ball, it moves. When you push down on a spring it gets
tighter and tighter (you can feel it push back on you). When you rub your hands together you get
heat! The energy you put into the system
must be put out some way. This is as
close to a truth in science as you can get.
When radiation energy comes in contact with any
matter (matter is the science term for a solid, liquid or gas) it has 3
possible interactions. The first is to
simply pass on by. Some energy waves
have such tiny wave lengths, and are so high energy they kind of just miss the
matter, or the just ding off the matter and do not interact with it in any
meaningful way. Think about an X-ray passing through your soft
tissue.
The next option is reflection. We use this all the time. Think about wearing a white t-shirt in the
blazing sun. The shirt reflects most of the light hitting you and keeps you cooler.
Finally the most dynamic and confusing of all is Absorption. This means the energy hits the matter and is
absorbed. The most obvious version of
this case is when you leave your car with black leather seats in the sun. As the sun beats down on the seats, the
smaller wave length light we can’t see likely passes through it, some of the light is reflected (giving it the
shininess), and most of it is absorbed and then emitted as heat. The seats get
hotter and hotter as they absorb more and more energy. This is how liquids and solids generally
absorb radiation – it hits them and they convert this energy into heat.
Gas on the other hand has a weirder interaction
with radiation. Since it is so spread
out it cannot easily release the energy as heat, it must find another way to convert the radiation energy. The radiation must cause some sort of rotation, or vibration so that the energy
has somewhere to go. The following Gifs
show Carbon Dioxide (CO2) and Methane (CH4) common greenhouse gasses vibrating.
This adds the final piece to our greenhouse
puzzle. Gases need some specific
requirements to absorb radiation energy.
First they need a structure that allows them to vibrate or spin or some
way to absorb some energy. If these were
just something like Nitrogen gas (~70% of our atmosphere) the energy simply
passes by or bounces off of it – there is nowhere for the energy to go in the
system.
Next we
need the correct wavelength to stimulate the movement. As you look at the CO2 and CH4 molecules
wiggle back and forth, imagine you are holding one end of a jump rope and I am
holding another. I’m just going to hold
my end and you are going to move your arm up and down. If you go too slow it will look like this:
If you go too fast the rope will be going up and
down all along it in no pattern. However
when you get exactly the right speed you will get this motion:
You see one peak as your arm goes up and one
valley as your arm goes down. This synching up of energy input wavelength
and its response’s wavelength is exactly what is needed for a gas to absorb
radiation. This yields charts like
the following:
Gray where CO2 absorbs radiation, white
where it doesn’t
This shows the “Absorption spectrum” for CO2. The horizontal axis is wavelength. The vertical axis is % of energy
absorbed. As you look around 2000 nm,
4000nm, and 20knm you see almost all radiation is absorbed. These are the wavelengths of radiation that
cause CO2 to vibrate and twist and wiggle in the right way. Anywhere that is not gray CO2
will absorb NONE of the energy.
Putting it
All Together
We now have everything we need to model the
planet. Here is a model with no
greenhouse gasses.
Under this model the temperature of the Earth
would be pretty stagnant. Yes some days
there would be more clouds so the Earth reflects more radiation, other days less
clouds and a warmer earth. Aside from
our previously discussed “Milankovitch Cycles”, the sun would always supply “W”
radiation, the Earth would absorb “X” radiation. The earth would supply “Y” heat and radiate
away “Z” energy. The temperature of the Earth would always be
“X+Y-Z” and those numbers would change little throughout the years.
Here is the actual picture of our system:
The new addition is the absorption of Earth’s
radiation by the atmosphere. This energy
is then absorbed and re-radiated back towards earth in the process we described
above. THIS IS THE GREENHOUSE EFFECT.
Let’s stack the Sun and Earth’s radiation spectrums with the 4 major
greenhouse gas absorption spectrums.
I hope you can see the issue at hand. These
four gasses all absorb energy at the same wavelength the Earth emits radiation. They also happen to not interact with
sunlight at all. This is where the
“Greenhouse” property is attained. As the sun beats down the earth will
heat. As the earth heats it radiates
energy. This energy is absorbed by
greenhouse gasses and is then radiated back toward the earth. This heats the earth more. This means more radiation is coming off the
earth (greater amplitude). This causes
even more energy to be transferred to the greenhouse gasses and the process
spirals on.
In engineering we call this a positive feedback
system. You put some amount of energy in
and the system builds and builds and builds until it goes out of control. We almost never design a system to do
this. The human body uses positive
feedback for one process… child birth. It’s pretty easy to see why this is not an
ideal situation for the earth. It’s also
easy to see that adding more gas to the system adds more energy returned to the
earth – it makes the process more drastic.
Problems
with Greenhouse Gasses and Man Made Global Warming
Here is a pretty easy wrench you could throw into
this whole situation with just the past 4 blog posts. The southern hemisphere’s summer is currently
occurring when the earth is closest to the sun (Axial procession from the
Milankovitch Cycle) which will naturally cause the Southern Hemisphere to heat
up with no humans necessary. As the Southern
Hemisphere gets hotter naturally more and more water vapor will be in the air
as the oceans see more energy. Since
water vapor is both the most bountiful and absorbing of the greenhouse gasses
the planet will start a greenhouse process with this alone. This heating of the planet causes more water
vapor to get into the atmosphere, causing a greenhouse effect all on its
own. More water vapor causes more heat,
causes more greenhouse effect. Since there is no way to stop Axial procession
nor water evaporation you cannot blame people for global warming?
This scenario is what these first 4 blog posts
have been preparing for. From our last
blog post we know we can reconstruct climates and temperatures of the past;
from “Being John Milankovitch” we know how to trace large increases in global
energy; from the first blog post we have the long term results of those
thousand year old changes. Scientists
need to take everything they have learned and are honestly still deciphering
about the past and apply it to the climate they are seeing today. They need to compare CO2 in the past to CO2
in the present etc.
Image Credits:
http://www.bbc.co.uk/staticarchive/df504ecc44f0690dc88a53b7532182d8dc2e80ba.gif
http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/section_11/62049a0bf48e2cb0b024d238707590e3.jpg
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