Climate Change 101; Some
Fundamental Science
Joseph P. Straley
Department of Physics & Astronomy
University of Kentucky
straley@pa.uky.edu
October 25, 2012
Where does our energy come from?
… from burning fossil fuels.
Chemistry of fossil fuel use
1 ton coal + 2.7 tons oxygen
= 3.7 tons carbon dioxide + 2000 KWH
= $200 worth of energy
Supplying the world’s energy for a year
requires burning the equivalent of
9,000,000,000 tons of coal, and
produces 34,000,000,000 tons of CO2
Carbon release to the environment
This produces 34,000,000,000 tons of
CO2 every year.
What happens to it?
What happens to the CO2?
It gets stored in various reservoirs:
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The atmosphere
Ocean surface water
Plants
Deep ocean
Mineralization (e.g. making limestone)
Reservoirs for CO2
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The atmosphere
Ocean surface water
Plants
Deep ocean
Mineralization (making limestone) too slow
Reservoirs for CO2
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The atmosphere
Ocean surface water
Plants
Deep ocean 500 year time scale
Mineralization (making limestone) too slow
Reservoirs for CO2
• The atmosphere
• Ocean surface water
• Plants
The atmosphere, the ocean surface, and the
plant reservoir are closely coupled: CO2 freely
moves among them, and is shared in a fixed
proportion.
Demonstration!
The plant reservoir is not a permanent
storage. Plants die and decompose (or are
burned). We are cutting down forests faster
than they are growing
Implication of the demonstration
The CO2 content of the atmosphere is
cumulative of all burning of fossil fuels. It is
a permanent change (on a 500-year time
scale). The amount of CO2 in the
atmosphere should be steadily rising, and
this will continue as long as we continue to
burn fossil fuels.
Increasing carbon dioxide
Did humans cause the increase?
It seems likely. We know how much coal
and oil we burn in a year, and thus how
much CO2 was produced. We know how
much air is in the atmosphere (atmospheric
pressure is the weight of the air). The result
is that if half of the 34,000,000,000 tons of
CO2 stayed in the atmosphere, the CO2
concentration would increase by 3.3 parts
per million per year (3.3 ppm/y).
Did humans cause the increase?
It seems likely. We know how much coal
and oil we burn in a year.
Any questions?
Why does it get cold at night?
“Because the sun is not shining.”
Carla objects, “Then why isn’t it cold in a closet?”
Bertie says,
The temperature steadily decreases at night.
Why?
Demonstration!
The leaky bucket model
The water corresponds to the energy
coming from the sun. Energy (and water)
are “conserved.”
The water level corresponds to the
temperature
What are the leaks?
Where does the
energy go?
Energy transfer mechanisms
• Conduction: requires contact
• Convection: requires motion of air or water
• Radiation: it’s light, and can take energy
wherever light can go.
But it doesn’t have to
be visible light that
carries the energy
away.
All objects emit light
• The power emitted and the color depend
on the temperature
• Near room temperature the emission is not
visible (infrared light), and low power
• Sufficiently hot objects glow –
• Red hot == 500 C
• Yellow hot == 1000 C
• White hot (like the sun) == 6000 C
Demonstration!
The temperature of the earth
Energy comes from the sun
in the form of visible light
All of this energy is reemitted,
so that the energy of the
earth stays the same from
day to day.
Both input and output are
nearly constant in time.
This determines the average
temperature of the earth
The temperature in Lexington
Any questions?
“The temperature of the earth”
Different parts
get different
amounts of sun.
Weather
moves the
energy
around.
Can we define
a temperature
for the earth?
Temperature of the earth
It varies from
place to place
and from day
to day, but the
changes are
gradual. So it
is possible to
construct an
average.
The average temperature of the earth is 13 C (55 F).
The temperature of the earth
When it is night one place it is day another.
When it is winter here
it is summer other
places. Averaging
the temperature of
many places can
give a result (55 F)
that does not change
much from day to day.
The temperature of the earth
There’s a problem with the theory: physics
predicts the rate of radiation of infrared light.
When you put the numbers in (ignoring the
atmosphere), the
earth comes out
out
too cold (0 F! ).
This means that
something is
a
blocking some of the holes in the bucket.
The earth is warmer than expected
The earth is warmer than expected, because
some of the energy emitted from the ground
is absorbed by the atmosphere (plugging
part of the “leak”). But
the visible light from the
sun is not affected. This
raises the temperature
of the earth from
0 F to 55 F.
Any questions?
There is an average temperature of the
earth that is determined by the balance of
sunlight coming in and infrared light leaving;
but something is blocking
some of the infrared light
leaving, which makes the
earth warmer than expected.
And a good thing, too!
What is plugging the leaks?
• The atmosphere is transparent in the
visible, but not in the infrared. This is due
to the presence of “greenhouse gases”:
Greenhouse gases
• Water vapor -- 2/3 of the effect
• Carbon dioxide (CO2) -- 1/4
• Methane and other gases -- 1/10
http://en.wikipedia.org/wiki/File:Atmospheric_Transmission.png
Greenhouse gases, again
• We have no control over the amount of
water in the atmosphere. Water
evaporates from the ocean, and comes
down as rain and snow, spending less
than a week in the air.
• Carbon dioxide accumulates in the
environment. This should cause the earth
to get warmer.
• Methane doesn’t last very long (a few
years).
Any questions?
Some infrared radiation
is being absorbed by
water vapor, carbon
dioxide, and some
other gases. The
atmosphere should be
making the earth
warmer, just as a
blanket keeps you
warm.
Is the temperature increasing?
Is the temperature increasing?
Climate change is not entirely
due to greenhouse gases
• Burning stuff makes smoke. Smoke
settles on the ground and makes it darker,
so that more sunlight is absorbed.
• Burning stuff makes clouds. These reflect
sunlight and decreases the power input.
Thus we may be blaming CO2 for something
that occurs when we make CO2.
Can we calculate the effect of CO2
on the temperature of the earth?
This is hard, because of feedback effects.
• Increasing temperature  more water vapor
• Increasing water vapor  more clouds
• Increasing temperature  less snow and ice
• Increasing temperature  more CO2
The temperature rise is small, because CO2 is
only a small part of the atmosphere, and the
CO2 concentration hasn’t increased much …
yet.
Overlaying two graphs
Apparently, the earth is getting
warmer
It amounts to about 2 Fahrenheit degrees
over the last century.
However, the theory says that increasing
CO2 concentration in the atmosphere is
inevitable and permanent, and that this
should lead to a permanent temperature
rise. It is a distant but unstoppable problem
for our descendants.
Temperature predictions
Summary
• The earth has definitely gotten warmer in
the last century
• The increase in carbon dioxide in the
atmosphere is the likely cause; this theory
implies that the temperature will continue
to go up in the future.
• We need to discuss the implications and
develop a plan
Any questions?
• The earth has definitely gotten warmer in the last
century
• The increase in carbon dioxide in the
atmosphere is the likely cause; this theory
implies that the temperature will continue to go
up in the future.
• We need to discuss the implications and develop
a plan