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 energySupplying 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:

• The atmosphere• Ocean surface water• Plants• Deep ocean• Mineralization (e.g. making limestone)

Reservoirs for CO2

• The atmosphere• Ocean surface water• Plants• Deep ocean• Mineralization (making limestone) too slow

Reservoirs for CO2

• 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?

Bertie says, “Because the sun is not shining.”

Carla objects, “Then why isn’t it cold in a closet?”

The temperature steadily decreases at night. Why? Demonstration!

The leaky bucket modelThe 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 earthWhen 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 earthThere’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 expectedThe 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