Chapter 1 - The Earth and Sun
The term Climate Change is broad, with natural and artificial causes, and short and long term
impacts. Before we can move into the human activities that make climate change worse, we need to
explore the way that the Earth heats up and cools down.
I know that it’s not a perfect analogy, but think about an oven. If the oven is at room
temperature, we need to add an amount of energy to raise the temperature inside the oven to
350 degrees. The amount of energy we add will be called H. Okay, let’s assume that our room
temperature is always the same and we always add H amount of heat. Here’s how the cycle
works:
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The oven is at room temperature (70 degrees)
We add H amount of heat (it takes 15 minutes for all of the heat to be added)
The oven heats up to 350 degrees
We turn the oven off and it cools back down to room temperature in 2 hours.
We then repeat the whole process.
Every time we do this process, we start and end with an oven at room temperature.
Now, let’s change things a bit. Let’s add more insulation to the oven so that it holds its heat
more efficiently. If we add H amount of heat, after 15 minutes, the inside temperature will be
350 degrees. This time, however, if we wait 2 hours, the oven will not have cooled all the way
back down to room temperature. Due to the improved insulation, it only gets down to 80
degrees in 2 hours. But, if we do everything as we did before, at that 2 hour mark, we have to
add H amount of heat again. We’re not simply setting our oven to 350 degrees—we are adding
a specific amount of heat energy. This time, the oven heats up to 360 degrees. As we continue
to repeat our cycles, the oven will gradually have a higher temperature because it is not losing
heat as quickly, and we are adding the exact same amount of heat each time.
The situation is physically different in the Earth’s atmosphere, but the general idea is the
same. We are receiving the same amount of energy from the sun as we always have, but the
Earth is not cooling down as much as it did in the past, so our overall average temperature is
rising. The reasons for this change are both simple and complex. On the simple side, we can
say that we are adding “insulation” to the atmosphere as we did with our oven, but in this case,
we aren’t adding it intentionally.
We will start by looking at the type of energy that comes into the Earth’s system from
the sun and the type of energy that leaves the Earth and its atmosphere. Energy travels in
waves, and the length between the tops of those waves is called the WAVELENGTH. The
wavelength determines the type of energy it is. If you look at a rainbow, you see all of the
colors of the visible spectrum. Each color is simply energy (radiation) with a different
wavelength. Blue and violet light have the shortest wavelength, and red has the longest.
Beyond violet, there are shorter and shorter wavelengths which result in ultraviolet radiation,
then X-rays, and finally gamma rays. As wavelengths get longer than red light, we get infrared,
microwaves, and then radio waves. We can only see about 1% of the entire spectrum.
Some forms of radiation are completely harmless to people and animals, including the
visible spectrum and radio waves. Some are dangerous in high doses, such as ultraviolet
radiation which causes skin cancer and X-rays which can cause various cancers in very high
doses. Gamma rays are particularly dangerous and luckily are rarely a problem. But, why are
gamma rays rarely a problem? The Earth is protected by a magnetic field called the
MAGNETOSPHERE high up in the atmosphere. The magnetosphere is able to block the shorter
wavelengths that are dangerous to life. The “middle” wavelengths which result in visible light
pass through the magnetosphere.
Occasionally, the sun gives off blasts of charged particles that hit the magnetosphere
and cause various things to occur. These solar flares can disrupt satellite communication which
affects cell phone use, and very large solar flares could possibly destroy parts of the electrical
grid that delivers power to homes and businesses. One harmless effect of the charged particles
hitting the magnetosphere is the creation of the AURORA BOREALIS (in the northern
hemisphere) and the AURORA AUSTRALIS (in the southern hemisphere). These are commonly
called the Northern Lights and the Southern Lights. They are wavy patterns of light in the sky
that are nothing more than energy releases from the magnetosphere as it is hit with the
particles shot out during a solar flare. The picture below shows the Northern Lights.
Okay, so the magnetosphere protects us from being bombarded by X-rays and
ultraviolet radiation, and it allows visible light to pass through. That’s pretty convenient
because it allows us to see, but it is also important because it limits the total amount of energy
that reaches the Earth’s surface. The systems that we will study in this course have developed
over very long periods of time in a fairly constant energy system. If there were other
wavelengths that were allowed through, the Earth would go through significant changes. It is
important to know that these changes would not mean the end of everything that exists on
Earth. Our magnetosphere goes through periods where it flips its polarity, and when it is in the
final stage of flipping, it disappears temporarily. This flipping simply means that magnetic
readings for north and south will switch. Compasses will point to the south pole, instead of the
north pole.
Let’s stop here for a moment and think about all of this energy that reaches the Earth
from the sun. Physicists have studied energy and tell us that energy doesn’t just disappear.
Basically, there are four things that can happen to energy:
1.
2.
3.
4.
It can be used for some form of work (think of burning fuel in an engine)
It can be converted to matter (think of photosynthesis)
It can hang around and “heat up” a system
It can be released by the system
So, what happens to the energy we receive? To answer this, we have to understand the
type of system that the Earth has. Most people think of two types of systems: OPEN and
CLOSED. An OPEN SYSTEM is one which receives and gives off energy freely. The human body
is an open system because it receives energy from outside sources and returns energy and
matter outside of itself. A CLOSED SYSTEM or ISOLATED SYSTEM is one which does not receive
or give off any energy or other inputs from the outside. It is difficult to come up with a truly
closed system outside of a laboratory experiment. Some might say that the universe is the only
true closed system. Okay, then what is the Earth—open or closed? Both or neither. The Earth
is not a closed system because it receives all of its energy from the sun. But, it doesn’t quite fit
into the open system category because it is pretty self-contained. For this class, let’s call it a
Semi-closed system. We can look at the Earth and Sun together as a nearly closed system, if
that helps. Even though we get all of our energy from the sun, the amount of energy we get is
only one two-billionth of the sun’s total energy output.
The important thing here is that the Earth gets all of its energy from the sun—and we’re
talking about a lot of energy. It provides the light we see, the heat that keeps the Earth from
being a frozen ball in space, the energy that churns our atmosphere, the “food” for plants, and
an energy source for countless processes. There’s more incoming radiation than we need,
though. Why doesn’t the Earth just boil away into space? Think about our oven. The Earth,
like the oven, cools down at night when our energy input is low.
The sun doesn’t take days off, so we must be losing the excess energy somewhere, but
where? Some of the sun’s energy is immediately reflected back to space. Light colored areas
on the Earth’s surface reflect more light. We call this reflection of sunlight back to space
ALBEDO. If we look at the whole Earth, the average amount of sunlight reflected back from the
surface is 31% of what comes in. The picture below shows the albedo values for various types
of surfaces on the Earth. Lighter colored areas have higher albedo values. As this course
progresses, we will discuss possible changes in the Earth’s overall albedo caused by a warming
climate.
The reflection of sunlight is more complicated than shining a flashlight on a mirror.
When the sunlight comes through the atmosphere and hits the surface, it is in the form of
shortwave radiation (visible light). When it is reflected away from the Earth’s surface, it is in
the form of longwave radiation (such as infrared radiation). The picture below shows
shortwave radiation coming to the Earth’s surface and longwave radiation being reflected back
to space.
The Earth also gives off energy at night when it cools. The heat that’s given off is really
just energy, so it heads toward space as longwave radiation. This seems pretty easy. Energy
comes in. Some is used and some is released. We seem to have a nice balance, but there’s a
new problem. Our atmosphere has various gases. That’s a good thing because we need to
breathe, but there are other gases that aren’t so good because they mess up the energy
balance. Some of these gases, called greenhouse gases, block some of the longwave radiation
from leaving the system. Some of that blocked energy heats up the atmosphere and some is
reflected back down to the surface. This is called the GREENHOUSE EFFECT and results in global
warming.
Some particles actually have the opposite effect—they block shortwave radiation from
entering our atmosphere. Examples of these “albedo forcing” particles include volcanic ash.
Some huge volcanic eruptions have put so much ash into the atmosphere that they have
lowered global temperatures temporarily. The year 1815 is known as the “Year without a
Summer” because temperatures were so low that there was snow in the summer in many
normally warm areas, crops failed, and so on. This was caused by a series of huge eruptions
starting in 1812, and ending with the massive eruption of Mt. Tambora in Indonesia which was
the largest volcanic eruption in the past 1,500 years. This period also coincided with
abnormally low sunspot activity. Interestingly, we are entering a period of very low sunspot
activity, according to NASA, so we will discuss how this might impact global climate change in
the short and long term later in the term.
At this point, we know that a lot of energy comes to the Earth in the form of shortwave
radiation, and some (not enough) leaves as longwave radiation. This incoming energy heats the
Earth, so why is Mexico so hot and northern Canada so cold? The reason is that sunlight that
hits the Earth’s surface directly is more concentrated. If a spot on the Earth is facing directly
toward the sun, then the sunlight will be perpendicular to the surface. The areas near the poles
will get less concentrated sunlight and therefore less energy. The picture below shows how
the sunlight hits the surface at an angle.
Okay, well now something doesn’t make sense. The angle of the sunlight seems to
mean that every location on the Earth should get the same amount of energy each day
throughout the year. It’s cold in the winter in the northern hemisphere and hot in the summer,
and this is all flipped in the southern hemisphere. Plus, areas near the Equator don’t seem to
have extreme temperature differences between summer and winter. Take a look at the image
below. The odd graph on the left takes a while to learn to use, but it shows the amount of
energy received at various latitudes (distance from the Equator) throughout the year. The
smaller graphs on the right show the amount of energy received throughout the year in four
different locations. The top shows the North Pole, where it gets a lot of energy in the middle of
the summer and almost none in the winter. The South Pole is the opposite. New York has a
more gradual curve than the poles, and the Equator is almost a flat line. These lines show
energy, but they also indicate the temperature differences throughout the year. But, we’re still
stuck on the question of why this happens.
The differences in temperature are caused by the SEASONS. Okay, we all know what we
call our seasons, but knowing why they occur is a bit more complicated. To fully understand
the seasons, we have to look at three things: Rotation, Revolution, and Axial Tilt. ROTATION is
pretty simple. The Earth spins like a top. It always goes in the same direction and the same
speed. Actually, the speed may change very slightly. In 2004, there was a massive earthquake
off the coast of Indonesia that was so powerful that it actually changed the length of a day by a
fraction of a second. Before you laugh at a fraction of a second, think about the amount of
power that it would take to slow the Earth down, even a little bit. The length of a day is simply
one full rotation, so this event made the day a tiny bit longer by slowing the rotation.
REVOLUTION is describing as the Earth orbiting the sun. The Earth travels around the
sun in an almost circular path (it is slightly elliptical). One revolution is called a year. Rotation
and revolution aren’t enough to explain our seasons. The sun doesn’t have a cool side. We
have to add the final piece to the puzzle—AXIAL TILT. Imagine a globe like the one below:
The holder has a rod that goes through the North Pole and come out at the South Pole. You can
see that the globe is tilted. When you spin the globe, it is always tilted. The rod in the middle
of the globe is its axis. If a sphere spins, that “rod” through the sphere is described as the axis
of rotation. The Earth’s axis is always tilted. The important thing to remember about the
Earth’s axial tilt is that it never wobbles. We have a tilted planet that revolves around the sun
with the same tilt. This means that in the summer, the northern hemisphere is tilted toward
the sun, and when it is winter, the same tilt means that the southern hemisphere is tilted
toward the sun. This can be very difficult to visualize. The picture below is used in many
college classes, and students still scratch their heads trying to figure it out.
There may be a less fancy way to make this easier. Let’s go back to our globe. Below, we have
the globe and a yellow arrow that represents sunlight. Remember that the place with the most
concentrated energy is the place where the energy hits the surface directly. In this image, the
sunlight hits the southern hemisphere directly, so we are looking at December--the southern
hemisphere’s summer and our winter. The day when the sun hits the farthest southern point
directly is called the WINTER SOLSTICE.
We can flip this around and look at June, when we have summer and the southern hemisphere
has winter. The image below shows the sunlight hitting the northern hemisphere directly. The
day when the sunlight hits the farthest northern point directly is called the SUMMER SOLSTICE.
That’s a pretty neat trick with a globe and an arrow, but what about spring and fall? Now we
have to be creative and imagine that we are the sunlight heading for Earth. The picture of the
globe shows us our target. We’re going to hit right in the center which is a spot on the Equator.
There are two days each year when the sunlight hits the Equator directly. These are the
VERNAL EQUINOX (the first day of spring) and the AUTUMNAL EQUINOX (the first day of
autumn). On these days, every place on Earth has 12 hours of daylight and 12 hours of night.
Equal amount of daylight and darkness only occur on two days. For those of us in the
Northern Hemisphere, after the Vernal Equinox, we start to get longer days and shorter nights.
Our longest day is the Summer Solstice. Then, days start to get shorter again until the Winter
Solstice which is the day with the shortest period of daylight. This is not just about how much
light we have to do things outdoors. The amount of sunlight we get is related to the amount of
heat we get. When we have longer days, we have more energy, so we can grow crops to eat.
In the winter, farmers can’t grow food, so we have to wait for the seasons to change again.
Most of us are used to summer nights where the sun sets sometime around 9 pm and winter
nights when the sun sets around 4 pm. There are other places where the day lengths are more
extreme. In the middle of the summer in England, the sun doesn’t set until after 10:30 pm. If
we go all the way up past the Arctic Circle, the sun doesn’t set on the Summer Solstice. They
have 24 hours of sunlight. Some people call this area the “Land of the Midnight Sun.” The
picture below was produced by placing a camera on a tripod and opening the shutter to expose
the film throughout the day north of the Arctic Circle. The arrow points to the sun at midnight.
It is low in the sky, but it never sets.
Well, everyone in this class lives south of the Arctic Circle, so should we all have the
exact same amount of daylight? No, we have slightly different day lengths because the sun’s
angle is different at different latitudes. The picture below shows a person who looks at the sun
at noon on the solstices and equinoxes. She can measure the angle of the sun above the
surface. On the Summer Solstice, the sun is high in the sky at noon. On the Winter Solstice, the
sun is low in the sky. If you follow the dashed lines before and after noon, you will see that the
lines are longer in the summer. This means that the sun is up for a longer period of time.
We should have a pretty good idea of the system that provides energy for the Earth.
Our next chapter will move to our atmosphere, and the types of gases and materials that it
contains.