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Here side by side we see all of life...and life in its smallest package: a single cell.
Like an intricate fractal, when we zoom in and out, the structures of life don’t look
that much different.
What allows both these structures to survive is a thin layer of protection. The cell’s
membrane, or outer barrier, selectively filters what goes in and out of the cell.
For Earth, we have our atmosphere, or the envelope of gases held around the planet by
gravity, that acts as “the world’s biggest membrane.”
The physician and writer Lewis Thomas most famously made this comparison in 1973. Like
a cell membrane, the atmosphere filters what’s allowed in and out, like different wavelengths
of light.
Thomas marveled at how photographs like the famous “Earthrise” showed the sterile
surface of the moon with the “exuberant” Earth rising in the background. It seemed
miraculous that the Earth was so alive while everything around it seemed so...not.
Most of us take the atmosphere for granted and rarely think about what it’s made of
or how it moves or changes. We notice it when bad weather inconveniences us or appreciate
it when enjoying the outdoors. But it’s been 5 billion years in the making and it’s
how we and every other cell of life are able to survive.
I’m Alizé Carrère and this is Crash Course Geography.
Intro
Earth’s atmosphere is a unique reservoir that forms a protective boundary between outer
space and the biosphere, which is where all life exists on Earth’s surface.
Air is really a mixture of gases that are odorless, tasteless, colorless, formless,
and blend together so well they tend to act like a single gas.
The recipe close to the Earth's surface is 99% nitrogen and oxygen, slightly less than
1% argon, and a tiny percent of volume comes from minor gases, like carbon dioxide. But
this mostly consistent mixture starts to deviate as we get towards the outer edge of the atmosphere.
To better study the progression, we can break up the atmosphere into vertical layers or
“spheres” using several different characteristics.
The four layers we hear most about are the troposphere, stratosphere, mesosphere, and
thermosphere, and they come from studying the atmosphere based on its temperature structure.
Each layer has a different starting temperature that decreases or increases as we move towards
outer space.
Temperature even affects the thickness of the layers. The layer closest to the Earth
where all weather and most of the air molecules exist -- the troposphere -- can extend out
anywhere from 8 to 16 kilometers above the surface, depending on the season or latitude
of where you are on the globe.
That sounds kind of arbitrary, but it’s really just physics: when the air molecules
are cold, they huddle together, making the air denser and more compact. So in winter
or near the poles, the troposphere is thinnest. And it’s thickest where the air molecules
spread out, like in warm places at the equator.
When we move to the stratosphere, the next layer, temperatures tend to be layered and
get progressively warmer. Here we have the ozone layer, which is the section of the atmosphere
with the highest concentration of ozone, one of the minor gases in our recipe.
The ozone layer lets the wavelengths of light conducive to life to pass through, while filtering
out those that are harmful, like most of the ultraviolet waves. Absorbing UV rays is what
causes temperatures to increase in the stratosphere.
Then temperatures drop in the mesosphere and increase again in the incredibly hot thermosphere
where the few air molecules floating around out there can get to be 1,100 degrees Celsius!
Altogether, the atmosphere extends 480 kilometers above Earth’s surface. Which sounds like
a lot, but the diameter of the Earth is 12,756 kilometers. Compared to that, the atmosphere
seems like...the peel of an orange.
This thin layer of gas is so critical for life to exist, which is why it’s so important
to talk about early on in our journey into physical geography.
Without the atmosphere, none of the processes we’ll study in the hydrosphere, lithosphere,
and biosphere would function.
Energy from the Sun is constantly passing through the different layers of the atmosphere
as waves, landing on and being absorbed into the Earth’s surface to provide the heat
and warmth for life.
Really, that “energy” is electromagnetic radiation, or different wavelengths of energy
that travel away from the surface of an object. All objects -- the Sun, the Earth, our skin
-- are constantly emitting waves of electromagnetic radiation.
Very hot and high energy objects, like the Sun, emit vast amounts of energy as short
wavelengths, or solar radiation, in the form of light. Cooler objects, like the Earth,
emit much longer heat waves, or terrestrial radiation.
Since there's constant sunlight passing through the atmosphere, we might expect temperatures
to keep increasing, like when you're sitting under a blanket and get so hot that you want
to throw it off.
Fortunately, the Earth doesn't get swelteringly hot because the Earth and the atmosphere naturally
balance the shortwave solar energy that arrives with the longwave energy sent back to space.
This is the atmospheric energy budget, which is achieved with three common types of energy
transfers.
The first type of energy transfer is that radiation we’ve been talking about, which
is more generally the transfer of energy through waves. Like when we warm our hands over a
campfire. Insolation, or incoming solar radiation, reaches us by -- you guessed it! -- radiation.
Imagine we’re riding a sunbeam of solar radiation, trying to navigate the atmosphere
as it hurtles towards the surface. The atmosphere protects the Earth by filtering sunlight,
so there are quite a few obstacles to make it through.
If our sunbeam has 100 units of radiant energy, most of those units will be intercepted before
we make it to the surface. Let’s go to the Thought Bubble.
The outer layer of the Sun is extremely hot thanks to its intense energy, so our sunbeam
radiates at short wavelengths.
This radiant energy can pass easily through oxygen and nitrogen molecules because they're
basically little tiny windows letting short-wave energy in and out.
But other gases throughout the atmosphere absorb short-wave energy like a sponge filling
up with water.
In the stratosphere, ozone is a major obstacle.
And in the troposphere, water vapor in the clouds is the enemy.
Overall, 19 units of radiant energy from our sunbeam are intercepted by absorption.
Other particles in the air are pesky, too!
Dust, smoke, and volcanic emissions scatter radiation and change the direction of the
light’s movement without altering its wavelength.
8 units of energy from our sunbeam will get returned to space, while 20 units will get
scattered as diffuse radiation but persevere to reach Earth’s surface.
At this point, 53% of our sunbeam is still headed towards the Earth's surface.
Our next obstacles are clouds.
Thick clouds are actually capable of reflecting up to 80 percent of total incoming radiation,
like a mirror bouncing the energy back into space.
And even when we make it to the surface, our sunbeam isn't safe -- the ground can reflect
short-wave radiant energy too.
Snow and ice have higher albedos and reflect most of the solar energy that hits them, while
a black pavement has a low albedo and absorbs all the incoming solar energy.
On average 26 units of solar energy are reflected back into space by clouds or albedo on the
ground.
If we add it all up, just 27% of our original sunbeam reaches the Earth’s surface without
being absorbed, scattered, or reflected!
Thanks Thought Bubble! So after that rocky journey, 47 units (or so) of radiant energy
gets through to the ground as a combination of direct radiation that doesn’t get absorbed,
scattered, or reflected, and diffuse radiation that got scattered briefly but still makes
it through the atmosphere.
As my personal hero David Attenborough might say, that 47% is just enough for life on our
planet.
Any more and the surface might be too hot for life, like Mercury. Any less, and it would
be too cold for life as we know it.
After being absorbed into the surface, incoming radiation is eventually re-radiated by the
Earth as terrestrial radiation. Here the two other types of heat transfer have a role in
moving heat energy away from Earth’s surface, to the atmosphere, and out into space.
Heat is carried upwards from the Earth in convection currents. For example, insolation
heats water from the Earth’s surface, which evaporates, becomes water vapor, and condenses
into clouds in the troposphere.
As the water vapor condenses and changes from a gas to a liquid, the energy that gets released
heats up nearby air molecules.
This is like when water is boiled: convection currents let hot water molecules flow upwards
and cool.
And some heat is actually transferred by conduction, or through actual contact. Like when you go
to grab the hot handle of that pot of boiling water.
Heat is transferred to your palm through the physical contact you make with the pot.
Conduction is most important in the lowermost layers of air in contact with the ground,
but air is actually a pretty poor conductor of heat. So the small amount of heat transferred
through conduction ends up being carried further upwards by convection.
So the solar radiation coming in equals the terrestrial radiation, plus convection, plus
conduction coming out of the Earth. It's balanced!
The atmosphere actually traps quite a bit of the long-wave terrestrial radiation, re-radiating
and reflecting these heat waves back again in a continuous energy exchange. So the atmosphere
is actually heated from below.
Certain gases can absorb solar radiation on its way to Earth, but other trace gases like
carbon dioxide, methane, water vapor, and nitrous oxide are great at absorbing longwave
terrestrial radiation and sending it back to the Earth’s surface. This produces the
natural greenhouse effect.
In a greenhouse, the glass lets in insolation but doesn’t allow the warm air inside to
escape. These greenhouse gases do the same thing in our atmosphere.
Greenhouse gases get a lot of negative attention, but without the natural greenhouse effect,
Earth’s surface would be too cold for human life.
But we're running into problems because human activities -- like burning fossil fuels and
massive deforestation -- have increased the concentrations of greenhouse gases. So more
heat energy stays in the atmosphere.
This produces a warming trend which upsets the ecological systems of Earth. When the
atmosphere energy systems become unbalanced, there’s a cascading effect on other physical
and biological processes, from sea levels rising to changing distributions of plants
and animals.
Where we are on the globe plays a big role in how much energy is trapped by the greenhouse
effect or allowed to escape. In our previous episode we learned every location on Earth
doesn’t get the same amount of solar energy because of how the Earth tilts and moves.
The atmosphere ends up emphasizing this imbalance. For example, at the equator when the sun is
overhead, the incoming radiation only has to get through the vertical thickness of the
atmosphere, and a fairly large amount does get through.
But at high latitudes, the insolation doesn’t hit head-on and has more atmosphere to make
it through. So there’s more opportunity for scattering and reflection.
In theory, if the vertical atmospheric energy budget was all we had, tropical areas would
actually get warmer and the Arctic and Antarctic even colder. But this doesn’t happen.
Instead, large horizontal circulation systems -- like ocean currents and wind systems -- move
the excess heat the Earth receives at low latitudes to the poles. Soon we’ll see how
this energy transfer from the equator to the poles is one of the fundamental driving forces
behind the general circulation of the atmosphere around the globe.
In the next several episodes, we’ll explore how the atmosphere and its energy systems
are the basic ingredients for weather and climate. Like how the atmosphere ultimately
makes it possible for rice to grow in hot and wet places like Vietnam. Or why in cold
snowy places like Siberia, houses have steeply pitched roofs.
So much of how humans interact with our environment is shaped by how energy, heat, and water move
through the atmosphere.
Far from being a boring blanket of air, the atmosphere is an intelligent, sophisticated
shield that performs complex functions to make life viable on our planet.
Thanks for watching this episode of Crash Course Geography which was made with the help
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