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Our planet's atmosphere has been warming
recently and the cause is something
called greenhouse gases. But what exactly
are greenhouse gases and how do we know
they cause our planet to warm up? The
objects on this table are at room
temperature - each more or less as warm as
the objects next to it. But some things
are warmer than others like this cup of
tea. Because the cup of tea is warmer,
energy is flowing from the tea to the
room around it in the form of infrared
radiation. By using an infrared camera we
can watch the cup of tea cool as it
transfers energy to the room. Over time
it will reach the same temperature as
the other objects around it. In science
this transfer of energy is what we call
heat. Now let's step back and imagine our
planet. Our Sun is continually
transferring some of its energy to our
planet. Much of that energy travels right
through Earth's atmosphere and reaches
the surface where it is absorbed and
re-emitted into the atmosphere in longer
infrared wavelengths. If all of that
energy passed through the atmosphere
directly into space our planet would be
quite cold - on average around minus 18
degrees Celsius. But infrared energy can
interact with certain gases in the
atmosphere
which are known as greenhouse gases.
These gases interrupt the flow of energy
escaping from earth, holding some of that
energy in the atmosphere. It is a little
like throwing this towel over this
heating pad. The towel interrupts the
flow of heat, trapping the energy near
the surface of the pad. A simple
experiment will demonstrate which
atmospheric gases behave in this way.
Here are four cylinders filled with four
atmospheric gases - nitrogen, oxygen, carbon
dioxide and methane. Notice the gases are
transparent to visible light just like
the atmosphere, but in this experiment
we'll test which ones are transparent to
infrared light. This heating pad emits
infrared wavelengths and will simulate
our planet's surface. We've sealed the
ends of each cylinder with thin plastic
wrap because glass itself blocks
infrared radiation, but the thin plastic
doesn't. We'll start with a cylinder
filled with nitrogen gas and place it
between the infrared camera and the
heating pad. Looking down through the
cylinder we notice that the infrared
energy passes right through nitrogen
without being absorbed. If we try the
cylinder filled with oxygen gas we get a
similar result. But carbon dioxide is a
different story.
Notice how the image in the infrared
camera becomes much cooler. This reveals
that the carbon dioxide is absorbing
some of the infrared energy given off by
the heating pad before it can reach the
camera. And here's methane. Methane also
absorbs the infrared radiation so
greenhouse gases like carbon dioxide and
methane are gases that absorb energy,
trapping more energy in the atmosphere.
But to see if greenhouse gases actually
behave this way in the atmosphere we
have to move from our tabletop
experiment to the real world.
The first piece of evidence comes from a
satellite measuring the wavelengths of
infrared energy escaping from Earth's
atmosphere. Since the precise wavelengths
absorbed by each greenhouse gas are
known, the satellite should detect gaps
at those precise wavelengths. This curved
line shows what would be detected if no
gases were absorbing energy and this
squiggly line shows the real
measurements from the satellite. The
difference between the curved line and
the squiggly line reveals the energy
being absorbed by greenhouse gases. These
wavelengths here are missing because of
carbon dioxide and this dip is caused by
methane and these gaps show wavelengths
absorbed by water vapor and ozone - two
other important greenhouse gases. This is
the energy that doesn't escape through
the atmosphere and remains behind to
warm the planet. Next we have to see if a
change in the amount of greenhouse gas
in the atmosphere causes a rise in
global temperature by looking at things
over time. Here is a chart that shows a
40% rise of atmospheric carbon dioxide
over the last 60 years, and here is a
chart that shows global temperature over
the same period.
The average surface temperature of our
planet has been rising at the same time
as we've been putting more and more
greenhouse gases like carbon dioxide
into the atmosphere, so given our little
experiment and the evidence from real
world measurements, it is very difficult
to ignore the fact that greenhouse gases
are causing our planet's climates to
change. And once we realize this we have
to ask ourselves what are we doing about
it.
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