When last we talked about climate change, we left off with
the idea that more energy comes into Earth’s climate system than leaves. Heat
is essentially building up in the atmosphere, less heat is escaping back into
space than is coming in in the form of sunlight. The reason for this is a
change in the amount of greenhouse gasses in the atmosphere.
When we look up into the sky we see what we think of as
nothing. But far from it, the sky is full of matter, billions upon billions
upon billions of atoms are floating around up there. And these atoms, just like
many others, absorb infrared radiation, which is simultaneously being released
by every atom everywhere, every atom that isn’t absolute zero. The Earth is
radiating infrared radiation, as a result of having been hit with light from
the sun. That light (a narrow relatively high energy band of the
electromagnetic spectrum) gets converted into a longer wavelength, lower energy
form, infrared radiation, and infrared radiation is the form in which the Earth
loses the energy that it originally gained as light from the sun. If there were
no atmosphere, if when we looked into the sky what we really saw was nothing,
the Earth would be much colder than it is now because all the infrared
radiation it is emitting would easily escape into space. Much like a human
being, running around in the winter naked. Heat readily leaves the warm human
body, and with nothing there to stop it, the human rapidly cools. That naked
human can slow the escape of heat from her or his body by putting on clothing,
creating a warmer microclimate around the body, thus reducing their convective,
evaporative, and radiative losses.
The Earth loses energy by all of those means as well.
Sunlight comes in and warms the air. Warm air rises, transporting that heat
higher into the atmosphere, moving it closer to space. Likewise, water absorbs
that light energy and warms up, evaporating more readily. When water changes
phase from liquid to gas, it takes a great deal of energy with it. When that
water vapor makes its way up into the atmosphere and condenses into water
droplets to form clouds, it releases that heat, again, high in the atmosphere,
where it is more easily lost to space. The current imbalance in Earth’s energy
budget is due to the radiative losses (or lack there of), the fraction of the
sunlight that makes it to the surface of the Earth that doesn’t just go into
heating air or evaporating water. That light is simply absorbed by matter here
and reradiated back out as infrared radiation.
That infrared radiation doesn’t all make it back out into
space right away, because of green house gasses. The green house gasses are
water vapor, carbon dioxide, ozone, methane, nitrous oxides, and
chloroflourocarbons. All of them except the CFC’s are naturally occurring
gasses, and without them, the Earth would not be livable, so green house gasses
aren’t a bad thing. Each of them absorbs specific wavelengths of infrared
radiation, with water vapor absorbing the widest range of wavelengths (that is
why it is the most important green house gas!). There is a gap in the
absorption spectrum of water vapor though, from about 8 to 14 microns, a micron
is a micro meter, a millionth of a meter. That gap is essentially a hole that
infrared radiation can escape through, because any radiation with wavelengths
between 8 to 14 microns won’t be absorbed! Most of the green house gasses
overlap their absorption spectrums with water vapor, meaning they absorb the
same kinds of wavelengths that water vapor does. But carbon dioxide is
different. It absorbs wavelengths of around 12 and 13 microns, meaning, it
partially plugs that 8 to 14 micron hole in the water vapor absorption
spectrum. Because the hole that infrared radiation can escape through is
getting smaller due to being partially plugged by carbon dioxide, less heat can
escape the atmosphere. When less heat escapes, more heat stays around to warm
things up here on Earth.
Remember, there has always been a lag time between heat
getting radiated from Earth and heat getting lost to the atmosphere. That is
what makes Earth’s average temperature greater than zero, and that is a good
thing, at least for us. When climate is stable, that “lag time” is consistent,
which means the back log of heat leaving through the atmosphere is consistent.
What is happening now is, due to the extra carbon dioxide in the atmosphere, the lag time is
increasing, and the back log of heat is getting bigger, so Earth’s average
temperature is warming. And it is just due to the slight narrowing of this
atmospheric window. That is why we pay so much attention to carbon dioxide as a
green house gas, it is the atmospheric factor that is effecting the biggest
change in the heat balance of the Earth. Next time we will look at an example
from Earth’s history, of a period that experienced the same kind of rapid
warming we are beginning to, as it may give us a sense of what we can expect.
References:
Scroll down this page from Texas A&M University to see a
graph of the absorption spectra of several green house gasses: http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm
