Note: This program first aired on April 12, 2014.
So far we’ve talked about how the amount of energy reaching
the Earth can change, though differences in the amount of energy the Sun emits
and variations in what the Earth reflects back into space as light. The last
big player in climate change affects not how much energy reaches the Earth, but
how much is kept here.
Recall that the energy that drives the climate system comes
from the Sun arriving as light. Light is a high energy, short wavelength form
of radiation, and the Sun emits light because it is so hot. About 30% of the
light that hits Earth is reflected back into space, due to the reflectivity, or
albedo, of certain Earth surfaces. That leaves approximately 70% to actually
reach Earth and enter the climate system. Of that 70%, about 20% of the visible
light is absorbed by gasses in the atmosphere. We’re more familiar with the
phenomenon of atmospheric gases absorbing certain wave lengths of solar
radiation when we talk about the stratospheric ozone layer. The ozone molecule,
O3, is found at low but important concentrations 15 to 30 kilometers above the
surface of the Earth. It plays an important role in the history of life on
Earth, because it absorbs some of the non visible radiation that comes from the
sun, ultraviolet light. UV light is particularly damaging to cells (skin cancer
anyone?), and without the filtering effect of stratospheric ozone, it would
have been hard for life to evolve, or the life that did evolve, would look
pretty different.
It turns out that ozone absorbs visible light as well, as
does water vapor in the atmosphere, and even carbon dioxide to a limited
extent. Any non reflective particulates (dust, soot) can also absorb light.
This is where that first 20% of energy entering the climate system goes—its
absorbed by gases and particles in the atmosphere.
The last 50% of the light energy from the sun makes it to
the surface of the Earth and is absorbed there by everything down here-rocks
and soils, plants, water, and sunbathers. When that as light energy gets
absorbed it gets transformed into infrared radiation, otherwise known to us a
heat. The gases in the atmosphere, the rocks and soils, plants, water and
sunbathers aren’t hot enough to emit the energy as visible light, so it down
cycles into heat. That 70% of light energy from the sun that hits the Earth all
gets turned into heat, which is the form in which it is reradiated back into
space which is eventually where it will all end up.
When the gases in the atmosphere absorb light and radiate
out infrared radiation or heat they do so in all directions. Each molecule
sends out its little bit of transformed heat energy, and that energy can then hit
another nearby molecule, heating it up, before that second molecule radiates
out the heat, and so one. Because the gases in the atmosphere are up so high,
quite literally closer to space, it is much easier for the heat they are
emitting to make it back out into space. When we get high enough above Earth,
the atmosphere is no longer the atmosphere, there are so few air molecules
there we call it the exosphere. With so few molecules, when a molecule does
radiate some heat, the chances of another molecule absorbing it are much lower,
and the chances of it simply going into space are much higher. The net result
of this is that a high percentage of the light energy that is absorbed by the
gases in the atmosphere gets reradiated back into space, without ever helping
us out down here on the surface.
Down here closer to Earth all that heat radiation is a great
deal further from space. Again, radiation is happening in all directions, but
some of that direction is right back to the surface of the Earth. So simply by
distance it is harder for heat at the Earth’s surface to get out into space.
The physical process of heat radiation transferring from atom to atom, and the
randomness of the direction of travel all play into this. The net effect of
this is that of the 50% of the light energy of the sun that reaches the surface
of the Earth, a lower proportion is radiated back out into space (at least
immediately). This time lag in the functional reemmission of infrared radiation
from the surface of the Earth is the reason that we have a livable climate
here, why the average temperature of the Earth is something higher than
absolute zero. And it is due in part to this backscattering of infrared
radiation I’ve just described. But it is mostly due to gases in the atmosphere,
green house gases. The specific mechanics of how this process works will be our
topic next week.
