Prioritize...
Once you've completed this page, you should be able to
- discuss the so-called “greenhouse effect,” and the “greenhouse gases” that contribute to it, as well as its importance for life on Earth.
- describe the connection between the greenhouse effect and global warming, and make a distinction between the two.
Read...
So, we have this fundamental gap – out in space the Earth appears like it’s roughly 0F, but we know from living on the surface it’s generally warmer. Why? Well, recall that Planck’s Law told us that every piece of matter is emitting some form of energy. And the air molecules in our atmosphere are just that – it’s matter that can absorb and emit radiation itself. How does that help keep the surface temperature at a more comfortable level?
As you've learned, the earth's peak emission occurs at infrared wavelengths (from Wien's Law), so what happens to that radiation after it's emitted upward from the surface? Does it all go right to space? The answer is no; some is absorbed by air molecules, in particular, so-called “greenhouse gases,” such as water vapor, carbon dioxide, methane, and nitrous oxide. Note that I haven’t listed the oxygen and nitrogen discussed earlier – they are inefficient absorbers of longwave radiation and are quite happy to let that energy go back to space, not unlike how a clean window is quite happy to let shortwave radiation from the sun pass right through.
However, greenhouse gases are unique because they are a set of molecules that readily absorb longwave radiation that is emitted from the Earth. Of the greenhouse gases that exist within our atmosphere, water vapor is far and away the most abundant, followed by carbon dioxide (although recall that in the overall scheme of the atmosphere, these are trace gases – the percentages are very small). It turns out that some of the wavelengths that carbon dioxide and water vapor absorb readily (particularly those around 15 microns and a little larger) coincide with the wavelengths of Earth's peak emission.
Below is a chart of the absorption spectra of these “radiatively active gases” along with the atmosphere as a whole in the last row. Remember that shortwave radiation is anywhere between 0.1 and 1 µm. What this graph is showing us is that there is really not too much absorption in these wavelengths, so a lot of the curves are closer to zero than they are to one. The big exception is the third row, which is oxygen O2, and ozone O3. Ozone is the molecule in our atmosphere that most effectively absorbs shortwave radiation from the sun. This is actually a very important protectant for life on earth. Without ozone, a great deal of high intensity, ultraviolet (shortwave) radiation emitted by the sun would reach the surface, leading to far more frequent sunburns and skin cancer rates than seen today. We’ll talk about the ozone hole that formed a few decades ago and policies to protect our ozone layer later in the class.
Now focus on longer wavelengths, everything to the right of 1 µm. We see that all these gases (starting from the top, methane, nitrous oxide, ozone, carbon dioxide, and water vapor) all do some sort of absorbing in these wavelengths. That means, as the Earth emits these long wavelengths from the surface upwards, a select group of molecules in the atmosphere absorb some of this energy. But the molecules also must emit at these wavelengths – some of it is emitted upwards (towards space as before) but some of it is emitted downwards, back towards us at the surface. In fact, this downward emission is typically termed “down welling longwave” since it’s the terrestrial radiation that is emitted from the atmosphere back towards the surface of the Earth.
The fact that greenhouse gases absorb and emit infrared radiation so readily works out very well for humans. The very existence of these gases in our atmosphere is the key difference in why the Earth’s surface is actually that 62F number we discussed earlier versus the 0F we estimated it would be if there were no gases at all! So aside from all the oxygen we breathe, we can also thank our atmospheric blanket for making sure we don’t live in a total icebox!
The contributions of down welling IR from greenhouse gases to warming the planet is called the greenhouse effect. It is critically important to note that these gases are part of the natural Earth system – we’ll talk about human-induced global warming later, but it’s the increase in these greenhouse gases that is concerning, not their existence themselves!
Now we finally have all the pieces to explain the large-scale energy budget of the earth. When we average over the entire surface of the Earth, the sun provides approximately 340 W/m2 of incoming energy. Approximately 1/3 of that is reflected by either clouds or the surface back to space, thanks to our planetary albedo. Of the 2/3 left over (), is absorbed by the atmosphere on the way down (primarily the ozone we discussed earlier), and is absorbed by the surface. Most of the energy associated with the solar radiation absorbed in the surface is subsequently re-emitted by the surface as long wave radiation upwards into the atmosphere, although some of it also leaves the surface via other heat fluxes. Of the energy emitted by the surface, only approximately 5% of it makes it directly to space, with the other 95% being absorbed somewhere in the atmosphere, thereby changing atmospheric temperatures throughout. At the same time, all of the atmospheric volumes absorbing radiation from the surface are also emitting radiation according to their own temperatures. Eventually, the atmosphere exhausts to space via emission from gas molecules. When combined with the making it straight through, through to space from the surface, a total of 239 W / m2 is emitted to space as longwave radiation. This is quite close to the that comes into the Earth system from the sun and is absorbed by the Earth system!
Checkout this video (4:07 minutes) for a walkthrough of the energy flow diagram.
Walkthrough of the Energy Flow Diagram
PROFESSOR COLIN ZARZYCKI: Let’s take a look at this energy budget in a little more detail. When we account for the curvature of the Earth, we get 340 watts per meter squared that is coming down from the sun. We know that before any of that energy is actually absorbed by material in the Earth system, a large fraction of it is reflected back to space. In fact, 100 watts per meter squared, and note that 100 watts per meter squared is approximately 29% of 340. This is our planetary albedo.
Now, of that radiation that is reflected back to space, 75 watts per meter squared are reflected by clouds, and 25 watts per meter squared are reflected by the Earth’s surface. Now, if you do 340 minus 100, that gives us 240 watts per meter squared. That’s the amount of energy from the sun that must be absorbed by components in the Earth system. 80 watts per meter squared are absorbed directly in the atmosphere, and 160 watts per meter squared are absorbed by the surface.
So now we know that we have 240 watts per meter squared of energy that is absorbed by the Earth system. Where does that energy go? Well, we can look at the surface energy budget. So remember, in accordance with Stefan-Boltzmann law, anything that has a nonzero temperature—thankfully, everything on Earth—is emitting some form of longwave radiation. So the Earth is constantly emitting radiation in the form of things like thermals. This is kind of sensible heat, heat that is directly being transmitted to the atmosphere. We also have evapotranspiration, so heat that is associated with things like the phase changes of water.
We’ll talk a little bit more about this, but the vast majority of energy that is emitted from the surface is longwave infrared energy that is emitted again in accordance with Stefan-Boltzmann law. Of that 396 watts per meter squared, 20 of it goes to space, but 376 watts per meter squared are actually absorbed in the atmosphere. And that atmosphere is allowed to then emit radiation both upward and downward. In fact, this back radiation back to the surface that the atmosphere is constantly emitting, again because it is a nonzero temperature, is what leads to the greenhouse effect that we discussed. This keeps our planet at a warmer temperature than we would expect just based on the total amount of solar energy in and the amount of energy out.
Now, the atmosphere itself at the end of the day is going to emit 219 watts per meter squared. And so when you combine 219 plus the 20 watts per meter squared that are emitted directly from the surface to space, you get a total outgoing longwave radiation at the top of the atmosphere of 239 watts per meter squared. This is almost exactly the same as the 240 watts per meter squared that we’re bringing in. So why is it off by this small amount? Well, there’s this one little arrow that we’re going to talk about later in the semester with regard to additional energy that is trapped or put into the system. But otherwise, you’ve now been able to account for where energy is moving within the Earth system and how to balance the amount of energy coming in from the sun and the amount of energy that the Earth is giving off in terms of its longwave radiation.