The Scientific Basis for Anthropogenic Global Warming

In the past week, two prominent conservative writers - George Will and Charles Krauthammer - have published opinion pieces debating the merits of the global warming argument. I can't recommend either. Mr. Krauthammer makes the valid point that scientific theory is not dogma; it cannot be known for certain and must be open to criticism. However, that argument is a distraction and  irrelevant to our discussion of climate change. The evidence supporting anthropogenic global warming(henceforth AGW) is collected from almost every branch of science (physics, biology, chemistry, geology); to challenge it would require massive amounts of evidence to the contrary, which is not provided. George Will's column reads like an angry old man complaining about kids on his front lawn. He goes so far as to compare scientists who support AGW theory to Nazi propagandists; his piece has no merits. Yet, a thorough analysis of their columns will have to wait until everyone is on the same page with the science of global warming; so today's lesson will focus on the scientific theory for AGW. I plan on following this up with a look at skeptical criticism to engage arguments like Mr. Krauthammer's more directly, and then a final discussion of the (already) observable and theoretical effects of climate change. Before getting into the science, a little vocabulary. So buckle your seat belts.
Terms

AGW: Anthropogenic is an adjective meaning originating from human activity. Anthropogenic Global Warming is the theory that the average global temperature of the planet is increasing due to the increased concentration of greenhouse gases in the atmosphere caused by human activity.
Greenhouse Gases: Gaseous matter that has the ability to absorb infrared electromagnetic radiation.
Climate Change: The effect of AGW on a local climate. This is a term to use when discussing the impact AGW will have on any specific location.
I'd also like to have a quick talk about proof. There are two types of arguments: deductive and inductive. Nearly every scientific argument (with the exception of theoretical math) is inductive; it is built from the ground up with evidence and occasionally assumptions, and thus it does not set out to "prove" a fact, but to build a case supporting the probability that our understanding is accurate. So a good scientist will rarely say something is proven to be true, and will instead talk about how likely something is to be true. However, because scientists are not certain about something does not mean that they are not extraordinarily close to certain (in fact a statistician could tell you precisely how close to certain the scientific community is, but trust me, you don't want to talk to a statistician. They have poor social skills and will always try to figure out where you are on the bell curve).

Greenhouse Gases

So what exactly makes a gas a green house gas? As I've defined it, the gas must be able to absorb infrared radiation, but how does it do this and what does that mean? Don't freak out, we're going to need to talk about quantum mechanics to answer this, but I think it's actually pretty cool stuff.

Figure 1. EM Spectrum and Atmospheric Opacity

Figure 1. has a lot of information in it; we're going to go through it slowly. At the top, is the electromagnetic spectrum which is the model for representing all the different energy levels light exhibits. If light has high energy, it will have a short wavelength, and if it has low energy it will have a longer wavelength. You can think of this like someone bouncing on a trampoline; if they have a lot of energy they will bounce rapidly and the waves on the trampoline will be close together, but if they are tired the bouncing slows down and the waves spread out. Qualifying light like this allows a way to easily display how some interaction effects all types of light. In the chart above, the X axis plots out the various wavelengths of light from nanometers to kilometers, while the y axis shows what percentage of that light is blocked by our atmosphere. The dips in the graph are wavelengths of light that manage to penetrate our atmosphere and reach the surface of the planet. The peaks in the graph show wavelengths of light that are either absorbed or scattered away by matter in our atmosphere. Notice how high energy light is absorbed by the atmosphere (the upper atmosphere), so thankfully we don't have to deal with gamma radiation when we go outside. The small dip in the UV portion of the spectrum is what gives us a tan when sunbathing- and eventually cancer. The dip in the visible spectrum allows us to see, and the valley in the radio wave spectrum allows for clear communication signals through the atmosphere. Infrared is partially absorbed by the upper atmospheres, partially absorbed in the lower atmospheres, and partially reflected back out to space.

Now what's really cool, is that the effect light will have on matter is incredibly specific to the light's wavelength. Quantum Mechanics developed in part by trying to explain this phenomenon: light energy is packaged together in specific quant(um)ities. This means that high energy light doesn't necessarily have more of an effect on matter than low energy light does, they have completely different mechanisms for interacting. For example, when visible light interacts with matter, it induces an electron in the outer shell of an atom to move into an excited state. UV light does this too, but it's energy levels can target the electrons in a covalent bond, causing bonds to break. More energetic light than UV can penetrate further into an atom's electron shell, causing inner electrons to be ejected, and very high energy light will be powerful enough to target the atom's nucleus. Light less energetic than the visible spectrum is not powerful enough to to induce a change in an atom's electrons, but it still has very specific physical effects. Microwave radiation causes molecules to rotate (your microwave is causing water molecules in whatever is being heated to rotate and bump into each other, enough so that they start to boil- temperature is a macroscopic measurement of the microscopic kinetic energy of matter, or how much the molecules are moving). Finally, infrared radiation causes molecules to vibrate, and the molecules that vibrate best can absorb the most infrared energy and meet the qualification of a greenhouse gas.

Figure 2.  Molecular Vibrations of CO₂ and O₂.

Shape is important for a molecule's vibration; here's look at carbon dioxide and oxygen. In Figure 2., I have drawn three of the most easily visualized vibrational modes for carbon dioxide, though in reality there are a couple more. This shows that CO₂ can have the oxygen atoms vibrate around the carbon symmetrically, the carbon can vibrate between the two oxygen atoms asymmetrically, or the molecule can bend from a linear shape like an inchworm. On the other hand, Oxygen only can vibrate in a symmetrical stretch. Unlike the other vibrational modes, a symmetric stretch is not caused by infrared radiation (or molecular oxygen will not absorb infrared light. The explanation is rather complex conceptually, but it goes like this. Electromagnetic radiation produces an effect on an atom's electrons. In a symmetric stretch, the change in the distribution of electrons is also symmetric, and so the changes in electronic distribution cancel each other out and nothing happens. I don't really get it either, but more important than my understanding is that the effect can be observed.)

Figure 3. Infrared Absorption by Carbon Dioxide.
Provided by Van Bramer, Scott. "Carbon Dioxide, CO2." Carbon Dioxide, CO2. Widener University, 5 Jan. 1996. Web. 01 Mar. 2014.

Figure 3. shows what happens when infrared light passes through carbon dioxide. Ignore the funky units, they are wave numbers and equivalent to wavelength, but they make the math simpler for chemists which is why they are used here. This graph is very similar to the one at the topic of Figure 1. with a few distinctions. First this is zoomed in to only represent the infrared portion of the EM spectrum. Second, this is only for Carbon Dioxide, whereas the first was for then entire atmosphere. Last, this shows the ability of CO₂ to absorb light, while the first showed the inverse- light's ability to avoid absorption. So the peaks here are wavelengths of light that carbon dioxide will absorb. Notice that there are unique wavelengths for the unique vibrations and that the symmetric stretch is not observed with Infrared Spectroscopy (that's the fancy name for this type of light analysis.) This graph is all the "proof" (rather, evidence) needed to show that carbon dioxide is a greenhouse gas.

Greenhouse Gases and the Atmosphere.

So what is this increase in carbon dioxide doing to the atmosphere? This is where things get a little messy and where a lot of "skeptics" start to object. The problem is a lack of an applicable experiment to test what's going on. The only full test would be to find a planet equivalent to earth, and crank up the carbon dioxide there, and then have a control planet where the carbon dioxide remained constant, and then we would need a couple dozen more planets to establish statistical validity. Until that becomes feasible, the answer to this question will have to come a little indirectly.

The moon has no atmosphere but has about the same average orbit from the sun as the earth. The average temperature on the moon is about negative nine degrees Fahrenheit. A physicist could do the math by balancing the energy coming in from the sun with the energy reflected off the earth and show that the average temperature of the earth without the atmosphere would be about zero degrees Fahrenheit.* The average temperature of the surface of the earth is fifty seven degrees Fahrenheit. The atmosphere makes the planet that much warmer.

There were also times when earth's atmosphere made the planet much warmer than it does today. Before the evolution of single cellular life, the atmosphere is estimated to have have been about 35% carbon dioxide. The sun was also younger and burned about 25% dimmer. In spite of the earth receiving significantly less solar radiation, the atmosphere was still able to foster a habit suitable for the evolution of life, which implies that carbon dioxide had an extreme warming effect. Estimates of Earth's temperature during the age of the dinosaurs place it at around 20 degrees Fahrenheit hotter than today's because of a higher carbon dioxide concentration.

Venus is a extreme example of greenhouse effect. Estimates for Venus' surface temperature without an atmosphere place it at around 160 degrees Fahrenheit. The average surface temperature of Venus is over 900 degrees Fahrenheit. The atmosphere concentration of carbon dioxide is 96%. Also it's clouds are sulfuric acid- don't go there.

Now, the mechanism for carbon dioxide as a greenhouse gas has been establish. Some cases of  how greenhouse gases can manipulated their environment have been shown. All that is still required is evidence that this effect is happening now.

Figure 4. Fluctuations in temperature and atmospheric concentration of carbon dioxide over the past 649,000 years.

Figure 4. shows a clear correlation between carbon dioxide concentration and the temperature of Antarctica. We can assume that the Antarctic temperature is representative of the average global temperature. While, it is foolish to assume the correlation implies causation, I have already provided an accurate mechanism for the causation to occur, explaining the correlation. Unfortunately, though, this view skews the impact of man made carbon dioxide from the industrial revolution- could we zoom in?

Figure 5. Global Average Temperature vs Carbon Dioxide Concentration since 1880

Oh. Actually- that's kind of scary. Let's not look at that.


Skepticism is a critical part of the scientific process. There is a time and a place for it. It is necessary. It is not reasonable to be skeptical of something that has a broad consensus of agreement without substantial evidence to the contrary. Period. 97% of climate scientists are in agreement. The AAAS, ACS, APS, AMS, and AGU are in agreement along with other scientific societies. So please, skeptics, where is your evidence to the contrary?

Please, anyone skeptical, this is your time to shine. I will be addressing skeptical claims next time (after a minor intermission) and I would love your take on AGW. Otherwise I'll be going off of vocal skeptics in the media and it will be less fun. Anyone wanting to play devil's advocate to my argument is also welcomed to do so.

My argument borrows several facts presented in the fantastic textbook Chemistry in Context which describes the chemistry behind many everyday phenomena. Eubanks, Lucy Pryde. "3: The Chemistry of Global Warming." Chemistry in Context: Applying Chemistry to Society. New York: McGraw-Hill Higher Education, 2009. N. pag. Print.

*The moon's surface is more reflective than Earth's, so it would be a little colder.