April 14 - CO2 in Review

CO2 causes global warming. This is not a hypothesis, this is pretty much fact. Therefore, anthropogenic (a big word for man-made) CO2 causes global warming. This is not subject to much debate. What is subject to a lot of debate is how significant are the contributions of CO2 and, more specifically, anthropogenic CO2. Here is some science to help you decide...

The reason CO2 is a cause of global warming is that it is opaque to certain wavelengths of light. When radiation at those wavelengths hit a molecule of CO2, it is absorbed, causing the molecule to retain the energy... heat. Now, for a moment, imagine a bottle filled with CO2. If you pass that light through the bottle, some will hit the CO2 and some wont. That which hits the CO2 causes an increase in heat energy within the bottle. Now put twice as much CO2 in the bottle. Now when you pass the light through the bottle, twice as much radiation hits CO2, causing a proportional increase in heat energy. If you keep adding CO2 to the bottle, eventually nearly 100% of the light that is passed into the bottle hits CO2 molecules and fails to reach the other side. The CO2 has become effectively opaque to light at those wavelengths. Now, if you add more CO2 to the bottle, there is practically no further warming effect because nearly all of the light is already being absorbed.

Global average levels of CO2, prior to the industrial revolution are reported to be around 290 parts per million (PPM). Today, they are 380PPM. At 290PPM, the amount of heat added to the atmosphere is around 33 degrees Celsius. Furthermore, at those levels, the atmosphere is already a little better than 90% opaque to the wavelengths of light it absorbs. If you do the math, there, you will realize that warming due to complete saturation of CO2 could only produce a warming effect of about 3.3 degrees (actually a little less.) To see even 1.6 degrees of warming, CO2 levels would have to double from the pre industrial revolution levels to 580PPM. That doesn't seem so far away. but it still implies a delta of 2X what we have seen, thus far.

What we did not discuss is where that CO2 came from. It is kind of implied by the "pre industrial revolution" statement that the CO2 increase came from mankind burning fuels, but that is not actually proven. When talking about green-house effects, scientists talk about drivers and non-drivers. A non-driver is an effect. Basically the premise for non-drivers is that when climate changes, a change results in the quantity of the green house gas and the reverse is not necessarily so. A driver means that climate is affected by increases in the greenhouse gas.

The meaning isn't exactly clear until you are exposed to the idea that the CO2 in the air is not there because we put it there, exactly. The level of CO2 in the atmosphere could be called, in the language of control systems, an error signal. It is a byproduct of the production and consumption rate of many systems on the planet. The two dominant systems are dissolution and biological carbon fixation.

When water is exposed to a soluble gas, such as CO2, it absorbs it. It does so in proportion to it's pressure or, when it is a partial constituent of the gas mixture, what is called it's partial pressure. When the partial pressure goes up, the gas is absorbed into the water to correct the imbalance. If the gas in the water leaves it without re-entering the gaseous solution for whatever reason (it does for several), then the water will take up more, again, to correct the imbalance.

biological carbon fixation might be the only method of removing CO2 from the atmosphere that most of us have knowledge of. This idea is fairly simple, when a plant grows, it consumes CO2, seperates the carbon from the Oxygen, uses the carbon to build itself and releases the oxygen. So we are left with the idea that we need to plant a bunch of trees. What is far less well undertood is that we don't necessarily need to plant these trees. This is another feedback system that will tend to keep atmospheric CO2 in balance. When CO2 levels rise, plants are encouraged to grow and fix more carbon, thereby reducing atmospheric CO2 levels.

There are many systems that contribute to the CO2 balance, but the upshot is that CO2 enters and leaves the atmosphere at roughly equal rates. As the rates rise, the atmospheric concentration will change to reflect the state of the systems. So, this is not like the chlorofluorocarbon debate of years past where we were supposed to be directly injecting the gas into the atmosphere. CO2 enters and leaves the atmosphere at a prodigious rate. It does not stick around long.

This is too short a forum and I do not have the expertise to discuss all of the many systems affecting the CO2 balance. It is the existence of these systems that determine the forcing versus non-forcing categorization. However, which CO2 is, is far from clear. Some of the systems will release CO2 if atmospheric temperatures increase. This suggests that CO2 could be non-forcing. However, many climate scientists think that CO2 is forcing. This is not fact, but is still a matter under investigation.

These systems are somewhat understood, but the extent to which they are understood leave a lot of room for error. When you compound the inaccuracies associated with every model of every system, you wind up with a lot of unpredictability. Too, much, as of this day, to make the sort of predictions about the source and the future of CO2 levels that many are making today.

Why does any of that matter if, after all that, only a 3 degree or so rise in temperature could result in the worst case. It potentially matters because of other systems in play that regulate climate. Unfortunately the nature of these systems are far less related to very hard science like physics and therefore much more poorly understood and modelable. These systems predict a change in temperature based on changing temperature. The problem is that, in most cases, we can not know whether the change is positive and reinforcing or negative diminishing. To give a couple of examples we need to briefly visit where the radiation that causes the CO2 warming comes from.

Oh, you thought it was the sun? Well, that is sort of correct. Almost all energy at the earth's surface has the sun as it's original source, but usually we describe it as where it comes from directly. For instance, oil is storing energy originally provided by the sun but we do not typically call it solar energy, no? Right. Well, the source of the warming for CO2 is the surface of the earth, itself. Solar radiation rains down on the earth's surface, predominantly in the higher wavelengths of visible and ultra-violet light. This light, in turn, heats the earth's surface. The earth, thus heated, re-radiates that energy at lower, infra-red wavelengths which is then absorbed by the CO2. In other words, precious little of the energy stored in atmospheric CO2 is received from the sun.

"albedo" is a word that roughly means how reflective an object is. A high albedo means an object reflects a lot of incoming radiation. A low albedo means the object absorbs a lot of incoming radiation. Dark things have low albedos and light things have high albedos. When something with a low albedo is illuminated it tends to get warm. When something with a high albedo is illuminated it tends to reflect the illumination.

Remember that system from above where higher CO2 might stimulate plant growth which would thereby reduce CO2? Plants tend to have a lower albedo than their surroundings and therefore radiate more heat energy, producing an increase in atmospheric temperature. This is more true if rising temperatures cause plants to grow in previously arctic regions. So which is the greater effect? decreasing surface albedo or the greater carbon fixation capability of more plant life. Don't trust anyone who tells you they know, because this is very new science.

Another system that might contribute more heat to the atmosphere is the changing albedo of the planet due to melting polar ice. As more ground and ocean (with low albedos) spend more time uncovered by ice (with a very high albedo), they absorb more solar radiation and re-radiate it as heat. The question here, is, is the small change directly due to anthropogenic CO2 generation sufficient to cause the polar ice to melt. Again this is very unclear to climate scientists. Some say yes, others disagree.

But all of these systems are trumped by the water cycle. The most prevalent greenhouse gas in the atmosphere is water vapor. It is responsible for something like than 90 percent of atmospheric green-house effect (this is determined by how much heat it stores and it's prevalence in the atmosphere.) The problem is that water vapor also increases planetary albedo (due to cloud formation) and, increasing temperatures tend to produce more water vapor. Scientists presently have a lot of error in their models regarding precisely how water vapor affects atmospheric temperature, but it does seem to have a regulatory effect. This error leaves a lot of room for skepticism regarding conclusions based on climate models.

So the "takeaway" point, here, is that a little very well understood science about CO2 physics drives a lot of less well understood science about CO2 regulatory mechanisms which in turn drive very poorly understood science about biological and geological systems from which we try to infer information about past present and future planetary climate. The large amount of uncertainty in all of this should leave one with a healthy does of skepticism about what we are told today about climate change.