I will not even try do full justice here to the basic theory of AGW theory. I highly encourage you to check out RealClimate.org. This is probably the premier site of strong AGW believers and I really would hate to see AGW skeptics become like 9/11 conspiracists, spending their time only on like-minded sites in some weird echo chamber.
If you are reading this, you probably know that CO2 is what is called a greenhouse gas. This means that it can temporarily absorb radiation from the Earth, slowing its return to space and thereby heating the troposphere (the lower 10KM of the atmosphere) which in turn can heat up the Earth’s surface. You probably also know that CO2 is not the only greenhouse gas, and that water vapor, for example, is actually a much stronger and more prevalent greenhouse gas.
It is important to understand that the greenhouse gas effect is well-understood in the laboratory. No one really disagrees that, all other effects held constant in a laboratory, CO2 will absorb certain wavelengths of reflected sunlight. What may or may not be well-understood, depending on your point of view, is how this translates to the actual conditions in our chaotic climate. Does this effect dominate all other climate effects, or is it trivial compared to other forces at work? Does this greenhouse effect lead to runaway, accelerating change, or are there opposing forces that tend to bring the climate back in balance? These are hugely complex questions, and scientists are a long way from answering them empirically.
But wait, that can’t be right — scientists seem so sure! Well, some scientists, particularly those close to microphones, seem sure. Their proof usually follows one or both of these paths:
- Some scientists argue that they believe they have accounted for all the potential natural causes, or “forcings,” in the climate that might cause the warming we have observed over the last century, and they believe these natural forcings are not enough to explain recent temperature increases, so therefore the changes must be due to man. This seems logical, until I restate their logic this way: “the warming must be due to man because we can’t think of anything else it could be.”
- Scientists have created complicated models to predict future climate behavior. They argue that their models show man-made CO2 causing most 20th century warming. Again this sounds good, until one understands that when these models were first run, they were terrible at explaining history. Since these first runs, scientists have tweaked the models until they match historical data better. So, in effect, they are saying that manmade CO2 is the cause of historical warming because the models they tweaked to match history… are very good at matching history; and because the models they programmed with CO2 as the major driver of climate show that…CO2 is the major driver of climate. We will see a lot of such circular analysis in later chapters.
The best evidence we could expect to find (lacking a second identical Earth we can use as a control in an experiment) is to find a historic correlation between temperature and CO2 that is stronger than the correlation between temperature and anything else (and of course, even this would not imply causation). There is a lot of argument whether we have that or not, a topic I will cover in the next chapter. Of course, the lack of unequivocal evidence at this point does not make the AGW theory wrong, just still… theoretical.
Before we get to the historical evidence, though, there may be a few other facts about CO2 and warming that you don’t know:
- CO2 is a really, really small part of the atmosphere. Currently CO2 makes up about 0.0378% of the atmosphere, up from an estimated 0.0280% before the industrial revolution. (Just to give an idea of scale, if you were flying from Los Angeles to New York City, traveling 0.0378% of the distance would not even get you off the runway at LAX. AGW advocates are arguing that a CO2 concentration increase of 0.009% has heated the world over a half a degree C.
- The maximum warming should, by greenhouse gas theories, occur in the troposphere (the first 10km or so of atmosphere). Global warming theory strongly predicts that the warming in the troposphere should be higher than warming at the ground. We will see later that the opposite is actually occurring.
- The radiated energy returning to space consists of a wide spectrum of wavelengths. Only a few of these wavelengths are absorbed by CO2. Once these few wavelengths are fully absorbed, additional CO2 in the atmosphere has no effect whatsoever. Also, these absorbed frequencies overlap with the absorption of other gasses, like water, which further lessens the incremental effect of extra CO2.
What does this mean? In effect, the warming effect of CO2 is a diminishing return relationship. The first increase of, say, 100 parts per million (ppm) in the atmosphere has a greater effect than the next 100 ppm, and so on until increased CO2 has essentially no effect at all.
I once bought a house that had fuchsia walls in the kitchen and family room (really). I spent all night painting the rooms with a coat of white paint, and when I was done, I found that some of the fuchsia still showed through the white paint, making it kind of light pink. A second coat of white made the wall nearly perfectly white. The effects of CO2 in the atmosphere are similar, with the first “coat” making for the most warming and later “coats” having much less effect but still adding a bit. At some point, the wall is white and more coats have no effect.
This relationship of CO2 to warming is usually called sensitivity, and is often expressed as the number of degrees of global warming that would result from a doubling in global CO2 concentrations.
There are lots of values floating around out there for sensitivity, but a preponderance (I won’t say consensus) seem to center on an increase of one degree C for a doubling of CO2 levels from the pre-industrial figure of about 280ppm. Note that you will see numbers much higher than this, but these generally include feedback loops, which we will get to later. Without feedbacks, 0.5 to maybe 1.5 degrees seems like a fairly well accepted number for sensitivity, though there are people on both side of this range.
Luboš Motl provides a handy approximation of the diminishing return effect from CO2 concentration on temperature. I have taken his approximation and graphed it below.
This is a very crude approximation, but the shape of the curve is generally correct (if you exclude feedbacks, which we will discuss in MUCH more depth later). Other more sophisticated approximations generally show the initial curve less steep, and the asymptote less pronounced. Never-the-less, it is generally accepted by most all climate scientists that, in the absence of feedbacks, future increases in atmospheric CO2 will have less effect on world temperature than past increases, and that there is a cap (in this chart around 1.5 degrees C) on the total potential warming.
Note that this is much smaller than you will see in print. The key is in “feedbacks” or secondary effects that accelerate or slow warming. We will discuss these in more depth later, but typically AGW supporters believe these will triple the sensitivity numbers, so a non-feedback sensitivity of one degree would be tripled to three degrees. Remember, though, these three points:
· Warming from CO2 is a diminishing return, such that future CO2 increases has less effect than past CO2 increases
· In the absence of feedback, a doubling of CO2 might increase temperatures one degree C
· In the absence of feedback, the total temperature increase from future CO2 increases is capped, maybe as low as 1-1.5 degrees C.