well, my final...FINAL question is:
are we SURE that the CO2 increases temperature, and not the reverse?
convince me of that, and I am sold.
Both.
We are sure that increased temperatures, by themselves, increase atmospheric CO2 concentrations. CO2 solubility in water is inversely related to temperature. As temperature goes up, the oceans can hold less CO2 in solution and more is released into the atmosphere. As temperatures go down, the oceans can hold more CO2 and CO2 is absorbed from the atmosphere into the oceans. The relationship is not linear, however, and CO2 emission or uptake by the oceans is a slow process.
As far as the ice core data and the causal relationship between CO2 and temperature, the CO2 increases happened
after the temperature increases, by roughly 800 years. That hardly implies causality, at least in that case. It
implies that temperature changes have historically been caused by other factors, and that those temperature changes themselves caused outgassing or absorption of CO2 by the oceans.
We are fairly confident that increased CO2 in the atmosphere will increase temperatures; the real question is
how much. A
doubling of CO2 concentrations, by itself (meaning no other feedbacks in response), will increase temperatures by roughly 1.2 degrees C, assuming no water vapor in the atmosphere. Each further doubling of CO2 will increase temperatures by roughly the same amount. It is essentially a logarithmic function. It requires greater and greater increases in CO2 to produce the same temperature increase. Think of it as a tinted sheet of glass that absorbs 20% of the light that hits it. The first sheet will absorb 20% of the original light. If you put a second sheet behind it, that sheet will absorb 20% of the light that hits it, which is only 80% of the original light (because the first sheet absorbed 20% of it). The two sheets together absorb (1.0 * 0.2) 20% + (0.8 * 0.2) 16% = 36%. Adding a third sheet causes (1.0 * 0.2) 20% + (0.8 * 0.2) 16% + (0.64 * 0.2) 12.8% = 48.8%. Each additional sheet of tinted glass added absorbs less and less of the original incident light. And you can never absorb more than 100% of the original incident light. Here's how it playes out for CO2, with water vapor accounted for:
The real questions are about how water vapor influences the temperature increases caused by CO2 and how it influences the feedbacks. How big are they? Are they positive or negative? Many of the models assume positive feedbacks, and quite large positive feedbacks. Many of the models assume full absorptivity of CO2 in the atmosphere in spite of water vapor absorbing the same wavelengths of IR light that CO2 does:
This means that
some of the ~1.2 degree C temperature increase that would occur due to CO2 increases won't occur because water is already absorbing some of the IR spectrum that would be absorbed by the added CO2.
After figuring out how much warming CO2 increases will cause cirectly, the warming itself changes some of the climate variables. Cloud cover, water vapor, and other things change as a result of the temperature changes caused by CO2. These changes can themselves change the temperature and are referred to as feedbacks. As far as feedbacks go, the big feedbacks are water vapor in the air and changes in cloud cover. This is a very complicated issue, and where most of the disagreement lies. All of the models assume very large, positive feedbacks several times larger than the initial change caused by increased CO2.
It is assumed that increasing temperatures will increase the amount of water vapor in the air, and the water will absorb even more IR light along a significantly broader spectrum than CO2. The evidence either way is pretty sparse at the moment, but we have so far not seen the increases in water vapor that have been modelled, although this could simply be a lag issue.
Decreased low level cloud cover is associated with warmer temperatures. Modellers have interpreted this as meaning that warmer temperatures
cause decreased low level cloud cover, and have added this a further positive feedback in their models. Many people believe the modellers have the causation reversed; that reduced low level cloud cover
causes the increase in temperature, not the other way around.
The real argument isn't over whether increased atmospheric CO2 levels will cause warming, but how much warming is caused by increases in CO2, how much of the warming we have experienced is due to CO2 and how much is due to other natural variations, and more specifically, whether CO2 caused temperature increases will be catastrophic and require a fundamental re-ordering of energy production and usage throughout the world.
