There were no cars or industries during the Medivial warming period. How does Al Gore's movie address that?
The last chart shows what appears to be a 50,000 (or so) year cycle in CO2 levels. The industrial age starts around the top of the last cycle. How did those lower C02 levels affect the global climate?
I've also read that water vapor is the primary greenhouse gas and it's much more abundant that CO2.
Mr. Gore addresses this by discussing the differences between "natural trends" and the huge, sudden change we have seen (in geologic time). There is no "trend"....the line showing levels of CO2 is almost vertical when you compare the graph with the 450,000-600,000 years of data we have. The scary part is not so much where the temperature is currently, but rather how quickly the CO2 levels are changing when compared to the "trends" we have historically seen.
Another interesting fact, is that we are right in line with what scientists in the 1960's said would happen if CO2 levels continued to rise. Their predictions thus far have held.
Yes, water vapor is most of our greenhouse gases. However, that statement alone can be very misleading. In addition to the % of a gas trapped in our atmosphere, one must look at the atmospheric lifetime, and the GWP of each of the greenhouse gases. Water vapor may be prevalent, however it evaporates/disseminates (depending upon its location in the atmosphere) sometimes in as little as a few days. Although the GWP of water vapor is still unknown, science's "best guess" puts the GWP somewhere between 0.5-3.0. However, it is my understanding that level is per molecule, per year (and many water vapor molecules do not last that long...although, admittedly the number of molecules stays about the same).
When you compare those rates to the other greenhouse gases, you can see that other factors need to be considered. Here's an excerpt from Wikipedia:
"Two scales can be used to describe the effect of different gases in the atmosphere. The first, the atmospheric lifetime, describes how long it takes to restore the system to equilibrium following a small increase in the concentration of the gas in the atmosphere. Individual molecules may interchange with other reservoirs such as soil, the oceans, and biological systems, but the mean lifetime refers to the decaying away of the excess. It is sometimes erroneously claimed that the atmospheric lifetime of CO2 is only a few years because that is the average time for any CO2 molecule to stay in the atmosphere before being removed by mixing into the ocean, uptake by photosynthesis, or other processes. This ignores the balancing fluxes of CO2 into the atmosphere from the other reservoirs. It is the net concentration changes of the various greenhouse gases by all sources and sinks that determines atmospheric lifetime, not just the removal processes.
The second scale is global warming potential (GWP). The GWP depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO2 and evaluated for a specific timescale. Thus, if a molecule has a high GWP on a short time scale (say 20 years) but has only a short lifetime, it will have a large GWP on a 20 year scale but a small one on a 100 year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO2 its GWP will increase with time.
Examples of the atmospheric lifetime and GWP for several greenhouse gases include:
CO2 has a variable atmospheric lifetime (approximately 200-450 years for small perturbations). Recent work indicates that recovery from a large input of atmospheric CO2 from burning fossil fuels will result in an effective lifetime of tens of thousands of years.[10][11] Carbon dioxide is defined to have a GWP of 1 over all time periods.
Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 62 over 20 years, 23 over 100 years and 7 over 500 years. The decrease in GWP associated with longer times is associated with the fact that the methane is degraded to water and CO2 by chemical reactions in the atmosphere.
Nitrous oxide has an atmospheric lifetime of 120 years and a GWP of 296 over 100 years.
CFC-12 has an atmospheric lifetime of 100 years and a GWP(100) of 10600.
HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP(100) of 1700.
Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP(100) of 5700.
Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP(100) of 22000."
By looking at the above data, one can easily see that CO2 is NOT the "worst offender" of the bunch. However, when you consider the quantity of CO2 in the atmosphere, and the direct correlation between CO2 levels and temperature, it is easy to see why CO2 is an issue that needs to be addressed....it is an offender in number, potency, and lifespan.
The other reason for NOT "going after" water vapor is that it is an entirely natural occurrence....we are having no effect on it.
I read something that pretty much hit home with me the other day. It was a quote that said, "Considering the consequences of no action, and the role we are most likely playing in such consequences, the burden of proof should be placed on the nay-sayers rather than the other way around." I couldn't agree more.