OK, let's bring some data into the discussion:
http://www.grida.no/climate/vital/02.htm
(A graph, derived from Vostok ice core samples, of CO2 and temperature
fluctuations over the past 400k years).
> Recent high-resolution studies of historical CO2 concentrations and
> temperatures over hundreds of thousands of years have shown a modest
> correlation between the two. In a number of cases, CO2 level increases
> are not in phase with temperature increases and actually trail the
> increase in temperature by a short time i.e. increases in temperature
> preceded increases in CO2 concentrations. The more studies that are done
> of the geological record, the more it seems that CO2 concentrations are
> correlated with temperature increases, but are not significantly
> causative.
Based on the Vostok data, you are right, there is a very strong correlation
between temperature and CO2 concentrations, but it doesn't always appear to
be causal.
> With respect to absolute CO2 concentrations, it is also important to
> point out that our best data to date suggests that they follow a fairly
> regular cycle with a period of about 100,000 years.
Also correct -- the peak of each cycle is at about 290-300 ppm CO2.
> As it
> happens, current CO2 concentrations are within 10% of other previous
> cyclical concentration peaks for which we have good data.
Not correct. Mauna Loa data <http://www.grida.no/climate/vital/06.htm> and
<http://cdiac.ornl.gov/ftp/ndp001/maunaloa.co2> show that the current CO2
concentrations are at 370ppm, 18% *greater* than the *highest* recorded
value from the past 400k years. Furthermore, CO2 concentrations are growing
at 15ppm every 10 years, much faster than any recorded increase in the
Vostok data (though perhaps the Vostok data isn't capable of such fine
resolution).
> In other words, we may be adding to the CO2 levels,
No, we are *definitely* adding to CO2 levels. Look at the following chart:
http://www.grida.no/climate/vital/07.htm
(Shows CO2 concentrations since 1870, the "historical record").
Not only is the CO2 increase over 130 years unprecedented in the Vostok
record, it is clear that the rate of change is *increasing*, not decreasing.
There is no other compelling explanation for this increase, except for
anthropogenic input. You're really out on the fringe if you're debating
this -- even global warming skeptics generally concede this point.
> but it looks a lot like we
> would be building a molehill on top of a mountain in the historical
> record.  At the very least, there is nothing anomalous about current CO2
> concentrations.
Wrong again. Current CO2 levels are currently unprecedented over the past
400k years, unless there is some mechanism that allows CO2 levels to quickly
spike, and then return back to "normal" background levels (and hence the
spike might not show up in the ice cores).
Still, by around 2075-2100 we will have reached 500 ppm CO2, a level that
even you would have a hard time arguing away.
> Also, CO2 levels interact with the biosphere in a manner that ultimately
> affects temperature.  Again, the interaction is not entirely
> predictable, but this is believed to be one of the regulating negative
> feedback systems mentioned above.
Yes, clouds and oceans are a big unknown. Still, we know ocean water has a
finite capacity to store CO2, and if the world temperature doesn't increase,
but we all have Seattle-like weather all the time, the effects would be
enormous.
> Last, as greenhouse gases go, CO2 isn't particularly potent, although it
> makes up for it in volume in some cases.  Gases such as water and
> methane have a far greater impact as greenhouse gases on a per molecule
> basis.  Water vapor may actually be the key greenhouse gas, something
> that CO2 only indirectly effects through its interaction with the
> biosphere.
Correct.
Data on relative contributions of greenhouse gasses:
http://www.grida.no/climate/vital/05.htm
Note that methane concentrations now are *much* higher than pre-industrial
levels (many cows farting, and rice paddies outgassing), and methane is also
a contributor in the formation of atmospheric water vapor. Another clearly
anthropogenic increase in a greenhouse gas. I'm in favor of reductions in
methane levels as well.
Data on water vapor here:
http://www.agu.org/sci_soc/mockler.html
> CO2 was an easy mark for early environmentalism, but all the recent
> studies and data I've seen gives me the impression that it is largely a
> passenger on the climate ride rather than the driver.
I tend to think that holistic, and techical approaches would work best in
reducing global warming. I favor an energy policy that has a mix of solar,
wind and nuclear, with all carbon-based combustion using renewable sources
of C-H bonds. Aggressive pursuit of carbon sink strategies also makes sense
(burying trees deep underground, for example). Approaches that involve
reductions in lifestyle to a "sustainable" level are unrealistic --
Americans just won't do it (you'd be surprised at the number of climate
change researchers driving SUVs). But, as California showed during last
year's energy crisis, shifts in patterns of consumption are possible, and
improved efficiency is an easy sell.
- Jim