Some people will say "Watch what happens to your soda as it heats up, the ocean works the same way." as proof of the CO2 isn't changing temperatures.  It is true that colder water absorbs more CO2 but this is a reason NOT to warm up the oceans as warm oceans will act as a positive feedback.  The warmer the ocean gets the more greenhouse gases it will release.  And the cycle repeats itself.  If you are trying to reduce greenhouse gases the last thing you want to do is heat the ocean and cause it to release more greenhouse gases.


            The shifts from the ice age to a warm period took about 5,000 years to complete.  Since the supposed lag is only 800 years long this means that only the first 1/6th (800 out of 5,000) cannot be explained by CO2.  Something other than CO2 started the shift from the ice age to warm period.  We can call it a triggering event much like a trigger would fire Daniel Boone's musket.  A warming earth, just like a bullet, needs much more than a trigger to get moving.  In the musket a relatively weak trigger will either set off a stronger priming charge or spark a piece of flint.  The priming charge or flint will then ignite the gunpowder.  It is the gunpowder that does the bulk of the work.  The same sort of thing happens in nature.  Triggering events that are too weak to warm the planet by themselves but are strong enough to set other mechanisms in motion are the planets orbital (Milankovich) cycles and other cycles.   DO events also play a roll in climate change.  Such triggering events were actually predicted by James Hansen et. al. well before the ice core data even showed a possible lag.  From (Lorius et al., 1990):

"changes in the CO2 and CH4 content have played a significant part in the glacial-interglacial climate changes by amplifying, together with the growth and decay of the Northern Hemisphere ice sheets, the relatively weak orbital forcing"

Here James Hansen and others are predicting that the earths wobble (Milankovich cycle) are the triggering event.  Greenhouse gases and melting ice sheets (which exposes more sun absorbing rock and dirt) were the expected positive feedbacks.
            In any case there is some debate as to whether or not CO2 lags behind the temperature.  The most recent research (Loulergue et al.) suggests that the CO2 in the ice cores may be in unison or even leading the temperature increase.  Even though the paper has withstood several reviews it is too early to make a definitive statement on the timing.  The temperature record is retrieved from the ice which was formed from compacted snow.  The CO2 record is retrieved from the air bubbles trapped inside the snow.  Because snow takes some time to compact into ice the ice will always be slightly older than the airbubbles trapped inside it.  Estimating the exact number of years that the ice is lagging behind the air bubbles is currently under some debate.




        In summary there are three key concept to remember.The first is that the climate is like a Rube Goldberg machine.  One small event can set off a series of chain events.  The second is feedback vs. forcing. If a greenhouse gas lags then the gas *could* be a positive feedback that compounds warming.  Permafrost melting is one such example of this.  Here the trigger (a magically warming earth) unleashes powerful greenhouse gases from the melting permafrost.  These gases will lag behind the temperature increase but can still have a dramatic impact.  An alternative scenario is where the temperature is stable but some force is releasing greenhouse gases.  Fossil Fuel burning is an example of this.  In this case the greenhouse gases are not a positive feedback but instead they are a driving force that is the initial cause of a warming earth.  Here the trigger (fossil fuel burning is one such example) does not impact the climate directly.  The third concept is that if the greenhouse gases are increasing only during the last 90% of the warming period that means only the first 10% of the warming period can be guaranteed to be independent of the greenhouse gases.  The the 90% of the warming that occurred during a period of greenhouse gas increase must be analyzed through calculations and physics to determine how much of the warming is related to the greenhouse gas.


A few more sources: