That is precisely the problem that is often found between correlation and causation.
In fact, the temperature proxies seem to indicate frequently the temperatures beginning to rise slightly before the CO2
levels begin to rise. And, in many cases there is a significant delay between the fall of the temperatures, and the later fall of the CO2
The theory is that there is an external event causing a slight perturbation in temperatures. This in turn causes a shift in CO2
levels, which then amplifies the temperature shifts.
One of the leading theories is that slight perturbations in Earth's orbit caused by Milankovitch cycles
causes the temperature shifts which are then amplified by the atmospheric CO2
in a feedback loop. Again, one has problems with correlation vs causation. The average amount of sunlight striking Earth is relatively unaffected by the Milankovitch cycles. What they may change is the distribution of the sunlight to the northern hemisphere or southern hemisphere. The north has more land and the south has more water so it is possible that the slight perturbations by the Milankovitch cycles would in fact cause global changes in heat distribution.
One of the hotly debated subjects is the variable output of our sun. It is known that the sun's output varies on about a decade cycle. It is also believed that there are longer term fluctuations in the solar output. In particular, about 400 years ago the sun went through a low activity period called the Maunder Minimum
which also correlated with a cool period often termed the Little Ice Age
. Many climate theorists believe that the fluctuations in solar output alone were not sufficient to drive the temperature shifts of the past glacial cycles. And, again, other factors such as CO2
are believed to have contributed, or amplified the changes.
Certainly, what few people debate is that the recent atmospheric CO2
increase is caused by humans burning fossil fuels. And, indications seem to indicate a slight increase in global temperatures over the last few decades.
Anyway, much of this is still theory in which we're conducting an experiment with the world as our petri-dish.