The data from ice cores drilled in Antarctica, Greenland and elsewhere is largely undisputed, subject to some methodological issues around decay over time, etc. There are places on earth, like Antarctica where there is pretty constant cold, so ice builds up and stays in place for a very long time. The ice traps air bubbles, from which you can infer CO2 levels and it has other markers indicating temperature levels. But you won’t hear the climate nuts ever talk about ice cores anymore, they don’t support any of their talking points. Here is a summary post again revealing what these ice cores have to say about the last few hundred thousand years of our existence, basically all of the time that modern humans have been on the earth. (WUWT Post)
As you see in the charts, temperature has been significantly higher in the past than it is today, and has risen extremely rapidly in the past, so there is nothing unprecedented about either temperature levels today or the pace of the increase. In fact, during interglacials there are typically a number of episodes of sharp increases or decreases, which have a variety of causes. You can see that this interglacial period is mostly unusual because it hasn’t hit the high temperatures of other interglacials, but does not yet appear to have begun the sharp descent. Hard to attribute any of this to human activity as the industrial age began a mere 200 years ago at most.
And when you look at the CO2 levels compared to temperature, you see that sharp correlation, but as Willis notes, causation is completely unclear. If CO2 levels were the dominant factor, there never would have been a decrease of temperature and renewed glaciation. It seems very likely to me that warming is caused by factors other than CO2, but a warmer world has more CO2. And it seems just as likely that at some point the increase in CO2 actually leads to a decline in temperatures, through more clouds, more rainfall or other mechanisms, and then a decline in CO2. If humans cause a persistence of CO2 levels, it seems most likely that the impact is to delay cooling, not lead to a further temperature increase. Caution is warranted in attaching instrument-based data to the ice core data, because the instrument data is heavily adjusted and has a level of precision not found in ice core data.
I think we are close to the onset of the next glacial age, which will be an utter disaster for humans. We will want to pump all the CO2 we can into the air, if that is in fact the dominant factor in temperature. Unfortunately, I don’t think it is.