This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
Objection:
The CO2 concentration lags behind temperature by centuries in the glacial-interglacial cycles, so clearly CO2 does not cause temperatures to rise, temperatures cause CO2 to rise.
Answer:
A close examination of the CH4, CO2 and temperature fluctuations recorded in the Antarctic ice core records does in fact reveal that yes, the temperature moved first in what is, when viewed coarsely, a very tight correlation. But what it is not correct, is to say the temperature rose and then hundreds of years later the CO2 rose. These warming periods lasted for 5,000 to 10,000 years (the cooling periods lasted more like 100,000 years!) so for the majority of that time (90% and more) temperature and CO2 rose together. This means that this remarkably detailed archive of climatological evidence clearly allows for CO2 acting as a cause for rising temperatures while also revealing it can be an effect of them.
The current understanding of those cycles is that changes in orbital parameters (Milankovich and other cycles) caused greater amounts of summer sunlight to fall in the northern hemisphere. This is actually a very small forcing, but it caused ice to retreat in the north which changed the albedo. This change, reducing the amount of white, reflective ice surface, led to increasing the warmth more in a feedback effect. Some number of centuries after that process started, CO2 concentrations in the atmosphere began to rise and this also amplified the warming trend even further as an additional feedback mechanism.
You can also go here for a discussion by climate scientists of exactly this question but with greater technical detail and full references to the scientific literature.
So, it is correct that CO2 did not trigger the warmings, but it definitely did contribute to them, and according to climate theory and model experiments, greenhouse gas forcing was the dominant factor in the magnitude of the ultimate change.
One warning that this gives us for the future is that we may well see additional natural CO2 come out of the woodwork as whatever process that took place repeatedly over the last 650K years begins to play out again. The likely candidates are out gassing from warming ocean waters, carbon from warming soils and methane from melting permafrost.
This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
“CO2 Lags Not Leads” was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.