"This is a huge change in temperature," says John Eiler, a professor of geochemistry at Caltech and a coauthor of the study. "It shows that carbon dioxide really has been a powerful driver of climate change in Earth's past."

To be more accurate, it shows that CO2 and temperature are linked, something that was already widely known from other things like ice core studies. Unless they were able to tighten the ancient timelines (for either temperature or CO2 levels) down a lot more than they have been up until now, initial cause and effect are still on the table.

I can think of a lot of external forces that would cause atmospheric temperatures to change. I can also think of a lot of ways a rising temperature might cause CO2 to increase. It's harder (but not impossible) to think of anything in ancient times that, given a stable temperature, would suddenly cause CO2 to rise or fall by itself.

There might be a greenhouse feedback once CO2 starts to rise, but that just raises the question: Absent non-CO2 external temperature forcings, what would ever cause CO2 levels to drop again?