Now you are changing the timeline. Cambrian's temperatures of 7.5C are 540 million years ago when CO2 was in the 7,000 ppm range (which is no longer in the 4 doublings range but in the 5 doublings range).
The paper was about the last 420 million years not 540 million years
And if you want to go back a little farther to, let's say, 600 million years ago, pre-Cambrian, this is the time of the great SnowBall Earth period when it is proposed that the entire Earth froze over, even the oceans at the equator.
Why would the entire Earth freeze over. Simple, all of the continents were locked together at the time, over the south pole. Again, other things, other than CO2 are the main driver of climate.
Your Cambrian example of 7,000 ppm is actually one of the actual dangers of too much CO2. At 7,000 ppm, humans would find it very difficult to breath. CO2 asphyxiation would become a significant problem for most mammals. Of course, we can never get to those levels since we would have run out of fossil fuels about 1,000 years short of ever reaching those numbers. In addition, O2 levels were higher back then which would have provided some compensation.
Maybe. The peak is about 7,000, and there's got to be some pretty significant error bars on that. I'd have to read the actual paper (as suggested above; haven't had a chance yet) to get a better feeling for how they arrived at the average climate sensitivity value.
The paper was about the last 420 million years not 540 million years.
Excellent point, and there are actual measurements (Royer) back that far, according to the figure.
And if you want to go back a little farther to, let's say, 600 million years ago, pre-Cambrian, this is the time of the great SnowBall Earth period when it is proposed that the entire Earth froze over, even the oceans at the equator. ... Why would the entire Earth freeze over. Simple, all of the continents were locked together at the time, over the south pole. Again, other things, other than CO2 are the main driver of climate.
Also (I believe), significantly (30%?? could be way off, working from memory) lower solar output. It also seems to me that I saw something recently questioning the "depth" of the Snowball Earth cold. Don't quote me.
CO2 asphyxiation would become a significant problem for most mammals. Of course, we can never get to those levels since we would have run out of fossil fuels about 1,000 years short of ever reaching those numbers. In addition, O2 levels were higher back then which would have provided some compensation.
There weren't even any land animals in the Cambrian, were there?
Good thought-provoking questions here. Again, thanks for finding that plot.