The basic idea behind global warming is that CO2 is one of the main (if not the main) variables that dictates the temperature of the Earth. This is so prevalent that when a researcher says that the CO2 levels are at the highest level in 15 million years, it makes the news. Somehow there is the perception that a certain CO2 level somehow causes a certain temperature. This is of course ridiculous.
The idea from this is based on the bad science idea that correlation equals causation. With that idea in mind the worst and most misleading chart of all time is the one made famous by Al Gore.
The reason it is so misleading is because it gives the impression of a correlation, but doesn’t actually provide any information. I am going to correct that little problem.
To start off I am going to provide the warmist view that if CO2 levels had never risen above 280 ppm from 150 years ago, the Earth’s temperature would be 0.8 to 1.0 °C cooler than they are now. So the temperature anomaly based on the zero anomaly would be about -0.5 °C. Depending on which temperature set a person uses that might vary a little bit, but hopefully no one will argue that idea too much.
So the CO2 level to temperature correlation is about 280 ppm to -0.5 °C.
With that in mind let’s look back to the Eemian Interglacial that took place from 135,000 to 115,000 years ago. If you look at the chart above that is based on the Vostok ice core data. The Vostok uses hydrogen isotope data (instead of the more common oxygen isotope) as the temperature proxy data and trapped air bubbles in the ice for the CO2 data. If I take a closer look though, the results are rather different from what a person would expect.
According to this, the CO2 level of 280 ppm correlates to 2-3 °C. So according to the very famous chart that shows CO2 determines temperature, the same level that 150 years ago meant the Earth should be -0.5 °C, meant that the Earth should be 3.0 °C warmer. Same chart, same source, two very different results.
If one were to extrapolate the temperature from from the 2.5 °C warmer and apply the IPCC forcing and climate sensitivity values, then the Earth should be almost ready to boil (or ~4 °C for the modern anomaly). It is interesting that the projected IPCC warming matches what the Earth was like when the CO2 level was at the natural 280 ppm.
Of course, the Vostok temperature is not the really good temperature reconstruction from the Eemian. The really good one is the EPICA temperature reconstruction. Unfortunately the CO2 doesn’t cover the entire period, but it does cover about half of it. So I will show both the EPICA and the Vostok CO2 level with the EPICA temperature reconstruction.
The EPICA CO2 level peaks right at 290 ppm, with a temperature anomaly of 4-5 °C. So if CO2 levels to determine temperature, then the Earth is really, really in lots of trouble. Of course, the Earth was hotter than the IPCC projections for doubling CO2 when the CO2 level was a mere 290 ppm.
In fact, if the CO2 forcing and climate sensitivity of 3 °C/doubling is correct, then the Earth should not be 5 °C until the CO2 level is ~900ppm. The problem is that a careful analysis for the fundamental chart of global warming shows that 5 °C happens when the CO2 level is more than 3 times lower than 900ppm.
Once again the theory of global warming runs into another inconvenient fact. The past CO2 levels to not match up with the projections of the IPCC. The Earth was much warmer with lower CO2 levels in just the last interglacial. It isn’t just the last interglacial that doesn’t match up with the theory of global warming. The interglacial from 335,000 years ago shows more of the same.
The closer one looks at the idea, the less the idea the CO2 levels dictate the temperature make sense. The Earth has been warmer with lower CO2 levels in the recent past. It isn’t that one record doesn’t match up, it is that all the records don’t match up.
CO2 at best was a vague, but poor proxy for ocean temperature. The worst thing that CO2 emissions have done is make it a worse proxy than it was before. The paleoclimate data provides no evidence that it is the Earth’s thermostat.