Due to IPCC the climate sensitivity of CO₂ very likely is 3.0 ± 1.5 ℃ for each doubling of the concentration of atmospheric CO₂.
Counting on earlier global warming using the Arrhenius formula [ΔT=λ·α·㏑(C/C₀)], global mean vs emissions of atmospheric CO₂, assumed mostly depending on CO₂, gives:
- 1970-2018 0.9 ℃ 325ppm,408ppm: sensitivity 2.74 ℃
- PETM 5 ℃ 700ppm,2000ppm: sensitivity 3.30 ℃
The Paleocene–Eocene Thermal Maximum (PETM) is associated with extreme changes in Earth's carbon cycle due to volcanism about 55 million years ago. The warm period lasted for about 200,000 years. (Wikipedia)
Of course, there is an amount of uncertainty in the PETM figures and Earth wasn't quite the same since it was ice free etc, but these two examples support the information from IPCC with a most likely value of about 3 ℃.
The circumstances, the cause of PETM and the coincidence of the today emissions and observed warming, indicate that calculations like this is relevant.
My question is:
Are there other geological events that can be used to estimate the climate sensitivity?
I did an error, the concentration of atmospheric CO₂ 1970 was 325 ppm.
The formula I use is: Climate Sensitivity = ΔT∙㏑ 2/㏑(C/C₀)].
I think that the Arrhenius formula is valid only for a solid sphere without (especially vertical) currents. Calculating the climate sensitivity for global land
from the range of the Keeling curve gives sensitivity≈3.48 ℃.