# How do we apply CO2 sensitivity (3C ± 1.5C for every doubling) to current CO2 levels

https://en.wikipedia.org/wiki/Climate_sensitivity#Estimating_climate_sensitivity

This indicates that current $$\small\mathsf{CO_2}$$ levels have a temperature equilibrium of 2.2 $$\small\mathsf{^oC}$$ disagreeing with statements that we are still below 1.5 $$\small\mathsf{^o}$$C. What did I do wrong?
280 PPM * 2 = 560 ppm
410 PPM / 560 ppm = 73%
73% * 3 $$\small\mathsf{^o}$$C = 2.2 $$\small\mathsf{^o}$$C

• Please always provide significant content of links within your post, since links are object to change, eventually leaving your question a stub. Also, we haven't reached equilibrium yet, have we?
– Erik
Oct 28, 2019 at 14:51
• @Erik The key content is in the question itself. The links merely provide the reference to this content. Yes it would seem that we have not yet reached equilibrium, yet when we apply this answer for the 1.5C claim the 1.5C claim seems to be refuted. It would certainly not be my first guess that my math correctly refutes a consensus of climate scientists. I would instead assume that I made a mistake. Oct 28, 2019 at 15:10

You have done multiple things wrong here. One is that you have used a linear interpolation when you should have used a logarithmic interpolation. But that's a different question, one that you have already asked. I'll address that there. That said, a logarithmic interpolation would indicate that the warming due to a 73% increase would be 2.37 °C rather than 2.2 °C.

But that math is incorrect. We have not seen a 73% increase in $$\text{CO}_2$$ levels over preindustrial times. We have seen a 46.4% increase. The second thing you have done wrong is to incorrectly apply a linear interpolation. The correct value, assuming a linear interpolation is correct, is $$\left(\frac{410-280}{560-280}\right)3^{\circ}$$, or 1.39 °C.

We have seen more than a 1.39 °C increase from preindustrial times due to the 46.4% increase in $$\text{CO}_2$$ levels over preindustrial times. The correct math is a logarithmic interpolation: $$\log_2\left(1+\frac{410-280}{560-280}\right)3^{\circ}$$, or 1.65 °C.

We have not quite seen a 1.65 °C increase. So what's going on? Another thing you have done wrong is to implicitly assume that the current climate represents the steady-state response to the current $$\text{CO}_2$$ level. It does not.

Climate instead changes rather slowly. Suppose we stopped adding ever more $$\text{CO}_2$$ to the atmosphere but instead maintained $$\text{CO}_2$$ at the current level of 410 ppm. Temperatures will continue to rise for quite some time because the climate has not yet reached its steady-state response to the rather sudden rise from 280 ppm $$\text{CO}_2$$ to the current level of 410 ppm.

• @Fred - I've reverted your changes. Just as to-day used to need a hyphen but no longer does, preindustrial no longer needs a hyphen, look it up. The change in how I wrote $\text{CO}_2$ versus how you wrote it is a bit trivial. Jan 6, 2020 at 11:32
• I only included the hyphen to be consistent with the way it was written in the third paragraph.
– Fred
Jan 6, 2020 at 12:06
• @Fred - Thanks. Jan 6, 2020 at 12:14
• I recalculated it for current 420 PPM CO2 and got 1.755 degrees C. This means that we already overshot the 1.5 degrees C goal (Paris accord) right? Sep 12 at 21:52