The last IPCC report referred constantly to global temperatures going up by three degrees C due to CO2 doubling. Deep in the report is the fact that pre-industrial normals are 280 ppm (parts per million) and the current is only 388, a rise of about 38%. In the same time period, global temperatures have gone up about 0.5 degrees, half of what one would expect from IPCC estimates for doubling. By the magic of the internet and some clever cutting and pasting, we can not only look at their data, but compare it ourselves.
Let’s start with the CO2 which has been measured, according to the IPCC, at Mauna Lao Observatory in Hawaii since 1958:
We can also look at the IPCC global temperature graph from their report:
The quick conclusion is that CO2 has been rising and so have global temperatures, so the two must be related. The IPCC graph is a bit misleading because the blue line is a 5 year running average that conveniently ends just before a cooling trend started. I have very unscientifically extended their graph for this next picture. In brief, if we cut out the portion of the IPCC temperature graph from 1960 on, scale it so that a 0.5 degree rise is about 1/2 the height of the CO2 rise that was supposed to generate a one degree increase and superimposing them, we get this:
What jumps out of this comparison right away is that while global temperatures are in fact increasing slightly, the acceleration we would expect from CO2 isn’t evident. In fact, it seems more like temperatures are leveling off and perhaps decreasing, despite CO2 being at the highest levels ever measured. If we back up and look at the even bigger picture however, we can extrapolate the CO2 graph down to 280 ppm and steady before 1920, and then compare to the entire IPCC temperature graph since 1860:
Now more information jumps of the graph. First of all global temperatures seem to have fluctuated by about 1/2 of one degree before CO2 levels started to rise significantly. Second, there has been a general warming trend for the last 150 years which seems to be decelerating right when CO2 levels are thought to be at their maximum in terms of accelerating it. Does that mean that CO2 and rising temperatures are not connected?
NO IT DOES NOT! What it means is that there are multiple processes occurring on the planet, some of which contribute to warming and some to cooling. Sorting out which processes are responsible for what is hard to untangle, but a 3 degree rise in temperature for doubling of CO2 seems to be an over estimate.