HERE'S A REAL LIFE EXPERIMENT WITH 750000 PPM CO2

 It is interesting to find that there are some areas of the world where CO2 bubbles up into the atmosphere naturally creating a local atmosphere which has an extremely high level of CO2 of up to 75%. At this level it would be impossible to breathe, so I expect there are warnings to keep people out. 

Here is an article to explain the effect of such conditions on the local temperature. Surprisingly the effect is zero - no difference from a nearby area with levels which are at the normal value of around 400 ppm.