I agree with that point. Earth was in a state of CO2 starvation especially during ice ages. Nature responded by evolving certain grasses that can utilize CO2 more efficiently thereby leading to even more starvation (of other plants).
CO2 is heavier than air.
True but they measure CO2 throughout the atmosphere at various altitudes and find general coherence. The measurements are so well repeated it is now old established science that nobody disputes. There are some variations to be sure, but those are mainly local from more plants, fewer plants, ocean vs land, human created CO2 pockets, etc. There is no consistent finding of more CO2 at the surface than aloft.
You should see the places these idiots put sampling tubes for CO2.
Obviously you are referring to Mauna Loa. Here are the daily average CO2 measurements at Mauna Loa for the past year:
There is some effect from the volcano from time to time, but not much, The overwhelming sinusoid pattern comes from the absorption of CO2 by plants in the northern hemisphere from spring through fall and the release of that CO2 from fall through spring. There is also some contribution from the warming global average temperature in northern hemisphere summer releasing CO2 in the ocean surface, the reverse in NH winter. The sinusoid also demonstrates decent mixing of CO2 throughout the atmosphere since there is not much seasonal vegetation on the top of Mauna Loa.
CO2 has a bonafide hockey stick, probably the highest in millions of years although that can't be proven since the ice cores have such poor resolution. But certainly a sharp rise since about 1950 using a combination of high res ice cores from the last 20k years from Greenland and the atmospheric measurements.