surprised no one has posted the Mauna Kea graph yet showing actual real atmospheric CO2 measurements - the annual variation is caused by differnece between winter and summer co2 emissions by the biosphere in (most plant Biomass is in the northern hemisphere - much less in the southern)
Looking further back over the last 300 years or so we have this as a record of atmospheric CO2 obtained by measuring CO2 concentrations in bubbles frozen in Ice at the polar regions - each year a layer of fresh snow traps air bubbles which acts as a record of the atmospheric CO2 for that year - The mauna kea direct measurements are the thicker black lines at the right hand of the graph.
Looking back over the period of the last few ice ages we can use the Ice core records to show the change in CO2 levels in the atmosphere - again by looking at bubbles trapped in ice. -
generally Ice ages tend to see CO2 levels of 200ppm, and warm bits between ice ages about 300ppm. Effectively the current atmosphere is nearly 400ppm the difference between the normal 300ppm and current 400ppm is due to human fssil fuel burning - although another big slug of CO2 has also found it's way into the oceans as well in recent years - ( a tiny amount compared with the size and chemical composition of the oceans, but enough to shift the alkalinity slightly)
Bottom line is these shifts in CO2 levels in the atmosphere are nowhere near enough to cause toxicity problems to humans, but are enough to cause big changes in the amount of solar radiation the earths atmosphere traps, and hence the earths temperature.