If 1 atm is now 760mm Hg at sea level what was it before we embarked on burning the millenia of stored fossil fuels over the past 100-200 years. If we burn another 2 trillion tons over say the next 20 yrs how would it affect the atmospheric pressure and composition and would the change affect our ability to breath, especially at higher altitudes and would it also be warmer with more air?

Like how much air do we need to breath on a balmy Mt Everest and can haemaglobin changes accomodate these variations in partial pressure if the increase is mainly due to CO2? If the result is a lower haemaglobin level does that mean thinner blood and longer lives?