How much carbon dioxide in the atmosphere can be attributed to human respiration?
Well, assuming Wiki is correct, the average person exhales roughly 900 grams of CO2 every day, or around 330kg per year. Multiply this by the roughly 6 billion people on the planet, and you get what amounts to a tenth of a percent (or 2E12 kg C) of all the carbon in the atmosphere (3E15 kg C).
In short, its no big deal.
Even if we were all to die tomorrow, and every bit of carbon stored inside all our organic molecules was turned into CO2, humanity ex mortem would be responsible for releasing approximately 75 gigatonnes of carbon into the atmosphere, or 2.5% of the current atmospheric carbon pool.
In short, its still no big deal.