AGW deniers ask "Hey, we breathe CO2; does that mean we're contributing?" supposing that this makes AGW absurd. The question it begs, though, is more one of, how does CO2 emission via respiration compare to that of power generation, and what are the differential concerns?
World population: ~ 7 billion humans
Avearge human CO2 production: 2T / year
CO2 produced by human respiration: 14GT / year
Coal burned: 7.075 billion short tons
Carbon intensity: 2.93 kg of carbon released per kg of coal burned
CO2 produced by coal combustion: 18.80 GT / year
Oil burned: 87 million barrels / day
Oil density: 0.703 g/cc to 0.832 g/cc (using 0.7675)
Carbon intensity: (using 3.1015)
CO2 produced by oil combustion: 8.876 GT / year
Natural gas burned: 3015443467227 m³
Natural gas density: 0.0008 g/cc
Carbon intensity: 2.79
CO2 produced by natural gas combustion: 6.73 GT / year
Looks like a denier's dream, right? So what's up here? Why do climate scientists think that the combustion of fossil fuels is contributing to AGW, while breathing is not?
It all comes down to life cycle. We know where the carbon goes, but where does it come from?
For fossil fuels, it comes from in the ground; it's been dormant there for untold aeons, completely separated from the carbon cycle.
For humans, it's quite different. When you eat, you're eating vegetation that got its carbon largely from the air, or you're eating animal flesh that got its carbon from vegetation that got its carbon largely from the air. Either way, every carbon atom in every molecule of CO2 you exhale was fairly recently in a molecule of CO2 anyway. Human respiration is, to this effect, carbon-neutral.
While total CO2 synthesized by human respiration is comparable to total CO2 from fossil fuel combustion, respiration has little effect on the carbon dioxide concentration of the atmosphere, since it's just giving back what it took. So if you think that you have a strong point when you say something like "We all exhale CO2!", and that you'll have convinced anyone, do us all a favor: don't hold your breath.