No it wouldn't. When you burn carbon (coal) and produce CO2, you get about 400 kJ for each mole of CO2 produced. When you burn hydrogen, you get about 300 kJ f

No it wouldn't. When you burn carbon (coal) and produce CO2, you get about 400 kJ for each mole of CO2 produced. When you burn hydrogen, you get about 300 kJ for each mole, but one molecule of water H2O contains half the oxygen of one molecule of CO2. So you get 600 kJ for 2 moles, which contain the same amount of oxygen as one mole of CO2. In other words, when you burn hydrogen you produce 50% more energy than when you burn carbon for the same amount of oxygen removed from the atmosphere. Since the start of the industrial revolution we've increased the CO2 in the atmosphere from 280 ppm to 420 ppm, or an increase of 140 ppm (parts per million). Which means we've reduced the oxygen in the atmosphere by 140 ppm (each molecule of CO2 produced removes a molecule of O2). A quick google shows me there's 20.947% of oxygen in the atmosphere. Which means before the industrial revolution we had 20.947% + 0.014% = 20.961% oxygen. If we wave a magic want, and rewrite history so that all coal and fossil fuels were replaced by pure hydrogen, then we would have removed only about two thirds of the oxygen, or about 90 ppm. So we would have had more oxygen, not less. By about 0.005%, or 20.952%. In any case, I don't think any impact on life would be felt whatsoever if the concentration of oxygen is 20.952% rather than 20.947%.