On the other hand, when we burn fossil fuels such as gasoline, we are releasing carbon dioxide that forms from carbon atoms that had been removed from the atmosphere millions and millions of years ago by photosynthesis and had then been sequestered in the coal, petroleum and natural gas that forms when plants and animals die and decay. By burning these commodities we are increasing the current levels of carbon dioxide. Clearly then, by living and breathing we are not contributing to global warming through the release of carbon dioxide. But can we help reduce global warming by dying? Probably.

Why isn’t the carbon dioxide from breathing a concern for global warming? | Office for Science and Society – McGill University
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