The carbon dioxide we exhale does not contribute to global warming for the simple reason that we also take up an equivalent amount of carbon dioxide from the air, albeit indirectly. Everything we eat can be traced back to photosynthesis, the process by which plants take up carbon dioxide from the air and use it to produce the vast array of organic compounds needed for life. Our bodies can be regarded as living engines that require fuel and oxygen to produce the energy needed to sustain life. In that sense we are not all that different from a car. Both for us and for the car the source of oxygen is the air, roughly 20% of which is made up of oxygen. An internal combustion engine burns gasoline and spews out water, carbon dioxide and a few combustion byproducts. We, instead of gasoline, burn the carbohydrates, fats and proteins in food. Like gasoline, these organic compounds are converted to carbon dioxide and water, which we then exhale.
How is it then that we don’t worry about the massive amounts of carbon dioxide that are released with every breath taken by the billions and billions of people and animals that inhabit the world? Because every atom of carbon in the exhaled carbon dioxide comes from food that was recently produced by photosynthesis. Everything we eat, save for a few inorganic components like salt, was in some way produced by photosynthesis. This is obvious when we eat plant products such as grains, fruits and vegetables, but of course it is also the case for meat. The animals that we eat were raised on plant products. Indeed, a growing animal is basically a machine that converts plants into flesh. So, since all the carbon dioxide we exhale originated in carbon dioxide captured by plants during photosynthesis, we are not disturbing the carbon dioxide content of the atmosphere by breathing.
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. We no longer exhale carbon dioxide and it will be a long time before the carbon atoms in our body eventually make it back to the atmosphere as carbon dioxide. Of course, there are always plenty of new babies who start to respire as we expire.