People do release carbon dioxide during respiration and gas exchange. But this does not make a great amount of that plus, the plants requires them for photosynthesis.
However, nowadays people release a large amount of carbon dioxide into the atmosphere due to the operation of factories, and driving cars. The burning of fossil fuels, which generates electricity, is the main reason of the increasing carbon dioxide in atmosphere.
This can lead to greenhouse effect, since carbon dioxide can trap heat inside and still absorb more heat at the same time. And then, global warming occurs and it affects living organisms.
