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Carbon dioxide is currently about 0.04% of the earth's atmosphere. As explained in the Dictionary of Environment and Development, carbon dioxide is a generally "colorless, odorless, non-toxic, non-combustible gas." Other scientific sources note that it is also (1) the most significant manmade greenhouse gas and "contributes more" to the greenhouse effect than "any other gas" released by human activity, (2) "required for the photosynthesis of all plants," and (3) "vital to life" because "almost all biochemicals found within living creatures derive directly or indirectly from" it.