How are greenhouse gasses emitted?

First you have to specify the gases that you consider "greenhouse". Each gas is emitted in a different way. For instance, carbon dioxide is expelled by animals in their breath, released by lakes, produced by fires, or escaped from the ground. Methane is produced by bacteria as they consume dead plants, and in the guts of cattle, and in rare places, escaped from the ground. www.huffingtonpost.com/2013/11/20/door-of-hell_n_4311694.html