While carbon dioxide emissions, especially from transportation and power production, produce most of the greenhouse emissions on Earth, emission of methane from livestock production and land use changes have been targeted by those interested in reduction of greenhouse gas emissions because methane is a much more potent greenhouse gas than CO2.  Livestock produce methane naturally by enteric fermentation, the process cows, sheep, and deer use to digest their food.  Recently, oceanographers discovered large seeps of methane coming from the ocean floor.  These sources of methane are likely to be from methane-producing micro-organisms similar to those that live inside the rumens of livestock, although some may be from natural gas escaping from deposits below the ocean floor.  Scientists believe that the contribution to atmospheric methane from these ocean sources is not nearly as large as that from animal production, but the fact that they were unknown until just recently makes oceanographers wonder what other sources of greenhouse gases are still out there undiscovered.  You can read a story about this new discovery at The Motley Fool.