What is the Greenhouse effect caused by?

Why have methane-producing animals not caused a run away greenhouse effect?

• U.S. cattle production is currently considered a major source of greenhouse gases (GHG).  I doubt the appropriateness of this belief  because bovines like cattle and other large animals have been producing methane for millions of years before the industrial revolution and these animals have yet to cause a run away greenhouse effect, like we see on Venus.  We'd be in a better position to prevent global warming if we understood the processes that have prevented Earth from suffering a flatulence induced greenhouse effect.

• Answer:

  This is a very good question, and I think it can be addressed as follows 1) A run-away greenhouse effect would suggest a feedback - for example, if a warming climate led to more people rearing livestock, and the extra animals emitted even MORE methane, and the climate got EVEN warmer, etc. etc. you would get a positive climatic feedback, or run-away greenhouse effect. 2) This run-away greenhouse effect dominated the Earth's climate for the first half of the earth's history - for the first 2 billion years, the Earth had no oxygen, and methanogens (tiny microorganisms that produce methane) dominated the Earth, and the atmosphere had thousands of times more CH4 in the atmosphere. These methanogens took a back seat when cyanobacteria began to release oxygen in the Earth's atmosphere - the oxygen consumed the atmospheric methane, and the climate dramatically cooled! 3) Wild animals emit approximately 15 Megatons of methane a year (15 Mt/y). In comparison, human reared livestock emits up to 100 Mt/y. Other human sources include rice (30-60 Mt/y) and fossil fuels (100 Mt/y), and biomass buring is approx. 50 Mt/y. So in the grand scheme of things, wild animals, although they do emit methane (from flatulence, lol!), their contribution is likely to have been negligible in the past.  4) You are quite right in thinking of animals as contributors to a run away greenhouse effect - if you count methanogens! Methanogens in wetlands contribute ~170 Mt/y. This is the single largest source of methane. If the climate warms, methanogens emit methane faster! In turn, if they emit methane faster, there will be more methane in the atmosphere and the climate will warm even more!! The methane emissions from wetlands is suspected to be a very important contributor to climatic change events in the past.