The June 2008 issue of WATTPoultry USA has some statistics on gases emitted by livestock as a percentage of the total amount of human-related greenhouse gases. The big three are:

• Nitrous oxide - 48 percent
• Methane - 48 percent
• Carbon dioxide - 4 percent

This equates to about 18 percent of human-related green house gases comes from livestock production.

Note that nitrous oxide persists in the atmosphere for 120 years or 300 times longer than carbon dioxide. The radiant warming potential of methane and nitrous oxide is also much greater than carbon dioxide.

The article these figures were taken from, "9 Green Technologies for Poultry", notes that while the industry is aware of, and starting to deal with the problem of methane production, the problem of nitrous oxide is less well known:

This is a tremendous problem that we don't really think about, but it is truly an opportunity. Now we have a way to start thinking about changing the systems and our impact on the environment by looking at that particular compound. ~ Dr. Karl Dawson, p. 24.