Hundreds of billions of animals are farmed worldwide, and the percentage of GHG emission is only 18?
The widely cited 2006 FAO report, Livestock’s Long Shadow vastly underestimated the GHG emissions of livestock. Environmental experts of World Bank, Robert Goodland and Jeff Anhang pointed that out in their report.
They published Livestock and Climate Change in October 2009, saying that livestock and their byproducts produce 32.6 billion tons of carbon dioxide per year. The produced GHG emissions account for at least 51% of annual human-induced GHG emissions. And that is a most conservative estimate. The article concluded that the best strategy for reversing climate change is to replace meat with soy products or other alternatives. This would rapidly reduce greenhouse gas emissions and their atmospheric concentration, slowing down the speed of climate warming. This is much faster than replacing fossil fuel with renewable energy.
In this report, the uncounted, overlooked, and misallocated livestock-related GHG emissions are mainly: respiration, land use, and undercounted methane (as the bottom figure of this page). All animals inhale oxygen and exhale carbon dioxide. It is a big omission to neglect carbon emissions from respiration. According to this report, livestock account for 13.7% of carbon dioxide emission. It is higher than 13% from all transportation worldwide (Section 1-3).
The results of different research institutions vary greatly due to different evaluation bases. However, there is no doubt that the main cause of greenhouse gas emissions is livestock farming. Due to human activity of meat eating habit.

Further Reading: 1-3 Greenhouse Effect and Greenhouse Gases | 1-4 How Many Animals Are Slaughtered Worldwide Every Year?
Cited Report: World Watch Magazine
Relative Download: Diet and Environment Relative Download