http://awellfedworld.org/globalwarming/worldwatch

According to the United Nations report Livestock's Long Shadow, the livestock sector generates more greenhouse gases (GHGs) than ALL transportation combined.

While 18% is more than all transportation combined, the the UN report (which was partially funded by the International Livestock Research Institute) is actually a conservative estimate in many respects.

Follow-up research by environmental scientists from the World Bank Group analyzed and recalculated the UN numbers, concluding that animal-sourced food production generates more than 50% of human-caused greenhouse gases. 

In their Livestock and Climate Change article published in World Watch Magazine, they analyzed the UN report, noting in particular what was uncounted, under-counted, and misallocated. Details here.

The new analysis also recalculated the numbers using a 20-year timeframe, instead of 100 years like the UN report. A 20-year timeframe is more realistic given the dire and fast occurring consequences associated with climate disruptions.

Read details about the how the UN report was recalculated.

Read the rest of the report which includes the drawbacks of grass-fed beef.