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.
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.