Between farming and deforestation, agriculture is responsible for around 24% of greenhouse gas emissions.
The main offender from this category is cattle, which produce more carbon dioxide emissions than most single countries, save for China and the United States.
But other elements of farming that can be dangerous as well. The use of nitrogen fertilizer, for instance, releases nitrous oxide, a greenhouse gas that's 300 times more potent than carbon dioxide. In October, Gates and a cohort of billionaires invested millions in a genetically modified bacteria that could mitigate this problem.