Agriculture significantly contributes to the amount greenhouse gases emitted into the atmosphere. This is cause by several aspects of agriculture. For instance, livestock generate more greenhouse gases (methane from the wastes they produce) worldwide than all modes of motor-vehicle transportation combined. There's also the aspect of deforestation. Trees help reduce greenhouse gases in the atmosphere by taking CO2 from the air. Therefore, cutting down trees in order to plant crops/raise livestock is indirectly adding to the amount of greenhouse gases in the atmosphere. Agriculture also requires the uses of engines to harvest and transport crops (ex: tractors plowing the soil or trucks transporting crops to grocery stores), which creates greenhouse gases as well.