Apart from these moral reasons, the current method of raising animals in massive factory farms threatens the environment. Not only do these farms cause exposure to pollution and disease in rural areas, but they also contribute to climate change in several ways. First, cattle themselves are a source of methane, which has much more significant greenhouse warming effect than carbon dioxide (CO2). As they digest food, cattle produce around 18 percent of the methane in the atmosphere caused by human activity. In addition, tropical rain forest is burned and cut down in order to grow crops to feed cattle. This burning adds CO2 to the atmosphere, and the forest can no longer capture CO2 and produce oxygen. Lastly, raising animal protein, with its requirement of oil-based chemicals and fuel, uses more energy in all these ways, our hunger for cheap meat adds to the global warming problem.