Besides, deforestation also contributes to global warming. Trees use carbon dioxide and give off oxygen in their place, which helps to create the optimal balance of gases in the atmosphere. As more forests are logged for timber or cut down to make way for farming, however, there are fewer trees to perform this critical function. Population growth is another factor in global warming, because as more people use fossil fuels for heat, transportation and manufacturing, the level of greenhouse gases continue to increase. As more farming occurs to feed millions of new people, more greenhouse gases enter the atmosphere. Ultimately, more greenhouse gases mean more infrared radiation trapped and held, which gradually increases the temperature of the Earth's surface and the air in the lower atmosphere.