The gas responsible for the most warming is carbon dioxide. Humans emit carbon dioxide when the burn fossil fuels, such as the gas and oil we use to run our cars, heating units, and other household appliances that we use every day. Deforestation also increases carbon dioxide, as it eliminates some of the trees and plants that can convert the carbon dioxide into oxygen.
Methane is 20 times more detrimental to the environment than carbon dioxide, when its impact is measured over a century. Methane comes from bacteria breaking down organic matter that lacks oxygen. This happens more frequently underwater, when the organic matter is trapped and in the intestines of herbivorous animals, such as cows and sheep. Because we are so dependent on these animals for our food, they are responsible for a large amount of our methane emissions.
Nitrogen oxides are even more potent than methane, having 300 times the capacity to trap heat per unit of volume. Nitrogen oxides come from fertilizer, which is used frequently in farming and gardening. Chlorofluorocarbons and Hydrochlorofluorocarbons, or CFCs and HCFCs, are gases that occur in small concentrations in the atmosphere, but are so powerful that even in small amounts can do great damage to the atmosphere. They are found in some older refrigeration units.
Although the problematic global warming is mainly caused by humans, there are also some natural causes. Volcanic eruptions and sunspots are two natural occurrences that add to the global warming problem. Earth's imperfect rotation is another natural cause. Since the Earth actually wobbles slightly as it spins, it exposes the northern and southern latitudes to different levels of solar radiation at different time.