There are 2 ways that greenhouse gases enters our atmosphere. One of them is through natural processes like animal and plant respiration. The other is through human activities. The main human sources of greenhouse gas emissions are: fossil fuel use, deforestation, intensive livestock farming, use of synthetic fertilizers and industrial processes.

There are four main types of feedback  greenhouse gases: carbon dioxide, methane, nitrous oxide and fluorinated gases. The main feedback greenhouse gas is water vapor. Let’s take a closer look at the sources of each type of greenhouse gas:

Carbon Dioxide

There are both natural and human sources of carbon dioxide (CO2) emissions. Natural sources include decomposition, ocean release, respiration and volcanoes. Human sources come from activities like cement production, deforestation and the burning of fossil fuels.

42.8 percent of all naturally produced CO2 emissions come from ocean-atmosphere exchange. Other important natural CO2 sources include plant and animal respiration (28.56%) as well as soil respiration and decomposition (28.56%).1 2 A minor amount is also created by volcanic eruptions (0.03%).3 4

87 percent of all human CO2 emissions come from the burning of fossil fuels like coal, natural gas and oil. Other sources include deforestation (9%), and industrial processes such as cement manufacturing (4%).5

Human sources of CO2 are much smaller than natural emissions but they upset the balance in the carbon cycle that existed before the Industrial Revolution. The amount of CO2 produced by natural sources is completely offset by natural carbon sinks and has been for thousands of years.

Before the influence of humans, CO2 levels were quite steady because of this natural balance. Since the Industrial Revolution, human sources of CO2 emissions have been growing. Activities such as the burning of fossil fuels as well as deforestation are the primary cause of the increased CO2 concentrations in the atmosphere.

—from: http://whatsyourimpact.org/greenhouse-gas-sources