Greenhouse Effect : How do humans contribute to the greenhouse effect?

Some human activities primarily the burning of fossil fuels intensify the warming effect by releasing additional greenhouse gasses into the atmosphere. These gases include carbon dioxide, methane and nitrous oxide. They come from varying sources and have different heat-trapping capacities.

The chart represents the increase in greenhouse effect since mid - 1700s (when human greenhouse gas emissions began to escalate) 

Carbon Dioxide

CO2 accounts for about 380 of every million molecules in the air, and it stays in the atmosphere for at least 100 years. Despite being a relatively week greenhouse gas, so much CO2 has been emitted since pre-industrial times that it has had the largest impact on the enhanced greenhouse effect  CO2 is produced primarily by the burning of fossil fuel in cars, power plants, and industry and emissions continue to decrease each year. Plants and trees absorb CO2 so widespread loss of forests also affects atmosphere CO2 concentration. 

Methane

CH4 makes up less than two of every million molecules in the air and stays in our atmosphere for only about ten years, yet one of these molecules tarps 20 to 25 more energy in that decade than a CO2 molecules in a century methane is emitted when fossil fuels are produced. It also comes from livestock, decomposing waste, and some agriculture. 

Nitrous Oxide 

N2O accounts for only about 300 of every billion molecules in our air, but is about 300 times more powerful (molecule for molecule) than CO2 at enhancing the greenhouse effect. It also last last for about 100 years in the atmosphere the primary source of N2O is crop fertilization and other agricultural activities. It's also emitted when fossil fuels are burned and during sewage treatment. 

Others

Some greenhouse gases such as the chlorofluorocarbons found in coolants or emitted in much smaller amounts but are thousands of times more powerful than CO2. Other sources of these potent gases include perfluorocarbons from aluminum smelting and hydrocholorofluorocarbons from refrigeration and aerosol sprays. 

-national geographic