The man-made (or anthropogenic) component of the greenhouse effect is caused by man’s activities that emit greenhouse gases to the atmosphere.
The most important of these is the burning of fossil fuels. Fossil fuels contain carbon, and when they are burnt this carbon combines with oxygen in the atmosphere to form carbon dioxide.
Changes in land use are also important sources of greenhouse gas emissions. For example deforestation results in the emission of carbon dioxide to the atmosphere that was previously stored on the Earth’s surface in the form of trees and other vegetation, or locked up in soils.
We emit carbon dioxide to the atmosphere in enormous quantities because we burn so much fossil fuel and the changes in land use that we have implement are so widespread.
This is why carbon dioxide is the most important gas in the man-made component of the greenhouse effect.