The Earth already had greenhouse gases in its atmosphere before humans ever came along, as part of the natural carbon cycle.
But since the Industrial Revolution, humans have been adding even more carbon dioxide, methane, and other greenhouse gases into the atmosphere. That’s through activities such as digging up and burning fossil fuels or clearing forests. Carbon dioxide is the most important of these gases, and typically persists in the atmosphere for centuries.
This chart from Ecofys breaks down the major sources of man-made greenhouse gas emissions in 2010:
Fossil fuels: When fossil fuels are burned for energy, it produces carbon dioxide and other greenhouse gases. Roughly 25 percent of man-made greenhouse gases came from burning coal, 19 percent from the use of natural gas, and 21 percent from oil.
Land-use change: 15 percent of man-made emissions came from land-use changes. Forests, for instance, absorb carbon dioxide — so if they’re cut down and burned for farmland, it increases emissions in the atmosphere.
Agriculture: Another 7 percent of man-made emissions come from agricultural sources — methane emissions from livestock, say, or changes in the amount of carbon stored in soil.
The world’s top five greenhouse gas emitters in 2011 were China, the United States, the European Union, India, and Russia. There are many more charts — breaking things down by per capita emissions and historical responsibility — here.