How does carbon dioxide cause global warming?

When fossil fuel is burned, its carbon atoms largely wind up in atmospheric carbon dioxide (CO2) gas. CO2 blocks the ability of the earth to radiate heat in the form of infrared light, which normally penetrates the atmosphere and goes into outer space. Hence the planet retains more heat.

Because the carbon in fossil fuels (coal, oil and natural gas) has been hidden away for millions of years, releasing it now is increasing its levels in the atmosphere. More greenhouse gases means more heat is retained. This is causing global warming.