Global worming is caused by the retention of the energy of the Sun in the atmosphere. The Earth is warmed by light from the Sun. This light passes through the atmosphere and strikes the surface. Some of the energy is reflected back into space, while some is absorbed by the ground, water and air. The energy that isn't radiated back into space warms the Earth.
Modern climate change is driven by an increase in this warming. Certain molecules in the atmosphere, called greenhouse gasses, preferentially absorb the light as it's reflected from the surface. This process traps energy in the air-water system and drives average global temperatures ever higher. According to NASA, the primary greenhouse gases driving the process are water vapor, carbon dioxide and methane.
These gases are produced naturally by the biology of the Earth, and they are generally held in equilibrium throughout the biosphere. Since the beginning of the Industrial Age, however, humans have produced dramatic increases in the concentration of these gases. As industry and agriculture release these greenhouse agents into the atmosphere, the air retains more of the energy Earth receives from the Sun. This theoretical model for global warming has been confirmed by observations of ice cores and tree rings, which establish a direct relationship between the concentration of greenhouse gases and mean global temperatures.