First, it's important to understand that the atmosphere is extremely thin. "The late Carl Sagan used to say if you had a globe with a coat of varnish, the thinness of that varnish relative to the globe is similar to the thinness of the atmosphere to the planet," Gore says.
That atmosphere, which is just a few miles above the Earth's surface, traps some amount of the sun's energy, or light radiation, as it hits the Earth. "That's a good thing, the natural trapping of [the sun's radiation], because it keeps the temperatures within comfortable boundaries," Gore says. While other planets are either much too hot or much too cold, Earth is just like Goldilocks—just right for life.
As humans add pollution like carbon dioxide (also known as CO2) into the air, the Earth's atmosphere becomes thicker. The thicker atmosphere traps more of the reflected radiation, raising overall temperature. This process is what we call "global warming." Other major factors adding to global warming, Gore says, include the burning of coal, oil, gasoline and forests.