What Is Global Warming?

March 26, 2006 — -- Of all the different gases that make up Earth's atmosphere, greenhouse gases are an especially important group.

Greenhouse gases have one thing in common: They let visible light through but not heat. Greenhouse gases even let strong light that can heat you up, like sunlight, pass through.

To be precise, they let in energy in the form of visible light but don't let in energy in the form of invisible heat waves, or infrared light. They absorb those types of energy.

So when light from the sun -- visible light -- hits Earth's atmosphere, the greenhouse gases let it through on its way to the ground.

When that sunlight hits something reflective on the ground, such as white snow or a shiny metal roof, it bounces back as visible light -- right back though the atmosphere and back into outer space.

But when the sunlight hits something dark and nonreflective, its heat -- its energy -- gets absorbed (that's why people often wear reflective white when playing tennis on a hot day instead of heat-absorbent black). The sunlight's energy, which arrives as visible light, gets converted to heat in the ground.

This energy then radiates back toward space in the form of what was first called dark energy when scientists discovered it in France two centuries ago. We now call it infrared, or invisible heat waves. (Many night-vision scopes "see" by picking up these heat waves or infrared waves from living bodies.)

This is the kind of energy that the greenhouse gases don't let through but absorb, as would a black cotton shirt.

So this heat, which originally came from the sun to Earth in the form of visible light waves, is now trapped in Earth's atmosphere. Unable to get back to outer space, it warms the air further, which over time helps warm the ground and oceans as well.