What Is Global Warming?

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.

This natural greenhouse effect, discovered two centuries ago, is basically a good thing, as it originally warmed Earth enough to develop and sustain life.

But about 150 years ago, when the industrial age began, all the new engines that drove the new machinery started to burn fossil fuels -- coal, oil and gas -- in enormous quantities that far surpassed the wood fires that civilizations had previously used for warmth.

And all this new burning poured -- and still pours -- greenhouse gases into the atmosphere.

Chief among these is carbon dioxide (CO2), which spews forth from chimneys and exhaust pipes.

This extra injection from industry thickens the blanket of the atmosphere, so to speak.

It means Earth now absorbs -- and holds -- more heat than it radiates back into space.

"The planet is now out of balance with space," as one climate scientist put it.

So the average temperature of Earth's atmosphere has crept up in tandem with the increased concentration of greenhouse gases that people put into the air.

One hundred and fifty years ago, our atmosphere had about 280 parts per million of carbon dioxide; now that figure has reached 381 parts per million, an increase of 35 percent.

When the concentration of greenhouse gases gets to roughly 400 parts per million, say many scientists, we'll probably be close to the threshold beyond which the impacts of climate change become extremely severe.

The threshold cannot be precisely predicted; it is a continuum, and scientists cannot know exactly where such dangerous limits of greenhouse gas concentrations lie, since modern science has never witnessed this process before.

But since climate directly affects agriculture, water supplies and human health, these changes could become highly disruptive to civilization and security.

But scientists work hard to improve their estimates, and to keep up with the many dramatic changes that global warming has already produced in all regions of the planet.