Like HowStuffWorks on Facebook!

What is the greenhouse effect?

While the greenhouse effect keeps our planet warmer than surrounding space, it also throws the temperature balance out of whack.
Photo courtesy of U.S. DOE

We often hear about the "greenhouse effect" in the negative context of global warming -- it's why the glaciers are melting and climate patterns are undergoing dangerous shifts. But the greenhouse effect is not actually a bad thing in itself -- it's a crucial and positive part of Earth's energy balance. It's what allows Earth to stay warm enough for life to survive. Without it, Earth would feel something like Mars.

The process gets its name from the greenhouses that stay nice and warm for growing plants. This is basically the same process that keeps Earth nice and warm for sustaining life. One example of the greenhouse effect that most of us experience in everyday life is the warming of a car's interior when the vehicle is left out in the sun.

You've probably noticed that your car is always much hotter inside than the outside temperature if it's been sitting there for a while. The sun's rays enter through your car's windows. Some of the heat from the sun is absorbed by the seats, the dashboard and the carpeting and floor mats. When those objects release this heat, it doesn't all get out through the windows. Some is reflected back in -- the heat radiated by the seats is a different wavelength than the light of the sun that made it through the windows in the first place, and the window glass won't let as much of that wavelength through. So a certain amount of energy is going in, and less energy is going out. The result is a gradual increase in the temperature inside your car.

Earth's greenhouse effect keeps the planet much warmer than surrounding space. In this article, we'll find out how that happens and see what's involved in maintaining Earth's energy balance. We'll also see what has happened to throw that balance out of whack.

Using the car-in-the-sun example, we can look at Earth's atmosphere as a giant, spherical car window.

Trapping Heat

Climate is the average weather conditions for an area over a long period of time.

When the sun's rays hit the Earth's atmosphere and the surface of the Earth, approximately 70 percent of the energy stays on the planet, absorbed by land, oceans, plants and other things. The other 30 percent is reflected into space by clouds, snow fields and other reflective surfaces [source: NASA]. But even the 70 percent that gets through doesn't stay on Earth forever (otherwise the Earth would become a blazing fireball). The things around the planet that absorb the sun's heat eventually radiate a portion of that heat (radiation) back out at a different wavelength, like your car seats and dashboard do.

This absorption-radiation process keeps the Earth in radiative equilibrium: The sun's radiation continually strikes the Earth, warming it; the warm Earth emits some of that radiation back into space, cooling itself. The more solar radiation the Earth absorbs, the more radiation it releases.

Some of that released radiation makes it into space, and the rest of it ends up getting reflected back down to Earth when it hits certain things in the atmosphere, such as carbon dioxide, methane gas and water vapor -- the car windows. The heat that doesn't make it out through Earth's atmosphere keeps the planet warmer than it is in outer space, because more energy is coming in through the atmosphere than is going out. This is the greenhouse effect that keeps the Earth warm.

If there were no greenhouse effect on Earth, it would probably look a lot like Mars. Mars doesn't have a thick enough atmosphere to reflect much heat back to the planet, so it gets very cold there.

So the greenhouse effect is actually a good thing. And as with most good things, moderation is key. The reason we often associate the greenhouse effect with environmental damage is that since the Industrial Revolution, the greenhouse effect has fallen out of balance: Earth's atmosphere is trapping too much heat.

Trapping Too Much Heat










Related HowStuffWorks Articles

More Great Links


  • Global Warming. NASA Earth Observatory.
  • Greenhouse Effect. HyperPhysics.
  • Pearce, Fred. "Instant Expert: Climate Change." New Scientist. Sept. 1, 2006.