The Greenhouse Effect

Have you ever climbed into a car on a bright, sunny day with all the windows rolled up? The air outside is just pleasantly warm, but inside the car it's so hot you can hardly breathe.

The car's clear glass lets bright sunlight pour right in. But the seats and floor heat up from that light, and the warmth they give off can't easily escape. The heat gets stuck inside the windows.

Something similar happens in Earth's atmosphere, but the mechanism is a little different. A car heats up mostly because the warmed air can't leave, not because the glass traps heat itself. Earth has no glass wall to block heat. Instead, it keeps its temperature through gases like water vapor and carbon dioxide that catch heat and send it back down.

Sunlight from the sun passes through to the ground just fine. But the heat Earth tries to send back out to space gets caught and returned to the surface. This effect, which wraps the whole planet like a giant greenhouse, is the greenhouse effect.

The real secret behind this effect is the difference in wavelength (the length of a light wave) between sunlight and Earth's heat.

The sun is thousands of degrees hot, so it mostly sends out short-wavelength, high-energy visible light (the light we can see). Greenhouse gases in the air can't absorb this short-wavelength light, so it passes right through. That's why during the day, solar energy reaches the ground safely and warms the surface evenly.

The ground, on the other hand, is far cooler than the sun. So it gives off heat as infrared light, which has lower energy and a longer wavelength.

Greenhouse gas molecules like carbon dioxide and methane soak up this long-wavelength infrared and start vibrating energetically. Then they send the heat back out evenly in every direction, and some of it returns to Earth's surface. The result: heat can't easily escape to space and stays in the atmosphere.

Many people hear "greenhouse effect" and immediately think of global warming or pollution. But the greenhouse effect is a blessing that Earth needs to be a livable planet.

What would happen if the air had no greenhouse gases at all and the effect disappeared? The heat the ground absorbed during the day would drain into space overnight. Earth's average temperature would plunge from today's roughly 15°C (59°F) to about -18°C (0°F), and the whole world would freeze over.

The Moon has no air, so it climbs above 100°C (212°F) in the day and drops to -170°C (-274°F) at night. Earth, by contrast, has thick air and oceans that hold heat, plus the greenhouse effect on top. So the swing between day and night stays mild.

The problem isn't the greenhouse effect itself. It's that the comforter has gotten too thick. As humans burn fossil fuels, greenhouse gases have piled up, trapping even the heat that should be escaping, and the planet keeps getting hotter.