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An ordinary greenhouse keeps plants warm in winter without using a heater. It works because the Sun's rays shine through the glass walls and roof, warming the inside--and that same glass keeps the heat from escaping. The greenhouse effect warms the Earth (and other planets) in a similar way, although it's not exactly the same.
The Earth's transparent atmosphere lets sunlight through to warm the ground and the oceans (more when it's clear, less when it's cloudy). The Earth's warmed surface releases some of the heat in the form of infrared radiation -- a form of light, but invisible to human eyes. This infrared light wants to keep on going, right back out into space, taking the warmth with it. But some gases in the atmosphere -- most importantly carbon dioxide, but also methane, nitrous oxide, and water vapor -- won't let all the infrared radiation pass through. These "greenhouse gases" are very good at absorbing infrared light. They spread the heat back to the land and the oceans.
So greenhouse gases act sort of, though not exactly, like the glass in the greenhouse. They let sunlight through on its way from space, but intercept some of the infrared light on its way back out. As a result, the lower part of the atmosphere stays much warmer than it would otherwise. Greenhouse gases are a natural part of Earth's atmosphere, and it's a good thing they are! Without them, the average surface temperature of the Earth would be about 33 degrees Celcius (60 degrees F) lower than it is now, and life as we know it would be impossible.
Climate scientists know how the natural greenhouse effect works on Earth and other planets. That's why they worry about what is happening as humans change the balance of greenhouse gases, for example by burning fossil fuels that release more CO2 to the air than what is released naturally.
TED | Mar 7
Top climate scientist James Hansen tells the story of his involvement in the science of and debate over global climate change. He outlines the over...