What is the greenhouse effect?
The greenhouse effect happens when
sunlight warms the Earth, and molecules
in the atmosphere intercept the resulting
heat before it can escape to space.
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 is not exactly the same. The Earth’s transparent atmosphere lets sunlight through to warm the ground and the oceans (more when it is clear, less when it is cloudy). The Earth’s warmed surface releases some of that 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, or CO2, but also methane, nitrous oxide, and water vapor—will not 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 in 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 33ºC (60ºF) lower than it is now, and life as we know it would be impossible. On the other hand, Venus has a much thicker atmosphere than Earth, most of it CO2, and the surface temperature there is above 444ºC (800ºF). Part of that is because Venus is closer to the Sun, but almost 389ºC (700ºF) of it is a result of Venus’ powerful greenhouse effect.
Climate scientists know how the natural greenhouse effect works on Earth and other planets. That is 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.
How do we know?
One important way is from satellite data. Space provides an ideal environment to measure the Earth’s energy budget — the amount of energy coming in versus the amount of energy going out. For the energy coming in, we use sensors such as the Total Solar Irradiance Sensor, which first flew on NASA’s Solar Radiation and Climate Experiment. It will be continued on the National Polar-orbiting Operational Environmental Satellite System, or NPOESS. For the energy going out we use sensors such as CERES, which stands for Clouds and the Earth’s Radiant Energy System, flying on NASA’s Earth Observing System. CERES will also be continued on NPOESS.