In the discussion of Earth's annual energy balance, we saw that Earth absorbed approximately 51 units of solar energy but emitted 117 units of infrared energy. What prevents Earth from getting colder and colder?
What will be an ideal response?
ANSWER: Although Earth receives solar radiation only during the day, it constantly emits infrared energy both during the day and at night. The atmosphere above only allows a small fraction of this energy (6 units) to pass through into space. The majority (111 units) is absorbed mainly by the greenhouse gases water vapor and CO2, and by clouds. Much of this energy (96 units) is radiated back to Earth, producing the atmospheric greenhouse effect. Hence, Earth's surface receives nearly twice as much longwave infrared energy from its atmosphere as it does shortwave radiation from the sun. In all of these exchanges, the energy lost at Earth's surface (147 units) is exactly balanced by the energy gained (147 units), which is why the average temperature on Earth stays constant.
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