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Why do water vapor and carbon dioxide affect infrared radiation in Earth’s atmosphere?

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Water vapor and carbon dioxide affect infrared radiation in Earth’s atmosphere because they are capable of absorbing and emitting energy through the vibrational and rotational motions of their molecules.12

In the context of infrared spectroscopy, molecules possess quantized internal energy states associated with these motions. When infrared radiation—which corresponds to the energy differences between these states—passes through the atmosphere, these molecules absorb the radiation, effectively capturing heat that would otherwise escape into space.1342

Because these gases re-emit the absorbed radiation in all directions, including back toward the surface, they play a critical role in the greenhouse effect. This process is essential for maintaining Earth's climate; without the absorption of infrared radiation by water vapor and carbon dioxide, the planet would experience extreme temperature fluctuations and remain too cold to support life as we know it.452