system
Given the following post, respond as if you're commenting on Reddit:
human
Why doesn't the greenhouse effect deflect energy from the sun before it hits the earth?

I have been recently looking into the greenhouse effect out of curiosity, and one thing that I have found is that the greenhouse effect warms the earth as when energy from the sun hits the earth, most of it is absorbed, and the rest is reflected back into space. The energy reflected back into space, however, first goes through the atmosphere, and some energy is reflected back into the Earth.

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My question is that should some of the energy from the sun be reflected back into space, and because there is more energy coming from the sun that reflecting off the earth, why doesn't the earth become cooled from this effect?
gpt
The radiation that comes from the Sun is not the same as the radiation that is "reflected" by the Earth. And this causes the difference in transparency as the radiation tries to cross our atmosphere.

Taking a few steps back. All objects radiate electromagnetic radiation. The amount and frequency of the radiation depends on the temperature of the object. Hotter objects will radiate higher frequency (=more energetic) and higher intensities of radiation. This radiation is called thermal or blackbody radiation.

The radiation of the Sun matches the spectrum of a hypothetical black body with the same temperature as the surface of the Sun quite well. The solar radiation has most of its energy in the visible and infrared parts of the electromagnetic spectrum. The Earth is much cooler than the Sun and it's radiation is therefore shifted towards lower frequencies, which means further into the infrared range.

Now, as it turns out, what we call greenhouse gases tend to not absorb radiation in the frequency-range that is typical for the Sun, while it absorbs the radiation that comes from the Earth far more readily. After absorption, the molecule will reemit the absorbed radiation in a random direction. So while the absorbed radiation was traveling away from the surface of the Earth, the random nature of the reemitted radiation means that about a large part of that radiation is transmitted back towards the Earth.

So the reason that solar energy isn't reflected back into space in the same way as radiation from the Earth is reflected back towards the ground is because the Sun and the Earth emit radiation in different frequencies and molecules such as CO2 tend to absorb and reemit radiation from the Earth while letting solar radiation through.