co₂, infrared & global warming
All objects glow with infrared light. Warmer objects glow more brightly. They don't choose to (or not to) it's a natural physical phenomenon.
You can't see infrared light but you can feel it. Think of when you held your palms up toward glowing coals in a fireplace.
Visible and infrared light from the sun shine through air, unless something reflects, scatters or absorbs it. When you're at a beach, you can feel it when a cloud passes overhead.
The Sun emits more shortwave infrared than longwave (because it is extremely hot). The Earth emits more longwave infrared than shortwave (because it isn't so hot)
You can play with an online simulator and see how infrared emission changes with temperature.
Earth in infrared
The warmer regions on earth emit more infrared to space than the cooler regions. Infrared emission is the only way Earth releases energy received from the sun. If Earth kept all of the sun's energy, the oceans would boil away and life would be exterminated.
The atmosphere is between Earth's surface and space. The atmosphere doesn't let all longwave infrared pass through. Like a pair of semi-transparent sunglasses, it absorbs a portion of what passes through.
Since the atmosphere absorbs some (but not all) of the infrared passing through, a satellite can monitor how much infrared was absorbed.
The most advanced satellites can detect extremely small infrared changes. There have been six such satellites put into orbit, but only four are currently active. One active satellite, called AQUA, carries the AIRS infrared detector. AIRS has been measuring Earth's infrared emissions since 2002.
The AIRS detector has very high infrared resolution. Think of it like colors in a rainbow, except it's an invisible infrared rainbow. AIRS can tell us exactly how much red, yellow and blue shine through the atmosphere. This allows us to determine which atmospheric gasses are absorbing the infrared, since each absorbs at a different "color."
satellites can see the infrared signature of CO₂
To show how AIRS can identify a specific gas, let's zoom in on the range where where CO₂ absorbs [b]. The satellite records a lot of fine detail.
Separately, in a laboratory, pure CO₂ gas is checked by infrared with a high-resolution spectrophotometer. The resulting pattern [c] has peaks occurring in the same place as where AIRS finds them in the atmosphere. This confirms that AIRS can see the signature of CO₂ in Earth's infrared emission.
Unfortunately, despite all of the money spent on climate research, there have been no long-term satellite infrared studies quantifying effects of CO₂ rising from 372 to 412 ppm.
I am a chemical engineer, not a climate scientist, but I can analyze measurement data. AIRS infrared measurements are accessible online at GES DISC. I went to Freelancer and hired two programmers to write a custom program that downloads and reads AIRS data. 17 years of measurements = 14 terabytes. It takes almost eight days for my computer to read and process that much data. I am writing a manuscript describing how Earth's infrared is changing with rising CO₂. Follow me on Twitter for updates.
Update 10-26-19: Manuscript is finished and submitted to a peer-reviewed journal.
Update 11-25-19: Preprint is available online, not yet peer-reviewed: https://arxiv.org/abs/1911.10605
Update 9-25-20: Revised manuscript submitted to a peer-reviewed journal.