In class we watched a video demonstrating that carbon dioxide (CO2) absorbs infr

Question

In class we watched a video demonstrating that carbon dioxide (CO2) absorbs infrared radiation. In this video Pieter Tans, a scientist at the National Oceanic and Atmospheric Administration, sits in front of an infrared video camera, and the image captured by this IR video camera is shown as a visible-light image on a computer screen. Carbon dioxide (CO2) is then placed between Pieter Tans’s face and the IR video camera by injecting CO2 into a tube. As the amount of CO2 between the scientist’s face and the IR camera increases, the image of Tans’s face fades in the infrared camera.

For this week’s reflective reading assignment, we would like you to discuss what this video teaches us about the greenhouse effect. You can find a copy of the video posted on iLearn with this assignment. Why does the scientist’s face disappear on the infrared camera when the tube fills up with CO2? Based on this demonstration, how do you expect CO2 to interact with infrared radiation in the Earth’s atmosphere? More broadly, what does this demonstration show us about the greenhouse effect in the Earth’s atmosphere? For this last question, it might help to think about where infrared radiation found in the Earth’s atmosphere and which direction it is, on average, propagating.

Explanation / Answer

The scientist's face disappeared on the infrared camera when the tube filled up with CO2 because of the nature of CO2 molecules that trapped the infrared radiation entering the tube. CO2, being a linear tri-atomic molecule, effectively absorbs infrared radiation as the molecule is able to vibrate in four different ways upon its interaction with the radiation. Based on this demonstration, we expect CO2 to absorb and re-emit the longwave infrared radiation that is emitted by the Earth's surface to the atmosphere. The infrared radiation would be emitted back to the atmosphere as the CO2 molecule would excite or vibrate only in its ground state leading to absorption and re-emission of photons in the infrared radiation. The greenhouse effect in the Earth's atmosphere is primarily due to the interaction of outgoing longwave radiation with the CO2 molecules. As the CO2 concentration increases with an increase in its emissions from the Earth's surface, the atmosphere prevents more infrared radiation from moving out into space and thus causes more radiation and energy to accumulate within the troposphere in the form of heat.

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