How does carbon dioxide trap heat?
You’ve probably already read that carbon dioxide and other greenhouse gases act like a blanket or a cap, trapping some of the heat that Earth might have otherwise radiated out into space. That’s the simple answer. But how exactly do certain molecules trap heat? The answer there requires diving into physics and chemistry.
Simplified diagram showing how Earth transforms sunlight into infrared energy. Greenhouse gases like carbon dioxide and methane absorb the infrared energy, re-emitting some of it back toward Earth and some of it out into space. Credit: A loose necktie on Wikimedia Commons
When sunlight reaches Earth, the surface absorbs some of the light’s energy and reradiates it as infrared waves, which we feel as heat. (Hold your hand over a dark rock on a warm sunny day and you can feel this phenomenon for yourself.) These infrared waves travel up into the atmosphere and will escape back into space if unimpeded.
Oxygen and nitrogen don’t interfere with infrared waves in the atmosphere. That’s because molecules are picky about the range of wavelengths that they interact with, Smerdon explained. For example, oxygen and nitrogen absorb energy that has tightly packed wavelengths of around 200 nanometers or less, whereas infrared energy travels at wider and lazier wavelengths of 700 to 1,000,000 nanometers. Those ranges don’t overlap, so to oxygen and nitrogen, it’s as if the infrared waves don’t even exist; they let the waves (and heat) pass freely through the atmosphere.
A diagram showing the wavelengths of different types of energy. Energy from the Sun reaches Earth as mostly visible light. Earth reradiates that energy as infrared energy, which has a longer, slower wavelength. Whereas oxygen and nitrogen do not respond to infrared waves, greenhouse gases do. Credit: NASA
With CO2 and other greenhouse gases, it’s different. Carbon dioxide, for example, absorbs energy at a variety of wavelengths between 2,000 and 15,000 nanometers — a range that overlaps with that of infrared energy. As CO2 soaks up this infrared energy, it vibrates and re-emits the infrared energy back in all directions. About half of that energy goes out into space, and about half of it returns to Earth as heat, contributing to the ‘greenhouse effect.’
By measuring the wavelengths of infrared radiation that reaches the surface, scientists know that carbon dioxide, ozone, and methane are significantly contributing to rising global temperatures. Credit: Evans 2006 via Skeptical Science
Smerdon says that the reason why some molecules absorb infrared waves and some don’t “depends on their geometry and their composition.” He explained that oxygen and nitrogen molecules are simple — they’re each made up of only two atoms of the same element — which narrows their movements and the variety of wavelengths they can interact with. But greenhouse gases like CO2 and methane are made up of three or more atoms, which gives them a larger variety of ways to stretch and bend and twist. That means they can absorb a wider range of wavelengths — including infrared waves.
How can I see for myself that CO2 absorbs heat?
As an experiment that can be done in the home or the classroom, Smerdon recommends filling one soda bottle with CO2 (perhaps from a soda machine) and filling a second bottle with ambient air. “If you expose them both to a heat lamp, the CO2 bottle will warm up much more than the bottle with just ambient air,” he says. He recommends checking the bottle temperatures with a no-touch infrared thermometer. You’ll also want to make sure that you use the same style of bottle for each, and that both bottles receive the same amount of light from the lamp. Here’s a video of a similar experiment:
https://news.climate.columbia.edu/2021/02/25/carbon-dioxide-cause-global-warming/
