How can CO2 trap so much heat if it only makes up 0.04% of the atmosphere? ArenвЂ™t the molecules spaced too far apart?Д«efore humans began burning fossil fuels, naturally occurring greenhouse gases helped to make EarthвЂ™s climate habitable. Only after the Earth absorbs sunlight and reemits the energy as infrared waves can the CO2 and other greenhouse gases absorb the energy. In other words, вЂњenergy coming into our planet from the Sun arrives as one currency, and it leaves in another,вЂќ said Smerdon.Д¬O2 molecules donвЂ™t really interact with sunlightвЂ™s wavelengths. Why does carbon dioxide let heat in, but not out?Д®nergy enters our atmosphere as visible light, whereas it tries to leave as infrared energy. YouвЂ™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.

He recommends checking the bottle temperatures with a no-touch infrared thermometer. вЂњIf you expose them both to a heat lamp, the CO2 bottle will warm up much more than the bottle with just ambient air,вЂќ he says. How can I see for myself that CO2 absorbs heat?ДЄs 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. That means they can absorb a wider range of wavelengths - including infrared waves. 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. Smerdon says that the reason why some molecules absorb infrared waves and some donвЂ™t вЂњdepends on their geometry and their composition.вЂќ He explained that oxygen and nitrogen molecules are simple - theyвЂ™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. Those ranges donвЂ™t overlap, so to oxygen and nitrogen, itвЂ™s as if the infrared waves donвЂ™t even exist they let the waves (and heat) pass freely through the atmosphere.Д«y 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. 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. ThatвЂ™s because molecules are picky about the range of wavelengths that they interact with, Smerdon explained. Oxygen and nitrogen donвЂ™t interfere with infrared waves in the atmosphere. (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.

When sunlight reaches Earth, the surface absorbs some of the lightвЂ™s energy and reradiates it as infrared waves, which we feel as heat. Credit: A loose necktie on Wikimedia Commons 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. Simplified diagram showing how Earth transforms sunlight into infrared energy.

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