By now, nearly everyone is aware of the ongoing debate regarding whether climate change, which naturally goes through cycles of higher and lower average global temperature, is being accelerated by human activity, referred to as "global warming". What is perhaps less commonly understood is why carbon dioxide is the primary culprit blamed for global warming and why it is a greenhouse gas to begin with.
Greenhouse gases increase the temperature of the Earth by absorbing and storing infrared radiation emitted by the Earth's surface in vibrational motion. As it turns out, not all chemical species are able to absorb infrared light. Whether a molecule can absorb radiation of a certain type is dictated by what are known as selection rules, which for infrared absorption state that the electric dipole moment of the molecule must change during the vibrational motion and the vibrational quantum number can only change by one unit per absorption event. As a consequence, monatomic gases (which cannot vibrate) and homonuclear diatomic gases—gases consisting of two atoms of the same type—are unable to absorb infrared light, and are called infrared inactive.
The Earth's atmosphere consists primarily of around 78% nitrogen, 21% oxygen, and 0.93% argon by volume. The remainder is made up of trace gases, such as 0.04% carbon dioxide, 0.0002% methane, and very small amounts of hydrogen, helium, neon, and others. These numbers are for dry air; in addition to the gases listed above, there is also a variable amount of water vapor contained in air, averaging around 1% of the total composition.
Based on the discussion above, most of the chemical species in the atmosphere are infrared inactive, meaning that the primary greenhouse gases are water vapor, carbon dioxide, and methane. In fact, carbon dioxide has four vibrational modes, or four different ways of vibrating, and only two of these—the C-O asymmetric stretching mode and the O-C-O bending mode—are infrared active. In terms of global warming, the bending mode is the more important of the two because the Earth emits less infrared radiation at the wavelength required to induce asymmetric stretching.
Although on average the atmosphere contains far more water vapor than carbon dioxide, the amount of water in the atmosphere is regulated by temperature itself, meaning the higher the temperature of the Earth, the more water in the atmosphere, and the more warming. In contrast, the amounts of carbon dioxide and methane in the atmosphere are determined by sources and sinks for these gases. More carbon dioxide and methane in the atmosphere increase the temperature of the Earth, which drives more water into the atmosphere, increasing the temperature even more. Finally, methane is roughly 30 times more efficient at absorbing infrared radiation than carbon dioxide, but there is about 100 times less of it in the atmosphere, and methane breaks down in the atmosphere after about 8 years. In contrast, carbon dioxide can remain in the atmosphere for hundreds or even up to thousands of years.
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