You Asked: How Does Carbon Dioxide Get So High Up Into the Atmosphere?
September 21-27 is Climate Week in New York City. Join us for a series of online events and blog posts covering the climate crisis and pointing us toward action. This week we are dedicating our You Asked series to tackling reader questions related to climate change.
The following question was submitted by a reader, and the response comes from climate scientist Ángel Muñoz.
How does CO2 get high up into the atmosphere? With a specific gravity of about 1.5, it should fall to the earth as it cools when rising. The airlines tell us that it is -40 degrees at 30,000 ft. So why or how is it that some CO2 works its way up into the upper atmosphere?
Carbon dioxide is a gas. The density of a gas increases as temperatures get colder. So, because temperatures decrease as we reach higher altitudes, gases become denser at higher altitudes. Denser objects tend to sink, pulled down by gravity. (In fact, the force of gravity pulling gas molecules towards the Earth’s surface is what maintains our atmosphere.) Different gases also have different molecular weights. CO2 is heavier than oxygen, so we might expect every CO2 molecule to sink below a layer of oxygen molecules. Generalizing this idea to the other gases in the air, we might deduce that this would result in a perfectly stratified atmosphere with separate layers of each type of gas.
We can see an example of a stratified atmosphere inside a bottle of wine. When the bottle is sealed, air between the surface of the wine and bottom of the cork includes both oxygen and CO2. Because CO2 is heavier than oxygen, gravity forces CO2 molecules to form a “layer” beneath the oxygen molecules, helping to separate the wine from the oxygen. Desirable properties of wine, such taste and smell, begin to change once the liquid is fully exposed to oxygen. Without the stratification inside the sealed bottle, we wouldn’t have that cushion of CO2 to protect the wine from oxygen, giving unopened wine a much shorter shelf life or even turning it into vinegar over time.
Earth’s atmosphere is not like the air inside a sealed wine bottle. Atmospheric gases are well-mixed, not stratified. This due to the force of diffusion. Gas molecules want to move, and they will expand to fill the volume within which they are contained. Confined to a tightly sealed container such as a corked wine bottle at constant temperature of about 52-57 degrees F, gasses have no room or enough “excitement” to expand and move around. They settle into layers based mostly on their molecular weights. However, the Earth’s atmosphere is much more expansive than a wine bottle. CO2 does not break down until about 80 kilometers from the Earth’s surface, giving atmospheric gases a huge expanse to occupy. Excited by the heat radiating from the Sun into the atmosphere, molecules move rapidly. As they bang into each other (for example, at 63 degrees F, CO2 molecules crash together about 7 billion times per second), the gas molecules intermingle, rather than settling in stratified layers. It is mainly diffusion that allows CO2 to integrate at altitudes higher than what its molecular weight alone would suggest, although other processes, like strong updraft and downdraft air currents, are also involved.
Similarly, upon uncorking that wine bottle for the first time and bringing it from the cellar to a warmer room temperature, the trapped gasses become a part of the larger atmosphere. Gas molecules mix, and after the bottle’s vacuum seal has been broken, replacing the cork means well-mixed molecules remain in the bottle once you replace the cork yourself. Oxygen is now able to reach the wine, eventually causing the wine to taste “off.” Anyone who has opened a bottle of wine to “let it breathe” before drinking it knows that some amount of oxygenation can improve the wine’s taste, but eventually oxygenation will ruin those desirable qualities. So, remember to responsibly consume a bottle of wine within a few days for best flavor. And remember that even in an airplane at 30,000 feet, gas molecules in an open bottle of wine will mix just as they do in the rest of atmosphere!