Before talking about why water vapor is the most abundant of the greenhouse gases, it is a good idea to know what makes a gas a “greenhouse gas” (GHG) in the first place. It’s also wise to know what a GHG does. Beyond that, it’s just a matter of comparing the relative amounts of GHG’s already present in the atmosphere and their rates of production to see why water vapor has the highest concentration of all. Finally, there’s the question, “Is abundance what matters most?”
You’ve likely heard that carbon dioxide is an important GHG, and maybe methane and a few others have crossed your path, as well. To be a GHG, a gas must meet just one requirement – it has to be able to absorb infrared radiation. All warm objects, the Earth in particular, emit infrared (IR) light as a way of releasing energy. The loss of this light energy to space is the natural cooling process that keeps the Earth at a steady temperature. GHGs absorb IR and can send it back toward the Earth, hindering the cooling process. More GHGs mean less cooling. All it takes for a gas to be able to absorb IR is that it must be made of three or more atoms. (IR has the right amount of energy to cause these molecules to vibrate and bend.) Any gas that has three or more atoms is a GHG. A gas with only one or two atoms can’t bend and is not a GHG. It’s that simple.
Looking at the composition of the atmosphere it’s plain to see that there are only a few major players. When dry air (no water vapor at all) is measured at sea level, two gases account for a little more than ninety-nine percent (99 percent) of the atmosphere. Nitrogen (about 78 percent) and oxygen (21 percent) are these two. Both are diatomic gases – they contain only two atoms each. In other words, the most abundant gases of all aren’t GHGs. The gas argon makes up most of the remaining one percent, but it’s a monatomic gas – only one atom – so it isn’t a GHG, either.
Finally, carbon dioxide (CO2) shows up in fourth place with about 0.03 percent of the total volume of dry air. With three atoms, carbon dioxide is a GHG, and it makes all the other GHGs in dry air seem relatively insignificant. Compare: Carbon dioxide at about 300 ppm to sulfur dioxide (1 ppm), methane (2 ppm), nitrous oxide (0.5 ppm), ozone, nitrogen dioxide, and ammonia (all less than 0.01 ppm). Looking at concentration alone isn’t quite fair, since some gases are “stronger” GHGs than others. Hydrofluorocarbons, for example, are known as potent GHGs, though they aren’t present in anything beyond trace amounts, but that goes beyond a discussion of abundance.
Up to this point, all measures were made on “dry air.” That’s convenient, because water vapor does not stay in the air in a constant concentration. You’re probably aware of this since some days have higher humidity than others. The amount of water vapor in the air depends on temperature (warm air can hold more water vapor) and on the availability of local water sources. In warm, wet areas, the concentration of water vapor can reach a maximum of about four percent (four percent). In cooler or drier areas, it can be very low, approaching zero (zero percent). On average, the atmosphere will be between two and three percent water vapor. For the Earth as a whole, then, water vapor is roughly one hundred times as abundant as carbon dioxide.
It’s important that readers don’t take the abundance of water vapor to mean that other gases like carbon dioxide don’t matter. The main sources of water vapor in the atmosphere are the Earth’s oceans. The amount of water vapor in the air is temperature-dependent, so as long as temperature is stable, water vapor’s contribution as a GHG is also stable. On the other hand, carbon dioxide’s concentration is steadily increasing. The Mauna Loa observatory (not located at sea level) has been measuring atmospheric carbon dioxide concentrations since 1958. Over the years, they’ve witnessed the average carbon dioxide concentration rise from about 320 ppm to about 400 ppm (first observed in May of 2013). Unlike water vapor, carbon dioxide has no maximum concentration in the atmosphere.
Aside from natural sources, humanity now contributes significantly to carbon dioxide concentrations, both directly (e.g., burning fossil fuels) and indirectly (e.g., clearing forests). Increasing the total concentration of GHGs in the atmosphere means that less heat can escape from the planet and results in a slow warming of the Earth (global warming). Climatologists are concerned more about carbon dioxide than water vapor not because it is more abundant, but because it is increasing. Further, while humans are very good at adding GHGs like carbon dioxide to the atmosphere, current technology cannot reverse the process to remove carbon dioxide at a meaningful rate.
In the end, water vapor is easily the most abundant greenhouse gas. The oceans constantly supply moisture to the air, replenishing losses to precipitation. While most prevalent, the average concentration of water vapor is fairly stable, though it will increase if the Earth’s average temperature climbs. Other greenhouse gases, carbon dioxide in particular, can increase beyond their current concentrations as the result of human activity. For people concerned about an impact on the climate, relative abundance is not the key factor. Instead, what matters more is the overall change in concentration.