Is water vapor camouflaged greenhouse gas?

By Bhaskar Anand

A few days back, I was going through the website of the U.S. Environmental Protection Agency (EPA), where I ended up landing on a page about "Overview of Greenhouse Gases.” On that page, I found carbon dioxide, methane, nitrous oxide and fluorinated gasses were featured as greenhouse gases (GHGs). However, I was surprised that there was no mention of water vapor mentioned, although it is a GHG. Whether this omission was deliberate or fortuitous is a question of interest. This hush-hush predisposition to the exclusion of water vapor from featured GHGs made me inquisitive. This curiosity led me to look for potential reasons behind this marginalization of water vapor as a GHG. Here is why.

Water vapor is the most prevalent (95 percent) GHG in the atmosphere and is accountable for over 60 percent of the Earth's greenhouse warming impact. Carbon dioxide makes up only around 0.04 percent of the atmosphere, but water vapor can range from 0 percent to 4 percent. Water vapor is a potent GHG because it absorbs longwave radiation and reflects it to the surface, leading to global warming. Studies show that water vapor feedback roughly doubles the warming induced by carbon dioxide. For instance, if carbon dioxide causes a 1 degree Celsius change, the water vapor will cause the temperature to go up another 1 degree Celsius. When other feedback loops are included, the total warming from a potential 1 degree Celsius change caused by carbon dioxide is, in reality, as much as 3 degrees Celsius.

A negative feedback effect could also occur if the amount of water vapor in the atmosphere is increased. This could happen if the atmosphere has more water vapor, which causes more cloud formation. Clouds reflect sunlight, lowering the energy that reaches the Earth's surface and warming it. If solar heat decreases, the Earth's temperature will drop. More water vapor would have a cooling effect rather than a warming effect in that circumstance. On the other hand, cloud cover indicates more condensed water in the atmosphere, which has a more substantial greenhouse effect than non-condensed water vapor alone ― a cloudy winter day is warmer than a clear one. Hence, it can be said that both positive and negative feedback co-exist.

The greenhouse effect, which has kept the Earth's temperature at a level warm enough for human civilization to evolve over millennia, is governed mainly by non-condensable gases, particularly carbon dioxide, with smaller contributions from methane and nitrous oxide. Small amounts of produced gases, particularly chlorine- and fluorine-containing solvents and refrigerants, were introduced to the mix in the middle of the 20th century. The atmosphere cannot store these gases in excess since they do not condense at atmospheric temperatures and pressures.

Despite being a dominant GHG and accounting for over 60 percent of the Earth's greenhouse warming impact, water vapor has no direct control over the planet's temperature. Instead, its amount is controlled by the temperature. This is the case because the maximum amount of water vapor the atmosphere can hold is restricted by the temperature of the surrounding environment. When the temperature drops below a certain point and a volume of air holds its maximum amount of water vapor, some of it condenses into liquid water and forms clouds. Water vapor is a condensable GHG with a minimal long-term impact on global warming. However, slight warming due to non-condensable GHG increases the atmospheric water vapor and amplifies the ephemeral rewarming effect via a feedback loop. Since we are unable to regulate water vapor, other non-condensable GHG emissions need to be controlled if we do not want our cities to be submerged and a significant fraction of Earth's population displaced.

Bhaskar Anand (https://www.bhaskaranandjha.com) is a doctoral student working in the field of air quality and material application at the Department of Civil and Environmental Engineering of Hanyang University.