If water vapour is a key greenhouse gas, why is its abundance in the atmosphere not causing concern?

It’s true that water vapor is the largest contributor to the Earth’s greenhouse effect. On average, it accounts for about 60% of the global warming effect!

However, the truth is, water vapor does not control the Earth’s temperature, but is instead controlled by the temperature.

Let’s look at this in more detail.

The greenhouse effect that has maintained the Earth’s temperature at a level warm enough for human civilization to develop over the past several millennia is controlled by non-condensable gases, mainly carbon dioxide, CO2, with smaller contributions from methane, CH4, nitrous oxide, N2O, and ozone, O3. Since the middle of the 20th century, small amounts of man-made gases, mostly chlorine- and fluorine-containing solvents and refrigerants, have been added to the mix. Because these gases are not condensable at atmospheric temperatures and pressures, the atmosphere continues to pack in much more of these gases.

Thus, CO2 (as well as CH4, N2O, and O3) has been building up in the atmosphere since the Industrial Revolution when we began burning large amounts of fossil fuel.

On the other hand, water vapor has a short cycle in the atmosphere (10 days on average) before it is incorporated into weather events and falls to Earth, so it cannot build up in the atmosphere in the same way as carbon dioxide does.

But why do we claim that the amount of water vapour in atmosphere is actually controlled by the global warming phenomenon?

As more CO2 and other longer duration greenhouse gases are emitted into the atmosphere and the Earth’s temperature rises, more water evaporates into the Earth’s atmosphere and stays there a bit longer owing to the higher temperature. All these further increase the temperature of the planet (as water is also a greenhouse gas), thus creating a negative feedback loop.

Thus, if there had been no increase in the amounts of non-condensable greenhouse gases, the amount of water vapor in the atmosphere would not have remained what it was earlier.

The above are the reasons why water vapour is not considered to be the main cause of global warming, but is rather controlled by the global warming effects of greenhouse gases such as CO2.