Water Vapour

It is interesting that while the greenhouse effect and climate change are largely attributed to atmospheric carbon dioxide concentrations, water vapour is actually a more abundant and important (in terms of warming effect) greenhouse gas. Water vapour does not garner much attention as a greenhouse gas due to its short atmospheric lifetime and because its presence in the atmosphere is largely independent of human activity.

When compared to CO2 or nitrous oxide, the water (hydrologic) cycle moves molecules through the atmospheric system quite rapidly. Depending on air temperature, the atmosphere can retain a specific amount of water vapour until it is saturated. Once this point is reached, the water is released in the form of rain or snow. This cycle occurs on a relatively short timescale; water molecules may only be present in the atmosphere for a couple hours or days.

Other, man-made greenhouse gases, like carbon dioxide, last much longer in the atmosphere. Though carbon dioxide is removed from the atmosphere by photosynthetic plants as well as ocean deposition, molecules are able to persist for up to 200 years, during which time they continue to warm the atmosphere and contribute to climate change. The same can be said of nitrous oxide, which slowly decomposes in the atmosphere over a period of about 120 years.

The other, very simple reason why climate scientists are more preoccupied with CO2 than water vapour is because the hydrologic cycle is largely independent of human activity. It is completely feasible to remove carbon from our energy and transportation systems and to reduce our GHG emissions from other activities; but it would be impossible to prevent water evaporating from the planet’s surface into the atmosphere. The ineffectiveness of such a strategy is such that scientists have not even bothered exploring it. It makes more sense to address the emission of long-lived, human produced (anthropogenic) greenhouse gases.