365 Days of Climate Awareness 185 – Water Vapor

Water vapor, in terms of its insulating effect, is the most important greenhouse gas in the atmosphere. It’s not mentioned with the more commonly known gases like carbon dioxide, methane and fluorocarbons, because it isn’t considered “well-mixed”. That is, water vapor is not diffused evenly throughout the entire atmosphere. Its concentrations vary immensely in space and time, due to many causes including air temperature, insolation (sunlight), proximity of water bodies, the nature and topography of the ground, and latitude.

US trends in precipitable water vapor (PWV), 1870-2010 (acronyms explained below).

It precipitates (rains, snows or sleets) out, and is taken back up again through evaporation. Water vapor’s presence in the atmosphere is highly dynamic. The hydrological cycle, or water cycle, refers to this dynamism: the constant global movement of water molecules through the different phases, solid, liquid and gas, depending on their heat content. So for being of immense importance to the climate, water vapor is fundamentally different from the commonly named greenhouse gases.

Global changes in PWV, 1988 - 2011, mm/decade.

Water’s feedback roles in global warming are many and very complicated. There are two principal feedbacks, one positive and one negative. The positive feedback is the increase of water vapor content in the atmosphere with the warming of the air and oceans. Increased vapor leads to an increase in the greenhouse effect, reinforcing planetary warming. The negative feedback is the formation of more clouds from the increased vapor. The negative feedback is the clouds’ albedo, reflecting UV radiation back into space and lowering the energy added to the global system.

Global changes in PWV, 1995 - 2011, mm/decade.

Satellite imagery since the late 70’s and traditional in-situ and radiosonde measurements spanning back through the 20^th century have combined to show increasing concentrations of precipitable water vapor (PWV—that is, water vapor content at a particular location) both in raw amounts (g/m²), and as a function of increasing temperature. The time series plot shows three trends: 20CRv2, which refers to the 20^th century data reanalysis, version 2, the second (there is now a third) review of historical climate data by NOAA; SSMI, referring to Special Sensor Microwave/Imager data from a satellite; and the C-C fit refers to a model of vapor content as a function of changing temperature, with relative humidity held constant. All three show a marked trend, over nearly a century and a half, toward higher vapor content globally.

Percentage increase of PWF relative to increase in local temperature (i.e. air’s capacity to hold moisture), 1979 — 2014.

It must also be said that there is at times poor agreement between satellite data sets. Generally speaking, different geographic plots show a range of PWV trends over time, from neutral (no gain) to net gain, unevenly spread around the globe. Combined with in-situ measurements, however, the trend toward increasing vapor content with rising global temperature is well-established.

Tomorrow: clouds.

Be brave, and be well.