Here is some interesting news; according to data from NOAA’s Earth System Laboratory, atmospheric water vapor is on the decline globally.

You’ve probably heard many times how water vapor is actually the  most important “greenhouse gas” for keeping our planet warm, with an effectiveness far greater than that of CO2.

It is generally accepted that the rank of important greenhouse gases is:

• water vapor and clouds which causes up to 70% of the greenhouse effect on Earth.
• carbon dioxide, which causes 9–26%
• methane, which causes 4–9%
• ozone, which causes 3–7%

Note the range of uncertainties, on water vapor some say the percentage goes up to 90% with reduced numbers on the other three.

It is absolutely true that water vapor is the gas most responsible for the “greenhouse effect” of our atmosphere. Greenhouse gases let short-wave solar radiation through the atmosphere, but impede the escape of long-wave radiation from the Earth’s surface. This process keeps the planet at a livable temperature: Without a suitably balanced mixture of water vapor, CO₂, methane, and other gases in the atmosphere, Earth’s average surface temperature would be somewhere between -9 and -34 degrees Fahrenheit, rather than the balmy average 59 degrees it is today.

This graph then from NOAA’s Earth System Research Laboratory, showing specific humidity of the atmosphere up to the 300 millibar pressure level (about 8 miles altitude) is interesting for it’s trend:

Click for original source of the graph

For some background into atmospheric absorption efficiency of the electromagnetic spectrum, this graph is valuable:

Note the CO2 peak at 15 microns is the only significant one, as the 2.7 and 4.3 micron CO2 peaks have little energy to absorb in that portion of the spectrum.  But the H2O (water vapor) has many peaks from .8 to 8 microns, two that are fairly broad,  and H2O begins absorbing almost continuously from 10 microns on up, making it overwhelmingly the major “greenhouse gas”.

Here is another graph looking at it in a different way:

Click for a larger image

Note that water vapor plays quite a role in keeping the planet cool by absorbing some percentage of incoming radiant energy.

The yellow line is what we’d get without an atmosphere, and the blue  line what we get with it. Sunlit temperatures on the earth’s surface are substantially less than those on the moon (up to 123°C) because our atmosphere intercepts some incoming solar short-wave radiation as well as some outgoing long-wave infrared.

So when we see atmospheric water vapor dropping as shown in the NOAA ESRL graph above, you know it has to have an effect on our overall planetary energy budget, the question that will be argued is; “how much”?

h/t: Thanks to atmospheric physicist Jim Peden,  and also to Barry Hearn, and Alan Siddons for some of the graphs and background to this post.