A Window on Water Vapor and Planetary Temperature

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:


[UPDATE: After reading comments from our always sharp readers, and collaborating with three other meteorologists on the graph, I’m of the opinion now that this graph from ESRL, while labeled as “up to 300mb only” is misleading due to that label. The first impression I had would be from the surface to 300mb i.e. the “up” portion of the label, but on the second thought I believed the label was intended to be numerical meaning “zero to 300mb” or from the top of the atmosphere down as opposed from the surface up as we normally think of it. The values looked like anomaly values, but are inthe range of absolutes for that elevation also.

Thanks to some work by commenter Ken Gregory, looking at other ways this and similar graphs can be generated from the site, it has be come clear that this is a level, not a range from a level. The label ESRL placed “up to 300mB was intended to list the availability of all data levels. Thus there is no 200mb data.

This demonstrates the importance of labeling a graph, as without any supplementary description, it can be viewed differently than the authors intend. A better label would be “at 300mb” which would be unambiguous. ESRL should correct this to prevent others from falling into this trap.]

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: