posted on Sep, 13 2018 @ 05:19 PM
originally posted by: OneArmedBandit
CH4’s absorption bands overlap with those of water vapor. CH4 is thus irrelevant. In other words, any energy that the methane might have absorbed
has already been absorbed by H2O.
You are attempting to draw vast conclusions from a half-vast understanding of the information on that chart.
The fact that water vapor shows near 100% absorption at the same location on the spectrum that Methane also shows a near 100% absorption does NOT mean
that water vapor absorbs 100% of the IR energy passing through a volume of air. Those absorption bands are on a per molecule basis. The way to
interpret that chart is to realize that a quantum of IR radiation that has a wavelength right at one of the peaks for Methane would have a near 100%
probability of being absorbed by the Methane molecule if that quantum of light happened to hit the Methane molecule. Similarly, if that same quantum
of IR light happened to hit a water molecule in the air it would also have a near 100% probability of being absorbed by that water molecule.
Obviously, any body of air could have a mixture of Methane and water vapor in it and any IR of the correct wavelength passing through it would be
absorbed by whichever of those molecules it happened to hit.
If your interpretation were correct, then that chart would be saying that Methane absorbs 100% of the IR energy in the atmosphere at that wavelength
AND water vapor also absorbs 100% of IR energy in the atmosphere at that wavelength. That would clearly be impossible, since no mixture of Methane
and water vapor could absorb twice as much energy as is actually in the atmosphere.
The sum total of all greenhouse gases in the Earth's atmosphere does not absorb 100% of the IR energy passing through. If it did, Earth would
resemble Venus. Some fraction of the IR coming off the Earth's surface escapes to space without being absorbed. If that were not true, then adding
greenhouse gases to the air would not increase the temperature because all of the IR passing through would already have been absorbed.
It is true that water vapor is a strong greenhouse gas. It is estimated to account for around 60% of the total heat trapped by greenhouse gases on
Earth. The number varies from time to time and from location to location, because the column density of water vapor in the atmosphere is not
constant. That's because water vapor is a condensible gas. If the air temperature goes down at a particular location and time, then that body of air
will precipitate out the water content. If the air temperature goes up at a particular location and time, then that body of air can contain more
Carbon Dioxide and Methane do not precipitate out at normal Earth atmospheric pressures and temperatures, When they accumulate in the atmosphere they
create a detectable rise in the atmospheric temperature just because they are directly absorbing IR energy. However, that temperature rise has the
added effect of increasing the solubility of water vapor in the air, which is also a greenhouse gas. In other words, the increased concentration of
water vapor in the air due to an increase in the background concentrations of Carbon Dioxide and Methane creates positive feedback and amplifies the
greenhouse effect of those two non-precipitable gases.