Well, the net is an interesting place. I did some digging and found that are in fact hundreds if not thousands of articles dealing with the topic of radio propagation and atmospheric chemistry.
There is interesting chemistry in the middle atmosphere between 15Km and 90Km in altitude, and this overlaps the “D” layer of the ionosphere, extending from approximately 50Km to 90Km, which is responsible for much of the absorption of radio signals below about 10 MHz.
The concentration of water vapor in the middle atmosphere has been increasing over time and thought to be a consequence of global warming.
The concentration of ozone has been decreasing on the whole though the variation caused by changes in solar UV radiation over the 11 year solar cycle are greater than the rate of decline so during the ascending part of the solar cycle the ozone depletion is masked or even partially reversed, and during the descending part, exaggerated.
Above 15Km, the atmosphere gets hotter rather than cooler. This increase in temperature is primarily the result of heating caused by UV radiation being absorbed by ozone. It has been known that the suns UV output varies considerably over the solar cycle, as much as 200% at the shortest UV wavelengths and as much as 30-40% at 250 nm.
Lately, the Suns activity overall has been increasing. We’re presently at a solar minimum, but on top of that 11-year cycle there seems to be a longer term increase in UV output. Observations of the UV output of other similar stars shows that this is actually “normal” behavior for a star like our Sun. UV is produced in the upper reaches of the Sun’s atmosphere and is therefore more affected by magnetic changes than visible light from the photosphere.
So increase in water vapor which results in increased ionization increases atmospheric absorption up to about 10Mhz. Decreases in ozone allow UV to penetrate further increasing ionization at lower altitudes where the effect mainly is to increase absorption. And increases in the Sun’s UV output increases ionization at all levels which increases absorption at lower radio frequencies and reflections at higher frequencies.
While I was able to find information with respect to the role water vapor, ozone, and chlorine play in atmospheric ionization, I was not able to find anything regarding CO2 and methane, also increasing in concentration, so I don’t know what roles they may play.
Suffice it to say there is not only one but multiple factors affecting radio propagation. So where will this go in the future? Well at this point we really don’t know what will happen with the Suns activity. We can be reasonably certain global warming will increase for some time. Levels of chlorine will likely increase and ozone will likely deplete for some time. So I think it likely that we will see existing trends continue. Lower frequencies will become more useless for long distance communication and the maximum usable frequency will continue to increase on average.