In the first post in this series we began with an assertion that according to greenhouse theory, it is impossible to warm the planet when longwave radiation to space is increasing. In the second post we discovered that it is possible for the planet to warm while longwave radiation to space increases if shortwave absorption increases. The first post has been corrected.
If shortwave absorption is increasing, the questions become, “How?” and, “Where?”.
NOAA ESRL has a long time series of atmospheric precipitable water (TPW) extending back to 1948. This time series shows a very small increase to the present and can be characterized as moving from higher values after WWII to a nadir in the late 70’s/mid 80’s, and increasing again to the present; much like surface temperature.
In the graphic above this TPW series is compared with RSS satellite lower troposphere (TLT) data. The correlation is not impressive, but there is reasonable agreement.
RSS has its own shorter TPW time series that excludes polar regions, and it is compared with ESRL TPW above. The correlation is poor, as will result from the trends being inverse from 1989 to 1997; but once again there is reasonable agreement. The burning question is what the satellites would have seen between 1948 and 1988. A reasonable supposition is that they would have generally tracked ESRL, with a poor correlation.
When the RSS TPW is compared with RSS TLT above, the correlation is good, but the trends appear divergent.
The point of all this is that if our increased shortwave absorption from CERES takes place in the atmosphere, it is probably going to be from increased atmospheric water.
We colored this Wikipedia image blue in the water shortwave absorption bands a while back. Oxygen, particularly in transition to ozone is a significant absorber in the SW solar spectrum. We will see when we get to MODTRAN that there is significant ozone formation near the surface as well as the stratosphere. CO2 absorption in the SW spectrum is limited to small areas near 2000 nm, shown green.
In the bands where the surface irradiance is decreased by atmospheric absorption, but some light still makes it through; adding more gas will increase the warming of the atmosphere and further decrease the warming of the surface. In the water bands where surface irradiance is already zero, adding more water will have no effect on the surface. These bands are about 19% of the total absorption by water.
The amount of light can’t change much. Like the sun, the earth radiates at an intensity and wavelength governed by temperature. In this sense one can think of the earth as the sun for outgoing longwave, and the atmosphere is caught in a crossfire between the two.
The crossfire becomes particularly wicked for reflected shortwave from the sun. In this case, the graphic above must be turned upside down. The atmosphere gets a second chance to absorb the photons that made it through the first time, and the longwave from the earth and the shortwave from the sun are both heading for space. The longwave that makes it out reduces the temperature of the planet, the shortwave that makes it out does not.
Even after all the atmospheric absorption and reflection from clouds, over half of the incoming solar SW makes it to the surface. Something like 7% is reflected from the surface, leaving just less than half absorbed by the surface. The surface is warmed, and radiates longwave out. Seventy percent of the surface is water, but as the graphic above shows, the shortwave absorption by water is at its lowest at the peak intensity of solar SW radiation.
The peak of solar intensity is in the visible wavelengths, and any diver knows that visible light penetrates to considerable depth, depending on turbidity. The ocean is still absorbing SW, albeit less efficiently than it might.
We really don’t have the tools yet to determine whether the atmosphere or the surface is where the CERES increase in SW absorption is predominantly taking place. Sea surface temperature, atmospheric temperature, and atmospheric precipitable water are all rising; together, as the laws of physics decree they must.
According to NASA (and Mr. Trenberth), the surface is radiating more energy up as longwave than the TOA receives as shortwave. We can address this, and longwave radiation to space, with MODTRAN in the next post.