Guest Post by Willis Eschenbach (@WEschenbach on X, my personal blog is here.)
Well, I had some further insights and questions about the issue of sunshine hours that I discussed in my last post, but as is far too common on my planet, along the way I got seriously side-tractored, all the way over to the question of the atmospheric window.
And what is the atmospheric window when it’s at home? It’s the portion of the emitted thermal radiation in a range generally taken to be 8µm to 12µm. The curiosity of this “window” is that thermal radiation in this range mostly goes directly to space.
Figure 1 shows the atmospheric window. This is from MODTRAN Infrared Light In The Atmosphere, an online calculator of how much radiation makes it to space and how much is absorbed in the atmosphere
Figure 1. The “atmospheric window” is the area between the vertical red dotted lines at 8 microns (µm) and 12 microns. The amount of thermal radiation that would be making it to space at a given temperature is shown by the smooth colored lines. The solid blue line shows how much actually makes it to space under the given conditions (tropical atmosphere, no rain or clouds). You can see that the major absorption by “greenhouse” gases occurs in the region centered at 15 microns.
So … how much of the upwelling radiation goes directly out to space? Well, some research showed that, to my surprise, the CERES dataset has both clear-sky and all-sky values for the top-of-atmosphere (TOA) upwelling longwave in the atmospheric window of 8-12 microns.
My interest in this was to see how the amount of upwelling radiation that avoids the greenhouse gases has varied over time. Figure 2 shows those changes.
Figure 2. Changes in the radiation loss through the atmospheric window, 2000 – 2024
Having seen that, my next question was, how does this relate to the increase in upwelling surface radiation that accompanies the increase in temperature over that same period? Figure 3 shows that relationship.
Figure 3. Scatterplot, loss through the atmospheric window versus changes in the surface upwelling longwave.
What this says is that as the surface warms, a steady 20% of the increase in upwelling surface radiation goes out through the atmospheric window, and 80% is absorbed in the atmosphere.
So, how does all of this relate to the question of the sunshine hours? Well, I was using the sunshine hours as some kind of proxy for the albedo. Inter alia, I wanted to see what the relationship was between the available solar radiation (the amount of radiation making it past the albedo) and surface absorbed radiation. And to my great surprise, I found the following:
Figure 4. Anomalies, downwelling solar at the top of the atmosphere and at the surface.
As I pointed out in my previous post, we’re getting more available solar radiation at the top of the atmosphere, and we’re also getting more sunshine hours … but what surprised me was, over the last quarter century there’s been no increase in solar radiation at the surface.
Obviously, the difference is due to increased absorption of solar radiation in the atmosphere … and that meant that I needed to look at the changes in atmospheric absorption and what that means to various other climate parameters.
Finally, to close the circle, that is why I had to find out how much surface upwelling longwave was not absorbed in the atmosphere. To understand the radiation dynamics of the atmosphere, I need to understand all of the sources of radiation absorbed by the atmosphere … which will be the subject of my next post. Assuming, of course, that I learn something about the subject between now and then.
Late night here in the forest, with the moon peeking above the horizon of this most awesome of planets. Next, I’ll go outside, walk around a bit, and revel in the tree-filtered moonlight … best of life to each one of you.
w.
Yeah, I’ve Said It Before: When you comment, please quote the exact words you are discussing, so your subject is crystal clear.
