Guest Post by Willis Eschenbach
When folks tell me that the sunspot-related changes in total solar irradiance make changes down here on the surface of our amazing planet, I suggest that they take a look at the numbers.
From peak to trough over the sunspot cycle, the top-of-atmosphere total solar irradiance varies by about 1.2 watts per square metre (W/m2) … which, when averaged over the entire surface of the earth gives a change of about 0.28 W/m2. This is less than a tenth of one percent variation in total incoming energy.
But in fact, it’s less than that. Only about half of the sunlight makes it to the surface, so we’re down to 0.14 W/m2 change from peak to trough, less than a twentieth of a percent.
Now, downwelling radiation at the surface of the earth averages about 500 W/m2 on a 24/7 average basis. And out of that, we’re supposed to believe that a variation on the order of a tenth of a W/m2 is going to make a difference …
“Ah, you don’t understand”, folks inform me, “Yes, TSI only changes by a tenth of a percent over a solar cycle. But extreme ultraviolet (EUV) varies as a percentage much, much more than that!” … and you know what?
They’re right …
… but they’re also wrong. Let me explain why.
To start with, here’s the breakdown of the strength of the solar radiation by wavelength.
Figure 1. Spectrally resolved top-of-atmosphere sunshine. X-axis units are nanometres (nm).
In the middle is the visible spectrum, from about 380 nm to about 750 nm. Longer wavelengths than that are called “near infrared”. Wavelengths shorter than that are ultraviolet (UV).
And way over on the left, at 10 – 24 nm wavelength between the vertical red lines, is the tiny amount of extreme ultraviolet (EUV).
So that is the first problem. Even though it varies on a percentage basis more than the TSI, the EUV represents such a small part of the sun’s energy that it cannot be even seen at this scale.
The second problem is that the variation in EUV is much, much smaller in absolute terms than the variation in TSI. In Figure 2 I’ve compared the variations in the middle of the EUV spectrum (18 nm) to the variations in the blue part of the visible spectrum (~500 nm).
Figure 2. Monthly variations in solar output, measured in the EUV (red/yellow line) and in the visible spectrum (blue line)
As you can see, the variations in the EUV are very small compared to the variations in the visible spectrum.
In fact, the only reason that the percentage variations in EUV are greater than the percentage variations in TSI is that changes in EUV start from almost zero … so even a tiny absolute change in EUV is a large percentage change in EUV.
For those reasons, I hold that looking at EUV to explain surface climate variations is a blind alley … but as always, YMMV …
Best to everyone on a warm and quiet night,
AS ALWAYS: I politely ask that you QUOTE THE EXACT WORDS THAT YOU ARE DISCUSSING so that we can all understand the subject of your thoughts.