The headline comes from this article at NASA, sent in by reader “Mike D.”
The gist of the article is “that there’s nothing to report.” Says David Hathaway:
“There have been some reports lately that Solar Minimum is lasting longer than it should. That’s not true. The ongoing lull in sunspot number is well within historic norms for the solar cycle.”…Although minima are a normal aspect of the solar cycle, some observers are questioning the length of the ongoing minimum, now slogging through its 3rd year…Hathaway has studied international sunspot counts stretching all the way back to 1749 and he offers these statistics: “The average period of a solar cycle is 131 months with a standard deviation of 14 months. Decaying solar cycle 23 (the one we are experiencing now) has so far lasted 142 months–well within the first standard deviation and thus not at all abnormal. The last available 13-month smoothed sunspot number was 5.70. This is bigger than 12 of the last 23 solar minimum values.”
In summary, “the current minimum is not abnormally low or long.”
Let’s take a look at the actual data and see if the statements about the “normalness” of the sunspot number are accurate. And let’s keep in mind the real reason NASA made this press release, the purpose of which is never directly stated—can you see it?. I’ll come back to this later.
Here is a picture from NASA showing the “Yearly Averaged Sunspot Numbers 1610-2007.”
Solar cycle “number 1” peaked around 1760, the cycles and other behaviors before this time are ignored in the official counting. Well, that’s neither here nor there—the labels do not matter—but we should always remember that the sun’s sunspot activity has been taking place for at least 4 to 5 billion years, and we only have measurements on the last 400. Thus we are in a very poor position to say what is “normal” and what is not. We can, however, make statements conditional on the data observed so far.
Hathaway’s analysis starts with cycle number “1” and ignores the previous data, which, given the extended period of low to no sunspots from 1650 to 1700, actually weakens his case. This is because, conditional on all the available evidence, periods of time with no or low sunspots are not that unusual. These quiescent periods are more likely given all the evidence than they are just using the data from 1749. This is true based on the simple observation that all the data has more quiescent periods than does the later half. It is true regardless of the periodicities or other structures present. Because we have seen periods in the past with few or no sunspots is excellent evidence, after all, that we will see these periods in the future.
So why would he purposely ignore evidence that would have strengthened his case? Part of the reason is that there is the possibility that the data before 1749 is measured with error, and so should be discounted somewhat. However, this error is not especially large. The real reason has to do with the “Maunder Minimum” (shown on the graph), the period with few or no sunspots. This period does not fit the probability model Hathaway has in mind, so it is ignored. NASA says this about the Maunder:
For reasons no one understands, the sunspot cycle revived itself in the early 18th century and has carried on since with the familiar 11-year period. Because solar physicists do not understand what triggered the Maunder Minimum or exactly how it influenced Earth’s climate, they are always on the look-out for signs that it might be happening again.
But Hathaway thinks the “quiet of 2008 is not the second coming of the Maunder Minimum.”
Thus we have gone from “For reasons no one understands” to “the solar cycle is progressing normally.” The path from one statement to the other is indeed rocky. This is why I believe Hathaway’s statements are too certain. I believe that periods of low to no sunspots are more likely. I am not, however, disagreeing with Hathaway in the sense that it does not appear that we are in another Maunder: there is only scant, at best, evidence for this.
As a technical note: It is not clear that the uncertainty in length of time in months that the cycles last is best represented by a normal distribution, as used by Hathaway. Ignoring the Maunder makes his approximation a better one, but there is never a good reason to ignore part of the data it does not fit your expectations.
Anyway, back to the real purpose of this press release. Why are people so interested in the length of the solar cycle? Easy. Because for years, most climatologists insisted that the role the sun plays in the climate was minimal. That is to say, changes in the behavior of the sun were not thought to be related to changes in the Earth’s climate. The sun, which alone supplies all the energy that goes into creating the climate, was thought not to be important. Obviously, this attitude is starting to change. This press release is a tacit admission that some now admit some role of the sun in climatology.
I do not have time to talk here of the actual methods to predict sunspot number, which is an important activity in space weather. But take a look at the first picture in the press release and see if you can not anything odd.