Solar Cycle 24

We are near the end of solar cycle 23, and the beginning of solar cycle 24. The last century saw some of the strongest solar cycles ever observed, in accord with estimates that solar output increased slightly in the first half of the 20th century. In fact, the increase in solar output is part of the reason for the planetary warming observed in the early 20th century, but can’t explain the recent, stronger warming because, well, average solar output hasn’t really changed in 50 years or so.

The most obvious sign of the solar cycle is the sunspot cycle, and the most common sunspot index is the Zurich sunspot number, also known as the Wolf number. This is actually a composite of sunspot count and sunspot group count (a method devised by Rudolph Wolf in 1848); the sunspot number is ten times the number of groups, plus the number of spots. Here’s a graph of monthly average Zurich sunspot number from 1750 to the present:

The 11-year cycle is obvious. It’s also evident that each cycle has a different peak height, and even a different duration. Many investigators have sought precursors, or “early warning” signals, which can foretell when the next cycle will begin and how strong it will be. Many relationships have been looked for, and many possibilities have been raised, but most of the potential relationships have not been established with statistical significance. This doesn’t mean they’re not correct; it simply means we haven’t yet found out with sufficient precision to have confidence in them. Mainly this is because of a paucity of available data; only 22 full cycles have been observed with sufficient precision to determine their parameters accurately enough for study.

An accurate prediction of the solar cycle would be very valuable. But I know from long experience just how difficult such a prediction is. Solar cycles don’t follow simple patterns; although the solar cycle is dominated by a roughly 11-year period, the timing and strength of each individual cycle is different. Furthermore, no one has yet identified a reliable pattern to those cycle-to-cycle variations.

Of course that doesn’t stop people from trying. In fact lots of people have tried, and NASA has a committee for the prediction of solar cycle 24 (the Marshall Space Flight Center has a solar cycle prediction page). In April, the prediction committee issued a press release indicating that expert opinion was split between predictions of a weaker-than-usual and stronger-than-usual upcoming cycle.

One of the activities of this committee has been to collect together predictions from the scientific literature, as well as predictions specifically solicited by NASA, to get an idea of the range of estimate from various researchers. These are collected together in a table of predictions available from NASA’s Goddard Space Flight Center (in Greenbelt, MD, not to be confused with the Goddard Institute for Space Studies in New York City).

The only relationship I’m aware of which can be established with statistical significance is that a longer solar cycle tends to be followed by a weaker one, while a shorter cycle tend to be followed by a stronger one:

Other possible factors include the rise time from minimum to maximum, the fall time from maximum to minimum, the asymmetry of the cycle, and measures of magnetic activity, but as yet these have not been established.

The next cycle is later than expected; current estimates are that it may start as late as March of 2008. But this is not later than average, a March ‘08 beginning will mean that cycle 23 lasted 11 years, just about average. The collected predictions of the strength of the upcoming cycle span a range from very strong to very weak. The average cycle strength at maximum is an index of 115, and predictions cluster about that average value (click the graph for a clearer view):

Based on available information, NASA scientists have put together the following prediction for cycle 24:

I’ll emphasize again that nobody really knows whether the next cycle will be stronger or weaker than average. We simply don’t yet have the ability to predict the characteristics of upcoming solar cycles.

In spite of this, some global warming denialists insist that not only will the next cycle be a weak one, we’re actually headed for another “Maunder-like” minimum. The Maunder minimum was a period of reduced solar activity from about 1645 to 1715. During this period almost no sunspots were seen, and it’s estimated that total solar irradiance (the total energy output of the sun) was also less than average. This, some say, will lead to cooling of the climate, and make a liar out of global warming activists.

To me, this argument makes absolutely no sense. For one thing, climate scientists — the ones who claim that greenhouse gases are the cause of global warming — have never denied that solar variations influence climate. For another thing, even if we do see another Maunder-like minimum (which I doubt), that won’t change the fact that greenhouse gases also influence climate, and we’re well along the pathway to altering it rapidly and with serious consequences; a temporary respite due to reduced solar activity won’t change the infrared absorption properties of CO2 and other atmospheric gases. Then there’s the fact that even the diminished solar output of the Maunder minimum had only a slight influence on climate, according to paleoclimate reconstructions. Here are two of the most prominent estimates of northern hemisphere temperature from 1500 to 1900 (Moberg et al. 2005 and Mann & Jones 2003):

It’s apparent that the Maunder minimum did not cause temperatures to take a nose-dive, only a slight decrease.

In the history of serious sunspot observations (basically since the invention of the telescope), we’ve only observed one Maunder-like minimum, and only 22 full solar cycles have been accurately delineated. Those who claim we have enough understanding to say with confidence that we’re about to enter a new Maunder minimum, are fooling themselves. Don’t let ‘em fool you.

UPDATE UPDATE UPDATE

One reader is convinced that solar activity is headed for another Maunder-like minimum, which will cause global temperature to decline significantly. He also seems to think that Dr. Svalgaard’s prediction for solar cycle 24 is more reliable than others’. So I tracked down Dr. Svalgaard’s published work containing the prediction: Svalgaard et al. 2005, Geophysical Research Letters, 32, L01104. What, you wonder, is the prediction for the upcoming solar cycle?

Using direct polar field measurements, now available for four solar cycles, we predict that the approaching solar cycle 24 (2011 maximum) will have a peak smoothed monthly sunspot number of 75 ± 8, making it potentially the smallest cycle in the last 100 years.

If this turns out to be correct (and that’s a big “if”), then it’ll be the weakest solar cycle since cycle 14 (peaking in 1901 at an index of 64.2). But it’s nowhere near another “Maunder-like minimum.” Dr. Svalgaard’s prediction doesn’t support the idea, it contradicts it.

Regarding the ability to predict solar cycles in general, the first paragraph of the introduction to the paper states:

At present, our limited understanding of the solar cycle does not allow predictions of future solar activity from theory.

This belies confidence in predictions of upcoming solar cycles.