One of the clearest signs of the variance is the number of sunspots visible on the sun measured since the days of Galileo and the first telescopes on a daily basis. Sunspots are dark spots (cooler than the photosphere average, which lower irradiance by 0.1%). They are accompanied by bright flares and faculae which are warmer than the average surface (and increase irradiance by 0.2%). The net result of an active sun is that it is a little brighter (0.1%) and thus warmer.
These cycles of the monthly sunspot number is quite evident on the following plot with data from NOAA’s National Geophysical Data Center (NGDC).
The 11 year cycle can be seen to vary in its amplitude and length (in the range from 9-14 years). The longer cycles tend too be associated with and followed by cycles of smaller amplitude.
Cycle 23 which peaked in 2000 is in its dying stages. Cycle 24 has yet to assert itself, although there were a few months back a few small spots with cycle 24 polarity (the polarity reverses across the solar equator and from one cycle to the next (why there is a 22 year cycle).
This month (August 2008) had a sunspot number of 0 by most accounts. One solar observing site in Italy,, Catania, saw what it called sunpots on the 21st. The other 9 major centers did not see them as spots but either nothing or pores. Right now it is dispute but the official international sunspot assigning site in Belgium has gone along with Catania for now. A zero sunspot month would mark the first time since 1913, we have had a 0 sunspot calendar month (there have been a few 30 day intervals without sunspots as recently as 1954 but they have crossed months. So far the first 7 days of September have been spotless. Since the last 11 days of July were spotless, this would have been a span of 49 days without a spot, which would have ranked as the fourth longest span since 1854!
Following is a plot of the number of months with 0 sunspots by year over this period.
Note that cluster of zero month years in the early 1800s (a very cold period called the Dalton minimum - at the time of Charles Dickens and snowy London town and including thanks to the major volcanic eruption of Tambora, the Year without a Summer 1816), and again to a lesser degree in the early 1900s. These correspond to the 106 and 213 year cycle minimums.
There was a stretch of 21 straight months from October 1809 to May 1811 without a single observed spot. Two other long periods included 4 months from November 1822 to February 1823 and 4 months from August to November of 1823.
The superimposition of the 106 and 213 cycles would suggest that the next cycle minimum around 2020 could be especially weak. Even David Hathaway of NASA who has been a believer in the cycle 24 peak being strong, thinks the next minimum and cycle 25 maximum could be the weakest in centuries based on slowdown of the plasma conveyor belt on the sun.
"Normally, the conveyor belt moves about 1 meter per second—walking pace," says Hathaway. "That's how it has been since the late 19th century." In recent years, however, the belt has decelerated to 0.75 m/s in the north and 0.35 m/s in the south. "We've never seen speeds so low."
"The slowdown we see now means that Solar Cycle 25, peaking around the year 2022, could be one of the weakest in centuries," says Hathaway.
In this plot of the cycle lengths and sunspot number at peak of the cycles, assuming this upcoming cycle will begin in 2009 show the similarity of the recent cycles to cycle numbers 2- 4, two centuries ago preceding the Dalton Minimum. This cycle 23 could end up the longest since cycle 4, had a similar sunspot peak and similarly had two prior short cycles.
Note how this cycle has compared with the prior four cycles. Three by the 12th year were well on their way back to high activity levels. Only cycle 20, was a long and quiet cycle but its minimum was not as low and it was starting to recover by this point in time.
In a very interesting paper presented here, Livingston and Penn of the National Solar Observatory in Tucson observed “Sunspots are cool dark regions on the solar surface with strong magnetic fields. There have been few direct measurements of changes in the physical parameters of sunspots, but here we present a study which shows that sunspots are becoming warmer and have weaker magnetic fields. The number of sunspots visible on the Sun normally shows an 11-year periodicity, and the current sunspot cycle (cycle 23) had a maximum in 2001, and is entering a minimum phase with few sunspots currently visible.
Our data show that there are additional changes occurring in sunspots, independent of the sunspot cycle, and these trends suggest that sunspots will disappear completely. Such an event would not be unprecedented, since during a famous episode from 1645-1715, known as the Maunder Minimum, the normal 11-year periodicity vanished and there were virtually no sunspots visible on the solar surface (Eddy 1976). Recent studies of the appearance rate and latitudinal drift of sunspots (Hathaway et al., 2004) and of the solar magnetic field (Svalgaard etal, 2005) predict that the number of sunspots visible in future cycles will be significantly reduced. Finally the occurrence of prolonged periods with no sunspots is important to climate studies, since the Maunder Minimum was shown to correspond with the reduced average global temperatures on the Earth (Foukal et al., 1990).”
The graph shows the line depth of OH 1565.3 nm for individual spots. The upper trace is the smoothed sunspot number showing the past and current sunspot cycles; the OH line depth change seems to smoothly decrease independently of the sunspot cycle.
Solar scientist Leif Svalgaard noted on Solar Cycle 24 forum relative to this paper that “There was a tiny pore on Aug 22nd, 2008. Bill Livingston measured its magnetic field and tells me today that it was 1931 Gauss. You may verify for yourself that that falls straight on his projected line. BTW, he has many other data points now between the last data shown on the plot and this latest one, and they also confirm the trend.”
Other solar modelers have predicted this upcoming cycle 24 as being very weak, a forecast that is becoming more credible with each passing quite month. Clilverd in this paper in 2005 in the Space Weather Journal predicted a peak sunspot number of 40 for cycle 24, similar to those cycles in the early 1800s based on the phasing of primarily the 52, 106 and 213 year cycles. Note how successful this statistical model has been in predicting prior cycle magnitudes.
Archibald in this paper using regression of past temperature and an expected solar sunspot peak of 40 of less for the next few cycles predicts a drastic drop in global temperatures.
And his projection based on this solar forecast of temperature through 2030.
Read still more in this very interesting Anthony Watts “2014-2015 - These years are a repeating theme in solar forecasts” story on the sun featuring a letter by Mark S. Lawson, Online Opinion, Australia.
Follow the sun daily here and monthly progression here.