.For decades, the number of observed sunspots has been directly related to the amount of radio waves coming out of the sun -- particularly the 10.7cm wavelength radio waves. If you measure this 10.7 radio flux, you can calculate a "synthetic" sunspot number using a simple equation. The synthetic number has matched the observed number very closely since 1951, when we first started measuring the flux.
Here is a chart from solar physicist Dr. Leif Svalgaard:
Since 1990, the number of observed sunspots has started to decline from the normal relationship. (Note that when the number of sunspots "R" is less than 5 per month or so, the relationship "is less reliable" -- mostly because observed sunspots are counted in a screwball way with single spots receiving a count of 12). The thin curve on the bottom of the chart is the normal 11-year sunspot cycle -- so you can see that this decline is independent of the regular up-and-down behavior of the sun.
This appears to be another manifestation of what Bill Livingston (National Solar Observatory) has seen in his measurements showing a decline in sunspot contrast and magnetic field since 1990 (which I have posted on previously). Here is Dr. Livingston's raw data, charted by me, with dots indicating yearly averages (placed mid-year):
The data forms somewhat of a cloud, but the noisiness is due to the sun's behavior, not to measurement error. The trend lines are calculated second order polynomials of the entire data set, not just eyeball curves.
It has been hypothesized that this decline in solar activity, independent of the 11-year solar cycle, is similar to what happened during the Maunder and Dalton Minimums (1645-1715, and 1790-1830 respectively). These "little ice ages" were times of excessive cold, although scientists disagree about cause and effect.
If you are interested in Dr. Svalgaard's equation used to calculate synthetic sunspot number from the 10.7 flux for dates between 1951 and 1990, it is:
SSN = -0.00106 SF^2 + 1.40438 SF - 80.25 ; r^2 = 0.9928
For dates after 1990, the equation has become:
SSN = -0.00226 SF^2 + 1.53925 SF - 92.33 ; r^2 = 0.9908
These equations are derived empirically, and do not reflect any underlying process. Dr. Svalgaard's paper is here. (Note that Dr. Svalgaard has mentioned a typo. All references to 1951-1988 should be 1951-1990.) Drs. Livingston's and Penn's paper is here.
No one knows why the sun is doing this.
Not really related but interesting nonetheless, Dr. Svalgaard on Solar Cycle 24 has provided this animation of a sunspot and its magnetic field taken by the Japanese Hinode spacecraft. Click for a movie: