UPDATE 26 September 2010: I changed the final paragraphs correcting "upper atmosphere" to "lower atmosphere" and adding some additional information.
Dr. William Livingston and Dr. Matthew Penn, of the National Solar Observatory in Tucson are continuing to take measurements of both the intensity ratio of sunspots compared to the normal surface of the sun, and of magnetic fields in sun spots. The sunspots have been getting paler and the magnetic fields less intense in an almost linear trend since about 1990.
The sun is behaving in a way we have never seen during the modern era. It is hypothesized that similar behavior caused the dearth of sunspots during the Maunder Minimum of 1645 to 1715 AD. This lull in solar activity coincided with the Little Ice Age, a time of extremely cold temperatures, crop failures, famine, plague, and enormous suffering and death. However, it is still being debated whether or not the reduced solar activity actually caused the cold weather. (There were volcanic eruptions too, and the temperature decline seems to have started long before the solar decline, maybe.)
Here is the latest Livingston-Penn data through September 6th, 2010, graphed by me.
click for full size image
The measured values form clouds rather than obvious trends, because the data is quite variable. But the trends are there nonetheless. The lines are not just eyeballed and drawn in, but are rather mathematically-fit logarithmic curves based on the data, thus showing the true trends. The large dots are yearly averages, centered in the middle of each year, or in the case of 2010, in the middle of the months collected so far in the year.
The latest data fall right on the curve. If the trends continue, sunspot intensity ratios will go to 1 (i.e. no contrast) and sunspot magnetic fields will average less than 1500 Gauss sometime between the years 2015 and 2020. At these levels, there will be virtually no sunspots, regardless of the state of the 11-year cycle.
Decline in solar activity causes a decline in the strength of the interplanetary magnetic field. This causes an increase in the flux of cosmic rays reaching the earth. One theory is that increased cosmic rays reaching the lower atmosphere cause increased water vapor nucleation, and thus the formation of more clouds. More clouds reflecting sun away means a cooler earth. But comparing the 11-year solar cycle state to global cloud cover, we don't see a real correlation.
As in all climate studies, we just don't have enough data to say anything definitive about whether the future will be warmer or colder than "average".
But I am guessing that we could be in for some cold times, and even food shortages, in the next 10-20 years.
The frozen Thames River in London, 1677