For the first time since October 10, and only the third time since August, the Earth-facing side of the Sun is blank, showing no sunspots. All told, 2010 has only been blank 13% of the time, for a total of 46 blank days, with only 12 days left in the year. These numbers contrast sharply with 2009, when the Sun was blank 71% of the time, or 260 out of 365 days.

It is very clear that the solar minimum is now over, and that the Sun ramping up to its next maximum. Blank days should soon cease (today might very well be the last for years), and the number of sunspots should continue to increase through approximately 2013, when astronomers now expect the maximum to peak.

It will be a weak maximum, however, likely accompanied with cold weather. At least, this has been the pattern for the last ten centuries, based on the best data that scientists have. When the Sun produces sunspots, the Sun gets hotter, and though that increase in radiation appears slight, it seems enough to warm the Earth’s climate. This is what appears to have happened around the year 1000, during what climate scientists call the Medieval Warm Period.

And when the Sun goes blank, or produces fewer sunspots, the Sun dims, and the Earth’s climate cools. This is what happened in the 1600s and 1700s, when the Little Ice Age gripped much of the Earth. It also happened in the first two decades of the 1800s, the last time the Sun produced as few sunspots as it is now, and when at least one year was called “the year without a summer”. Interestingly, that cold period at the beginning of the 1800s was also a period of intense volcanic activity, which threw a lot of dust and material into the atmosphere and thus helped contribute to the cooling of the Earth.

The last half of the 20th century, however, has not seen that much volcanic activity, which has made the atmosphere today clearer than any time in the past five decades. It has also been a time of increased solar activity, with most of the solar maximums peaking at generally higher numbers. No wonder scientists have detected evidence of a slight warming in the climate.

However, that warming appears to be ending, and it is doing so at the same time the Sun is going spotless. Though we don’t yet fully understand the mechanics of how these two events are linked, it behooves us to pay close attention. No climate prediction or computer model will mean anything if it does not.

Meanwhile, solar scientists remain unclear about the causes behind the solar cycle’s ebbs and flows. They have a reasonable idea that the cycle is caused by the Sun’s magnetic dynamo as it flips from one polarity to another. But why this happens is still subject to debate.

More importantly, it remains a complete unknown how long the next sunspot minimum will be. The Sun could spring back to life, as it did in the 1850s, producing lots of sunspots. Or sunspots might fade out for a few additional decades, as they did in the 1600s.

Sadly, based on the state of our science today, this is a question that probably no one will be able to answer — until we actually see it happen.