Mini-Throughput: Maunder Minimum Edition

400-year history of sunspot numbers

In 800 BC, Chinese astronomers looked to the sky and noticed something odd about the Sun. There were small obscurations on the surface. This is the first known instance of humans noticing sunspots, areas on the Sun that are slightly cooler and slightly darker than the rest of the Sun.

Most sunspots, however, are not visible to the naked eye. In the early 17th century, Galileo, as well as three other independent astronomers, turned his telescope to the Sun and discovered that it had lots of spots. That began a 400-year journey of keeping a record of just how many sunspots our Sun had. Long before we understood the physics — they are where the Sun’s tangled magnetic field pokes through the surface — astronomers monitored the number and location of sunspots from year to year, unveiling an 11-year cycle in their numbers and locations.

But not long after their discovery, the sunspots suddenly vanished. For a period of 70 years, the Sun barely had any sunspot activity. A normal solar cycle sees peak sunspot activity of 100-200 spots. During this period of inactivity — called the Maunder Minimum — the Sun barely managed a few dozen a decade.

By the early 18th century, the Sun was back to normal and has gone through a steady cycle ever since. But ever since the Maunder Minimum was discovered by 19th century astronomers poring through old records, we’ve wondered what caused it and if it might occur again.

These days, the subject of the Maunder Minimum usually comes up in climate skepticism. During the Maunder Minimum, Europe experienced an unusually cold span of years, sometimes dubbed the Little Ace Age. The arguments goes that the Maunder Minimum reflected a time of lower solar irradiance and that this caused temperature to drop. The corresponding argument is that since the late 20th century saw high numbers of sunspots and high solar irradiance, that it is the Sun driving global warming, not human activity.

Of course, there is not a consensus agreement on whether the Little Ice Age was even a thing or was mostly confined to Europe. And the record indicates that volcanic activity was more to blame than anything the Sun was doing. Moreover, the argument that global warming is driven by the Sun has really fallen apart over the last twenty years. The last two solar cycles have been rather gentle and solar irradiance has dropped slightly. But the temperature of the planet has continued to climb because — spoiler alert — human activity is the largest single driver of rising temperatures and it has continued apace.

Solar irradiance (yellow) plotted with temperature (red) since 1880. Credit: NASA/JPL

But here’s the thing: we don’t really understand why the Maunder Minimum happened. We only have basic ideas of why the Sun’s magnetic field essentially turned off for several cycles only to go back to normal again. The conjecture is that, as stars slowly spin down, they can’t generate the powerful magnetic fields they did when they were young. So the Maunder Minimum is essentially the Sun’s magnetic field sputtering a bit on its way to running completely out of gas in another billion years or so.

Well, there may now be more support for that theory. For the first time, we have observed another star entering a Maunder Minimum. The lead author was an undergraduate at my university and you can read a great thread from my colleague Jason Wright going through the paper and how they came upon this result. It’s another demonstration of why data archives are so important.

We understand a lot about stars, but there’s still stuff we’re figuring out. Hopefully, this is another big step in figuring out the cycles and patterns of our Sun (and what, if any, impact they have on the climate), a quest we have been engaged in for four centuries.