New (Indirect) Evidence for a Cooling Sun?

By Michael D. Lemonick

Last month, a new spacecraft called the Solar Dynamics Observatory caught the Sun in an eruption of energetic particles known as a coronal mass ejection. Normally, this wouldn’t have been all that big a deal, although when an especially big burst of particles reaches Earth (this one wasn’t), it can disrupt telecommunications.

But the mass ejection, plus an uptick in the number of sunspots, suggested that the Sun was waking up after an unusually quiet, long-lasting and sunspot-free solar minimum. Here's a movie of one of the lonely new sunspots.

This could have been a big deal, because a spotless Sun is a slightly cooler Sun. During the period known as the Maunder Minimum from the mid-1600’s to the early 1700’s there were very few sunspots — and parts of the world, at least, experienced a “Little Ice Age” during which the Thames River routinely froze over in winter, and the growing season became shorter and more unpredictable throughout much of Europe.

Image of the Sun, taken by NASA's Solar Dynamics Observatory today (Sept. 16).
Image shows two tiny sunspots — one in the bottom left, the other about halfway between the center and the bottom edge. Credit: NASA Solar Dynamics Observatory.

The Maunder Minimum was clearly not the only cause of the Little Ice Age — the cold began as much as 400 years earlier — but it could very plausibly have contributed to the chill. So when the most recent solar minimum turned out to be longer and deeper than anyone expected, scientists speculated about a new Maunder Minimum — and speculated further that if it happened, the cooling effect could hold back the climate warming caused by greenhouse gases. (The notion that the warming during the second half of the 20th century was caused by a warmer Sun has been largely discredited).

Now a new paper by two physicists at the National Solar Observatory argues that despite the Sun’s recent stirrings, we still might be entering a long-term minimum after all. There weren’t literally zero sunspots during the Maunder Minimum, just very few, and so the fact that we’ve got some now doesn’t prove things are back to normal. And the physicists, Matthew Penn and William Livingston, have some evidence that they might not be.

Penn and Livingston have been measuring not the number of sunspots, but the intensity of magnetic fields within them. And that intensity, they say, has been going more or less steadily down for 17 years —starting long before the last solar maximum, which started in 2000 or so. The one caveat is that they have only a few measurements before 2000. After that time, the downward trend is indisputable, but it could be associated with the overall decline in sunspot numbers that comes after a maximum. 

Even if the trend were ironclad, it’s not clear what the decline in magnetic field strength really means, and it’s not clear exactly how much cooler the Sun would get if sunspots largely disappeared for decades.

And even if it all turns out to be true — that sunspots go away for the next, say, 60 years, and the Sun cools off enough to counteract greenhouse warming — it would only be a temporary reprieve. Greenhouse gases would, without intervention, keep building up in the atmosphere. When the Sun brightens again, the added heat-trapping effect would likely send temperatures shooting up rapidly.