A major part of the climate change that scientists have documented over the past few decades comes from human emissions of heat-trapping greenhouse gases. Not all of it, however. Natural climate cycles haven’t magically disappeared — the El Niño/Southern Oscillation, or ENSO, for example, is entering a phase that will likely boost global temperatures temporarily — and scientists are still discovering new ones.
The latest comes in a new report published in Geophysical Research Letters. It’s well known that the Sun varies slightly in brightness every 11 years, and while those changes pale beside the effect of human-generated greenhouse gases, according to the report, they’re enough to trigger unusually cold winters in Central Europe.
The smoking gun is the freezing of the Rhine river, something that doesn’t happen often because it’s difficult to freeze such a large, free-flowing volume of water. Those unusually cold winters might come along at random, but by looking back at records dating to all the way back to 1780, a team led by Frank Sirocko, of Johannes Gutenberg University in Mainz, Germany, determined that they closely follow the 11-year sunspot cycle. The fewer the sunspots, says the paper, the more likely the Rhine is to freeze.
Sirocko, who usually studies ancient European climate by looking at lake sediments, first began to wonder about the sunspot-river connection while watching the Elfstedentocht, a 120-mile-long skating race on the canals of the Netherlands. “I realized that they can only hold the race every 10 or 11 years,” he said in an interview — a spacing that seemed suspiciously close to the timing of sunspots.
Astronomers already know that sunspots affect the Sun’s brightness. Normally, sunspots wax and wane every 11 years, and the fewer spots, the less energy the Sun puts out. Back in the 1600s and 1700s, there were almost no sunspots for an 80-year stretch known as the Maunder Minimum, and it was unusually cold in Europe the whole time: the Thames River froze over frequently, and the growing season got shorter.
To test his theory, Sirocko looked back at Rhine shipping records, which duly noted times when the river was too icy for barges to travel. “It’s a very simple measurement . . . either there is ice or there is no ice,” he said in a press release. Between 1780 and 1963, he and his colleagues determined that there were 14 episodes where ice blocked the river in many different places, largely shutting down shipping. And 10 of those 14 happened at times when sunspot activity was at a minimum — too many for it to be just a coincidence.
According to the authors, the way it works is that the lower solar energy during a sunspot minimum reduces the heating of the stratosphere, which sets off a series of changes that ultimately alter a natural climate cycle known as the North Atlantic Oscillation, which governs weather on both sides of the Atlantic.
Sirocko emphasized that this is a local phenomenon, not a global one, and that the new study does nothing to refute the idea that generally rising temperatures are triggered largely by human greenhouse gas emissions. In fact, he says, while Europe continues to experience colder winters when sunspots are at a minimum, the average temperatures during those winters are less cold than they used to be — and the Rhine hasn’t frozen since 1963.
What the paper shows, in other words, is that the Sun is affecting climate, just as the skeptics claim. It’s just not the most important effect.