Thursday, June 30, 2011

Understanding the Sun

Earlier this month, reports emerged from the Solar Physics Division meeting of the American Astronomical Society that the Sun may be headed into magnetic hibernation, pushing the Earth into a "Little Ice Age" and resulting in a "sharp decrease in global warming". The historical record of sunspots can only take us back a few hundred years. To understand the Sun on longer time scales, it helps to study other stars that are older and younger. Without this broader context, we have no way of knowing whether the Sun is typical or peculiar.

Consider the Gallup Organization, which regularly conducts telephone polling around the world to track public opinion. Each poll typically includes interviews with more than 1000 people, representing the full range of demographics within a given region. Imagine how different the Gallup poll results might be if instead of contacting a broad cross-section of society, they only interviewed one person in the town of Gallup, New Mexico. Depending on the topic of the poll and the person they happened to choose, we might get a very biased view of "public opinion" from this single interview. That's the whole point of sampling opinion more broadly: the responses from the population provide context for those of any individual. When scientists study the Sun, observations of other stars provide this context. So what can other stars tell us about the solar magnetic cycle and the possibility of the Earth entering a "Little Ice Age"?

Observations of sunspots show that the Sun goes through a magnetically active phase every 11 years, causing it to brighten by about 0.1% and bombard the Earth's upper atmosphere with high-energy radiation and charged particles. For a 70-year stretch beginning around 1645, this magnetic cycle disappeared from the surface of the Sun. Historians note that the absence of sunspots during this time coincided with a period of unusually cold winters in Europe -- a time known as the "Little Ice Age". There were several volcanic eruptions during the same period that thrust large quantities of sulfates into the Earth's upper atmosphere, which is known to have a cooling effect on climate -- so it is unclear whether, or to what extent, this so-called "Maunder minimum" in the Sun actually influenced global temperatures.

Studies of large populations of stars like the Sun reveal two particularly interesting facts: (1) Stars typically spend about 15% of their lifetimes in magnetic hibernation like the "Maunder minimum", and (2) There is a clear set of relationships between a star's rotation period and the length of its magnetic cycle -- that is, for all stars except the Sun. In other words, studies of other stars tell us that the Sun is in a peculiar phase of its evolution. If we want to understand the general problem of how magnetic cycles are created and sustained in stars -- including why they occasionally go into hibernation -- a good place to start is with stars that are more typical. Otherwise, we risk fine-tuning our understanding to explain a special case (our Sun) that turns out to be peculiar.

The broader context provided by other stars suggests that the Sun may very well be entering magnetic hibernation -- with the last one starting more than 350 years ago, we are nearly due for another. But the logical jump from expecting another "Maunder minimum" to creating a new "Little Ice Age" is much more speculative. In fact, the latest research suggests that "the change in climate radiative forcing since the Maunder minimum is about one tenth of the change caused by man-made trace greenhouse gases". So whatever happens with the Sun in the coming decades, the planet is certainly going to get warmer.