Sun Makes History: First Spotless Month in a Century

Changing Sun, Changing Climate?
http://www.aip.org/history/climate/solar.htm

This essay is partly based, by permission, on an essay by Theodore S. Feldman (PSDI, Bedford, Mass), "Solar Variability and Climate Change," rewritten and expanded by Spencer Weart. For additional material, see Feldman's site (http://www.agu.org/history/SV.shtml).

Since it is the Sun's energy that drives the weather system, scientists naturally wondered whether they might connect climate changes with solar variations. Yet the Sun seemed to be stable over the timescale of human lifetimes. Attempts to discover cyclic variations in weather and connect them with the 11-year sunspot cycle, or other possible solar cycles ranging up to a few centuries long, gave results that were ambiguous at best. These attempts got a well-deserved bad reputation. Jack Eddy overcame this with a 1976 study that demonstrated that irregular variations in solar surface activity, a few centuries long, were connected with major climate shifts. The mechanism remained uncertain, but plausible candidates emerged. The next crucial question was whether a rise in the Sun's activity could explain the global warming seen in the 20th century? By the 1990s, there was a tentative answer: minor solar variations could indeed have been partly responsible for some past fluctuations... but future warming from the rise in greenhouse gases would far outweigh any solar effects.

The Sun so greatly dominates the skies that the first scientific speculations about different climates asked only how sunlight falls on the Earth in different places. The very word climate (from Greek klimat, inclination or latitude) originally stood for a simple band of latitude. When scientists began to ponder the possibility of climate change, their thoughts naturally turned to the Sun. Early modern scientists found it plausible that the Sun could not burn forever, and speculated about a slow deterioration of the Earth's climate as the fuel ran out.(1) In 1801 the great astronomer William Herschel introduced the idea of more transient climate connections. It was a well-known fact that some stars varied in brightness. Since our Sun is itself a star, it was natural to ask whether the Sun's brightness might vary, bringing cooler or warmer periods on Earth? As evidence of such a connection, Herschel pointed to periods in the 17th century, ranging from two decades to a few years, when hardly any sunspots had been observed. During those periods the price of wheat had been high, he pointed out, presumably reflecting spells of drought.(2)

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http://www.aip.org/history/climate/solar.htm

What's Wrong with the Sun? (Nothing)

July 11, 2008: Stop the presses! The sun is behaving normally.

So says NASA solar physicist David Hathaway. "There have been some reports lately that Solar Minimum is lasting longer than it should. That's not true. The ongoing lull in sunspot number is well within historic norms for the solar cycle."

This report, that there's nothing to report, is newsworthy because of a growing buzz in lay and academic circles that something is wrong with the sun. Sun Goes Longer Than Normal Without Producing Sunspots declared one recent press release. A careful look at the data, however, suggests otherwise.

But first, a status report: "The sun is now near the low point of its 11-year activity cycle," says Hathaway. "We call this 'Solar Minimum.' It is the period of quiet that separates one Solar Max from another."

During Solar Max, huge sunspots and intense solar flares are a daily occurrence. Auroras appear in Florida. Radiation storms knock out satellites. Radio blackouts frustrate hams. The last such episode took place in the years around 2000-2001.

During Solar Minimum, the opposite occurs. Solar flares are almost nonexistent while whole weeks go by without a single, tiny sunspot to break the monotony of the blank sun. This is what we are experiencing now.

Although minima are a normal aspect of the solar cycle, some observers are questioning the length of the ongoing minimum, now slogging through its 3rd year.

"It does seem like it's taking a long time," allows Hathaway, "but I think we're just forgetting how long a solar minimum can last." In the early 20th century there were periods of quiet lasting almost twice as long as the current spell. (See the end notes for an example.) Most researchers weren't even born then.

Hathaway has studied international sunspot counts stretching all the way back to 1749 and he offers these statistics: "The average period of a solar cycle is 131 months with a standard deviation of 14 months. Decaying solar cycle 23 (the one we are experiencing now) has so far lasted 142 months--well within the first standard deviation and thus not at all abnormal. The last available 13-month smoothed sunspot number was 5.70. This is bigger than 12 of the last 23 solar minimum values."

In summary, "the current minimum is not abnormally low or long."

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http://science.nasa.gov/headlines/y2008/11jul_solarcycleupdate.htm