Question

Explain the impact of the Maunder Minimum on global climate.

Answer 1

The Maunder minimum is the name given to a period of extreme solar inactivity that occurred between 1645 and 1710. Of particular interest is that this period of inactivity corresponds closely to one of the coldest periods of the so-called "Little Ice Age" in Europe, a time of long, cold winters that caused severe hardships in the pre-industrial revolution world. This has led scientists to extensively study the possible influences of solar activity on terrestrial climate, as well as examine other stars for evidence of activity cycle behavior similar to the Sun's.

Some of the first telescopic observations were made by Galileo in 1611, and he immediately noted the presence of dark blemishes on the Sun's surface; these were the now well-known sunspots. (Several of Galileo's contemporaries saw sunspots as well, but Galileo is the most famous and usually gets the credit for "discovering" sunspots.) Today we know that the number of sunspots rises and falls in a roughly 11-year cycle; this is one of the most obvious manifestations of the solar activity cycle.

Although sunspots were observed telescopically in 1611, it was not until 1843 that an amateur German astronomer, Heinrich Schwabe, noticed a periodic rise and fall in their numbers. That it took over 200 years for astronomers to notice something so seemingly obvious is some cause for wonder, but it may be partly explained by the nearly complete absence of sunspots for 70 of those years, between 1645 and 1715.

For reasons not yet understood, the solar cycle operated at a greatly reduced amplitude during that time. Evidence suggests it did not cease entirely, but the sunspot number—an index representing the total level of sunspot activity at a given time—during the late 1600s was reduced by a factor of 10-20 from its typical value during "normal" cycles. This perplexing aspect of the sunspot record was formally pointed out by the astronomers F. W. G. Sporer and E. H. Maunder in 1890, and it is now known as the Maunder minimum.

The existence of the Maunder minimum is interesting on purely astrophysical grounds, because it suggests that the regular rise and fall of sunspots observed from 1715 all the way through to the present day may not be a permanent, or even typical, aspect of solar behavior. It is possible to create a rough reconstruction of the sunspot record prior to the invention of the telescope, using indirect indicators of solar activity, and there is evidence for other Maunder minimum-like periods intermittently from about A.D. 1250 through 1715. The solar cycle as observed today, is therefore not the state in which the Sun spends all—or even most—of its time. Having only observed one Maunder minimum, we have no idea whether the Sun spends 10%, 50%, or 90% of its time in such a state.

Even the "normal" 11-year cycle seems to have longer-term behavior. Different cycles have different strengths, with some of them showing more sunspot activity than others. The strengths of the cycle peaks seem to follow a roughly 80-year period of very strong cycles, slightly weaker ones, then back to stronger ones, and so forth. With detailed sunspot records extending only a few hundred years, it is difficult to confirm or disprove this hypothesis. Combined with evidence for multiple periods of nearly complete inactivity, it becomes impossible to say whether the solar activity cycle, so extensively studied in the last 30 years, is normality or an aberration.



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