Although the Sun appears to shine at a constant rate, this rate is, in fact, variable. At times, dark spots — called sunspots — appear on the Sun's surface. These sunspots, surrounded by particularly luminous areas, are a result of the Sun's magnetic field. During periods of high solar activity, the magnetic field produces more sunspots than usual and the Sun shines slightly brighter.

The Sun's magnetic field reverses every 22 years, and the sunspot cycle reaches a maximum every 11 years. This regular cycle of maxima and minima has been well documented since the mid-eighteenth century, although the number of spots varies, with some maximum years producing nearly 200 sunspots while others yield fewer than 50.

In addition to the 11-year sunspot cycle, the Sun also appears to have long-term variations. From about 1650 to 1715, the Sun went through a phase of low activity and produced virtually no sunspots. During this time, astronomers observed the scarcity of sunspots and noted that auroras — another effect of solar activity — were also rare. Referred to as the Maunder Minimum, this period of unusually low solar activity coincided with a bout of unusually cold temperatures on Earth.

The Little Ice Age (LIA) was a time of lowered temperatures in the northern hemisphere — the summers were cool, the winters were long and cold, rivers froze, and sea ice was widespread. Captured in artwork, documented in papers, and preserved in tree rings and arctic ice, the LIA is known to have had extensive effects. However, the causes of the LIA are not well understood — it is likely that a number of different factors influenced this climate change.

The clear coincidence with the Maunder Minimum indicates that solar activity plays a role in determining Earth's climate. If the Sun changes its energy output — particularly the amount of ultraviolet light emitted, which impacts the ozone in Earth's atmosphere — it affects the climate.

In addition to historical records of low solar activity during the LIA, further evidence can be found in tree rings. Not only are the rings from the LIA closely spaced, they also contain high levels of carbon-14. Carbon-14 is produced in the atmosphere by cosmic rays — energized particles from space — and is incorporated into materials such as trees. However, cosmic rays are affected by the solar wind. When the solar wind is strong, it prevents cosmic rays from entering into Earth's atmosphere, resulting in less carbon-14 in the atmosphere. Alternatively, during years of low solar activity, there is less solar wind and more carbon-14 is produced.

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