Then, in 1889, Maunder read an article by another German astronomer, Friedrich Wilhelm Gustav Sporer (1822-95). Sporer was also studying sunspots and he had discovered something very interesting. Astronomers had been observing and recording sunspot activity for centuries, but Sporer found that very few had been observed between approximately 1400 and 1520. This period came to be known as the Sporer Minimum. It was a time of very cold weather. People called it a "Little Ice Age." The Baltic Sea froze over completely in the winter of 1422-23. There were famines. Norse colonies in Greenland were abandoned because their crops failed and the sea froze over, preventing them from fishing. The colonists were starving and the survivors had no choice but to return to Scandinavia.
A similar, but less dramatic, "sunspot minimum" was just commencing at the time Maunder was studying the records. The Dalton Minimum lasted from about 1795 to 1820 and marked another period of cool weather, during which there was one year, 1813, that came to be known as a year without a summer.
Tree rings and isotopes
What Maunder discovered, however, was a period lasting from 1645 to 1715 during which not a single sunspot was reported anywhere in the solar Northern Hemisphere and several periods lasting 10 years in which no sunspots were seen at all. Fewer sunspots were seen over the entire 70-year period than are seen now in a single average year. Auroras, or northern lights, were also rare. Nowadays, Scandinavians are treated to a display of them on most nights, yet during this period they were so uncommon that when one did appear people regarded it as an omen. In 1716 an aurora was seen in England and the Astronomer Royal, Edmond Halley (1656-1742), was so impressed he wrote a paper explaining it. It was the first he had ever seen. Auroras are caused by the interaction between particles of the solar wind and the atoms and molecules of atmospheric gases. When there are few sunspots the solar wind is weak and auroras are uncommon. Maunder described this first in an article he wrote in 1890 and again in another article published in 1922.
The Maunder Minimum, as it came to be known, was another period of cold weather. In London, the River Thames froze so firmly a fair was held on it. Mountain glaciers advanced. The area covered by sea ice increased. In some winters Iceland was completely surrounded by sea ice. Inuit people in kayaks were seen several times in the Orkney and Shetland Islands, to the north of the Scottish mainland, and on one occasion they turned up in the River Don, near Aberdeen. It was the coldest part of the Little Ice Age (see "The Little Ice Age" on pages 87-93). Maunder tried to persuade other scientists that he had discovered an important link between solar output and climate, but with little success. They thought he was relying too heavily on old records that were very probably inaccurate.
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