Concept
The Spörer Minimum is a hypothesized 90-year span of low solar activity, from about 1460 until 1550, which was identified and named by John A. Eddy in a landmark 1976 paper published in Science titled "The Maunder Minimum". It occurred before sunspots had been directly observed and was discovered instead by analysis of the proportion of carbon-14 in tree rings, which is strongly correlated with solar activity. It is named for the German astronomer Gustav Spörer. Solar variation can be quantified using sunspot counts, but this measure is only reliable for periods after records of sunspot observations were routinely made by western astronomers. For periods before sunspot records, solar activity can be found from proxy methods, most notably the production of radioisotopes in the Earth's atmosphere from interaction with cosmic rays, which are modulated by the solar activity. The carbon-14 method used by Spörer to identify the minimum makes use of the fact that high solar activity is correlated with low production of carbon-14 in the atmosphere. Wilfried Schröder published a table of observed aurora borealis during the Spörer Minimum which showed that the solar cycle was active. Miyahara et al. likewise found the 11-year solar cycle was still prominently detected in the carbon-14 record even during the minimum. The amplitude of the 11-year cycle seems to have been modulated only from 1455 to 1510. Jiang and Xu look at sunspot records and aurora sightings from China during the period and suggest that a minimum from 1450 to 1560 is specious. They suggest dates for the sunspot minimum of 1400 to 1510. Like the subsequent Maunder Minimum, the Spörer Minimum coincided with a time when Earth's climate was colder than average. This correlation has generated hypotheses that low solar activity produces cooler than average global temperatures, although Jiang and Xu point out that while the period 1430-1520 (starting slightly before the Spörer minimum) was indeed colder than average in China, the period 1520-1620 (the second half of the minimum) was warmer than average.