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.
History of solar activity
For details on solar activity see: solar variation.
|Oort minimum ||1010||1050|
Solar activity is conventionally expressed in the form of 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 for the period around 1455-1510 A.D. 
Jiang and Xu look at sunspot records and aurora sightings from China during the period, and suggest that a minimum from 1450-1560 is specious. They suggest dates for the sunspot minimum of 1400-1510.
Possible correlation with climate
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.
A specific mechanism by which solar activity results in climate change has not been established, One theory is modification of the Arctic Oscillation/North Atlantic Oscillation due to a change in solar output.
- Eddy, J. A., "The Maunder Minimum", Science 18 June 1976: Vol. 192. no. 4245, pp. 1189–1202, PDF Copy
- "History of Sunspot Observations". Retrieved 4 February 2009. The Spörer Minimum (1420 to 1570), named after the German astronomer Gustav Spörer.
- Ilya G. Usoskin, A History of Solar Activity over Millennia, section 3, "The Proxy Method of Past Solar-Activity Reconstruction", Living Reviews in Solar Physics, 2013 (accessed 21 July 2015)
- Wilfried Schröder, Annals Geophys., 1994
- Miyahara, H., Masuda, K., Muraki, Y., Kitagawa, H. and Nakamura, T., 2006a, “Variation of solar cyclicity during the Spoerer Minimum”, J. Geophys. Res., 111(A10), A03103. abstract (accessed 12 July 2015)
- Yaotiao Jiang and Zhentao Xu, "On the Spörer Minimum", Astrophysics and Space Science, Jan. 1986, Vol. 118, 1-2, pp 159-162. Abstract and link (accessed 12 July 2015)
- "The Sun's Chilly Impact on Earth". December 6, 2001. Retrieved 2009-04-04.
- "The Medieval Warm Period". Retrieved 4 February 2009.
- Shindell, T.; Schmidt, A.; Mann, E.; Rind, D.; Waple, A. (Dec 2001). "Solar Forcing of Regional Climate Change During the Maunder Minimum". Science. 294 (5549): 2149–2152. Bibcode:2001Sci...294.2149S. doi:10.1126/science.1064363. ISSN 0036-8075. PMID 11739952.