Spörer Minimum

[1] 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.

[3] 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.

[5] 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.

This correlation has generated hypotheses that low solar activity produces cooler-than-average global temperatures,[7] 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.