History of sunspot number observations showing the recent elevated activity.
The Modern Maximum refers to the period of relatively high solar activity which began with Solar Cycle 15 in 1914. It reached a maximum in Cycle 19 during the late 1950s and may have ended with Cycle 23 in 2000 as Cycle 24 is recording, at best, very muted solar activity. This period is a natural example of solar variation, and one of many that are known from proxy records of past solar variability. The Modern Maximum reached a double peak once in the 1950s and again during the 1990s.
^Rigozo, N. R.; Echer, E.; Vieira, L. E. A.; Nordemann, D. J. R. (2001). "Reconstruction of Wolf Sunspot Numbers on the Basis of Spectral Characteristics and Estimates of Associated Radio Flux and Solar Wind Parameters for the Last Millennium". Solar Physics. 203: 179–191. Bibcode:2001SoPh..203..179R. doi:10.1023/A:1012745612022.