The revised International Sunspot numbers which will become official on July 1 do not show the often mentioned “Grand Maximum” in the late 20th century that had been apparent in the previous version. The term “Grand Maximum” had been used to refer to unusually high sunspot values during this period.
However, it should be noted that the revised numbers continue to show the unusually long period of high values. This doesn’t rule out the possibility that time-integration effects could be responsible for at least some of the global warming in the late 20th century.
Here are the views of Leif Svalgaard who participated in the development of the revised Sunspot numbers.
“The scenario of the initial post-Minimum recovery is still uncertain, as the
exact amplitude of the first cycles of the 18th century remains difficult to establish given the
scarcity of observations over that period. Still, the vanishing upward trend over the last 250
years questions the existence of a modern “Grand Maximum” in the 20th century (Solanki et al.2004; Abreu et al.2008; Lockwood et al.2009; Usoskin et al.2012,2014), which
resulted primarily from the erroneous transition between Wolf and Wolfer in the Hoyt and
Schatten GN time series. As this “Grand Maximum” concept rests on the occurrence of out-
of-range amplitudes of the solar cycle, it is definitely contradicted by the re-calibrated and
reconciled SN and GN series.
Still, although the levels of activity were not exceptional except maybe for cycle 19, the
particularly long sequence of strong cycles in the late 20th remains a noteworthy episode.
Indeed, the 400-year sunspot record and one of its by products, the number of spotless days,
show that such a tight sequence of 5 strong cycles over 6 successive cycles (from 17 to
22, except 20), which we can call the “Modern Maximum”, is still unique over at least
the last four centuries. Given the inertia of natural systems exposed to the solar influences,
like the Earth atmosphere-ocean system, this cycle clustering could still induce a peak in
the external responses to solar activity, like the Earth climate. However, we conclude that
the imprint of this Modern Maximum (e.g. Earth climate forcing) would essentially result
from time-integration effects (system inertia), since exceptionally high amplitudes of the
solar magnetic cycle cannot be invoked anymore. In this suggested revision, the estimated
or modeled amplitude of the effects, including the response of the Earth environment, can
be quite different, necessarily smaller, and should thus be re-assessed.
The recalibrated series may thus indicate that a Grand Maximum needs to be redefined
as a tight repetition/clustering of strong cycles over several decades, without requiring ex-
ceptionally high amplitudes for those cycles compared to other periods.”
www.leif.org/research/Revisiting-the-Sunspot-Number.pdf