evidence: It is equally dangerous to evaluate other solar/heliospheric/terrestrial indices from sunspot numbers by extrapolating an empirical relation obtained for the last few decades back in time. This is because the last decades (after the 1950s) were well covered by direct observations of solar, terrestrial and heliospheric parameters, corresponding to a very high level of solar activity. After a steep rise in the activity level between the late 19th and mid-20th centuries, the activity remained at a roughly constant high level, being totally dominated by the 11-year cycle without a long-term trend. Accordingly, all empirical relations built based on data for this period are focused on the 11-year variability and can overlook possible long-term trends (Mursula et al. 2003). This may affect all regression-based reconstructions, whose results cannot be independently (directly or indirectly) tested. In particular, this may be related to solar irradiance reconstructions, which are often based on regression-like models, built and verified using data from the last three solar cycles, when there was no strong trend in solar activity. [...] Lal D, Jull AJT, Pollard D, Vacher L (2005) Evidence for large century time-scale changes in solar activity in the past 32 Kyr, based on in-situ cosmogenic \(^{14}\)C in ice at Summit, Greenland. Earth Planet Sci Lett 234:335–349. https://doi.org/10.1016/j.epsl.2005.02.011 [...] Mursula K, Martini D (2006) Centennial increase in geomagnetic activity: latitudinal differences and global estimates. J Geophys Res 111:A08209. https://doi.org/10.1029/2005JA011549