About the recalibration of the sunspot record

TL;DR

This paper evaluates the justification of recent recalibrations of the sunspot record by analyzing continuous, homogeneous data from a single observatory, impacting solar activity and climate change studies.

Contribution

It provides an assessment of the validity of the recalibrated sunspot series using consistent observational data.

Findings

Recalibrated series significantly alter long-term solar activity trends

Homogeneous data from a single observatory questions the need for recalibration

Results impact reconstructions of solar irradiance and climate models

Abstract

In 2015 the calculation of the International (Zurich) sunspot number was cancelled, and it was replaced by a new "recalibrated" series. A new data series was proposed also for the Group sunspot number. As a result, these two series which originally differed mainly in their long-term trends, were "reconciled", and the long-term trends in both of them were minimized. These changes led to new reconstructions of the long-term variations of solar irradiance, and to reconsidering the impact of the solar activity variations on climate change. Here we use data for the number of sunspots and the number of sunspot groups from a single observatory with continuous and homogeneous observations in order to evaluate how justified these recalibrations are.