Sunspot-record update reveals no recent increase

Nature : The counting of sunspots is the longest observational record in science, dating back to 1610 when Galileo and others first saw the dark spots on the Sun. For the past four years Frédéric Clette of the Royal Observatory of Belgium in Brussels and his colleagues have been working to recalibrate two official sunspot lists: the International Sunspot Number and the Group Sunspot Number. The two lists occasionally differ because of variations in sunspot visibility or the ability of the observer. Clette’s team found a variety of systemic errors in both lists, including the failure to record sunspots due to the fading eyesight of one observer or to the fact that the observers were focused on something else. The team’s recalibrated metric revealed one major change—there was no “Grand Maximum,” an increase in sunspot activity, in the late 20th century. The finding suggests that solar activity and its impacts may have been overestimated. The recalibrated data also agree more closely with a third list kept by the US National Oceanic and Atmospheric Administration.