Scientists have raised doubts over whether sunspot activity has increased in recent decades – which some researchers have argued as a cause of global warming – following a re-analysis of a historical records that goes back as far as Galileo at the start of the 17th Century.
The so-called “grand maximum” of sunspots in the latter half of the 20th Century has been controversially touted as a possible cause of increased temperatures on Earth, but Frederic Clette of the Royal Observatory in Brussels said that there is little evidence for an increase in sunspot activity.
“We find no such grand maximum. There has been nothing exceptional about the level of solar activity,” Dr Clette told a meeting of the International Astronomical Union in Honolulu, Hawaii.
He and has team identified several sources of systematic bias in the two lists of sunspots that have been recorded over many years. One of the errors in recording could be attributed to the failing eyesight of an ageing observer in Switzerland who was seeing fewer sunspots over time, Dr Clette told the meeting.
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“In other cases, skywatchers were focused on making other solar observations, so if their notes do not mention sunspots this does not necessarily mean that none were present,” said a report in the journal Nature.
When Dr Clette and his colleague recalibrated the two lists after developing a method of choosing a main sunspot observer for a given interval of time while ensuring that observers from adjacent periods overlapped to give smooth transitions, the grand maximum of the latter half of the 20th Century disappeared, the journal says.
