Despite a previously established link between solar activity and climate, the Sun most likely did not help cause the upward trend in global temperatures observed during the past 300 years, a new study presented Friday at the IAU XXIX General Assembly has revealed.
As the researchers explained in a statement, from 1645 through 1715, sunspots were rare and the winters were on the harsh side. This event, known as the Maunder Minimum, revealed a possible link between solar activity and global climate trends, and led some to conclude that the sun was involved in the upward-trending temperatures that peaked during the late 20th century.
However, a discrepancy between two parallel series of sunspot number counts (the Wolf Sunspot Number and the Group Sunspot Number) have been a source of debate among experts, they said. In the new study, they correct the sunspot number to eliminate this discrepancy, nullifying claims that rising global temperatures cannot be directly attributed to increased solar activity.
The new correction of the solar activity record is known as Sunspot Number Version 2.0, and it explains that the apparent upward trend of solar activity that started after the end of the Maunder Minimum and extended through the industrial revolution was due to a “major calibration error.”
Sunspot, global warming link no longer supported by data
That error, which was found in the Group Sunspot Number, has since been corrected. As a result, the solar activity record now appears to have remained fairly stable since the 1700s. Now that the sunspot numbers have been corrected, the authors explained that existing climate models have to be reevaluated to account for this altered portrait of long-term solar activity.
So what was the reason for this discrepancy? Stanford University’s Leif Svalgaard, one of the researchers behind the study, said “because different observers at different times use different telescopes and counting methods, it is necessary to ‘normalize’ their counts, i.e. bring them onto the same scale. It is like one observation of weather reporting in Fahrenheit and another in Centigrade: one must convert one to the other.”
“The Group Sunspot Number was normalized to the counts reported for years 1874-1975 by the Royal Greenwich Observatory based on photographs of the Sun. But, because photographic techniques and emulsions were still developing during the early years, the counts were ‘drifting’ and were thus not a stable reference,” he explained to redOrbit via email. “We discover that by comparisons with several other visual observers, we can correct for the drift.”
Since the research was not focused specifically on climate issues, Svalgaard said that the team could not comment on other possible causes for climate change. But, he said, “Explanations that rely on simple correlations between changing climate and solar activity are not supported by the new sunspot series. Other explanations, or more complicated ones, must be responsible.”
(Image credit: Thinkstock)
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