Sunspots have long been used by climate skeptics to explain away a rise in global temperatures.
They argued that solar activity appeared to trend upwards over the past 300 years, on pace with global temperatures, peaking in the late 20th century. This, adherents say, demonstrates that the sun has played a significant role in modern climate change.
But now a group of scientists has found that one method for counting sunspots -- as a measure of solar activity -- was wrong. Called the Group Sunspot Number, this method showed a rise in sunspots from 1885 to 1945 culminating in what is called the Modern Grand Maximum.
By recalibrating that method in what they call Sunspot Number Version 2.0, Frédéric Clette, director of the World Data Centre, Ed Cliver of the National Solar Observatory and Leif Svalgaard of Stanford University, concluded that sunspot activity had been relatively stable since the 1700s and that there was no Modern Grand Maximum.
The team, which presented its results at the International Astronomical Union in Hawaii, said the findings closely mirror another historical record for sunspot activity, the Wolf Sunspot Number.
The latest findings make it difficult to explain the observed changes in the climate that started in the 18th century and extended through the Industrial Revolution to the 20th century as being significantly influenced by natural solar trends.
The new method may also force current climate evolution models to be reevaluated, given the greater understanding of long-term evolution of solar activity.
Sunspots grabbed the headlines a few weeks back, when Valentina Zharkova, a professor of mathematics at Northumbria University in England, told the Royal Astronomical Society's National Astronomy Meeting that sunspot activity could drop as much as 60 percent to 70 percent between 2030 and 2040 from the current cycle.
As a result, she predicted temperatures would also decline several degrees as they did in the 17th century. (She allowed the possibility that global warming could offset some of those decreases.)
Her conclusions have been met with skepticism from other solar scientists and those who study the climate. Most climate experts are suggesting any changes in the solar activity will have little impact on the Earth's climate.
A 2011 paper Geophysical Research Letters suggested a drop in solar activity would mean only a 0.5 degree drop in temperature while a paper out in June in Nature Communications found such a decline from 2050 to 2099 "is likely to be a small fraction of projected anthropogenic warming," delaying the effects of climate change by only two years.