Dark, unassuming blots on the Sun's surface, sunspots are an important scientific tool for studying the lifetime of the Sun all the way to the climate on Earth. Some of the data taken in the past 400 years is unreliable due to a lack of knowledge at the time or not following proper scientific procedures, however, so a team of scientists took it upon themselves to adjust and improve the data.
The new records alter the history of solar activity as we know it: the Maunder Minimum (an unusually inactive period for sunspots around the 1600s) is still evident in the data. However the Great Maximum (an increase in sunspots after the 1950s) has been smoothed out and almost vanished. This pours fresh doubt on theories that global warming since the Industrial Revolution can be accounted for by increased solar activity, rather than mankind's energy emissions.
The research was announced recently at the International Astronomical Union General Assembly.
Laure Lefevre, from the Royal Observatory of Belgium who has been working on the recalibration of Sunspot Number Series, told IFLScience why this work is crucial. “These sunspots are important because they are the main indicator of solar activity,” she said.
Sunspots occur in regions where the Sun's magnetic field is temporarily concentrated in one region. This inhibits the flow of hot matter to that region, making it slightly cooler and darker.
Thanks to the likes of curious scientists such as Galileo, we have observational sunspot data from 400 years ago. While this timespan of data is impressive, the accuracy of the observations wasn't regulated in the same way that it is today.
Fortunately, scientists are now trawling through this data to find any sources of calibration error, and hopefully correct them. The scientists have taken two official sunspot lists: the International Sunspot Number (the blue line in the graph below) and the Group Sunspot Number (red), in an effort to calibrate them. You can see their research in Space Science Reviews. The results of the two lists are largely similar, but occasionally there are noticeable inconsistencies in the numbers.
Unfortunately, the International Sunspot Number measurements include a couple of measurement blunders that the team had to account for. In one instance, an aging observer in Switzerland noted fewer sunspots over time – the same time that his eyesight was deteriorating.
Red line is the Group Sunspot Number data and the Blue line is the International Sunspot Number data. Top graph: data before calibration. Bottom graph: newly calibrated data. WDC-SILSO.
Using this information, the team created the bottom graph where the two data sets draw similar conclusions about the sunspot numbers in each cycle. The results can be seen on WDC-SILSO. This is especially true after the 1800s, with more data being available.
“Once you make the corrections, you can see that what people call the 'Grand Maximum,' the period after the 1950s... hasn't completely disappeared but it is very slight when compared to before,” said Lefevre. The Grand Maximum has been associated with global warming by climate change skeptics but with this new, smoothed data that theory looks increasingly unlikely.
Lefevre noted it was impossible to say with certainty what these results implied for climate changes on Earth, though. “The models have to be re-evaluated to consider the new data, but we don't know what it implies yet,” she said.
She added that “there is also an influence on the study of the long-term evolution of the Sun.” This indicates that any current models based on this data should be re-evaluated.
However other scientists have stronger views on this data. David Hathaway, a solar physicist from NASA, thinks the research will have big implications for the rise in global temperatures. He commented: “Previous work tying the increase in solar activity to the increase in global temperature here on Earth clearly overestimated the role of solar activity in global warming.”