Physics World:

A recalibration of data describing the number of sunspots and groups of sunspots on the surface of the Sun shows that there is no significant long-term upward trend in solar activity since 1700, contrary to what was previously thought. Indeed, the corrected numbers now point towards a consistent history of solar activity over the past few centuries, according to an international team of researchers. Its results suggest that rising global temperatures since the industrial revolution cannot be attributed to increased solar activity. The analysis, its results and its implications for climate research were discussed today at a press briefing at the IAU XXIX General Assembly currently taking place in Honolulu, Hawaii.

Measuring the sunspot number – or Wolf number – is one of the longest running scientific experiments in the world today, and provides crucial information to those studying the solar dynamo, space weather and climate change. Scientists have been observing and documenting sunspots – cool, dark regions of strong magnetism on the solar surface – for more than 400 years, ever since Galileo first pointed his telescope at the Sun in 1610. Scientists have also known about the solar cycle – an approximately 11-year period during which the Sun’s magnetic activity oscillates from low to high strength, and then back again – since the mid-18th century, and they have been able to reconstruct solar cycles back to the beginning of the 17th century based on historic observations of sunspot numbers.

Although solar activity has oscillated consistently, the timings and characteristics of individual cycles can vary significantly. Between 1645 and 1715, for example, solar activity did not pick up, and the Sun remained in an extended period of calm known as the Maunder minimum. Historically, this period coincided with the “Little Ice Age”, during which parts of the world including Europe and North America experienced colder winters and increased glaciation than today. These suggested that there exists a strong link between solar activity and climate change.

Until now, the general consensus was that since the end of the Maunder minimum, solar activity has been trending upwards over the past 300 years, peaking in the late 20th century – an event referred to as the modern grand maximum. The trend has also led some to conclude that the Sun may play a significant role in modern climate change. However, a long-running and contentious discrepancy between two parallel series of sunspot number counts has made this role difficult to pin down.