Surer sunspots should improve warming forecasts
By John Gribbin
THE Sun was much less active between 1700 and 1850 than astronomers have thought. Sunspot activity did not switch on abruptly at about the modern level around 1700, when the period called the Maunder minimum ended. Instead it has built up much more gradually to its present state. This may have important implications for our ideas about the Sun's cycle of activity, and for the modelling of global warming by climate researchers.
The Sun's activity follows an 11-year cycle. Soon after this was discovered in the 1840s, the Swiss astronomer Rudolf Wolf began collecting past records of solar activity, and eventually accumulated thousands that went back to the early 17th century. He used these records to establish an index of solar activity for each year, based on a count of the number of groups of sunspots and of individual spots seen on the Sun's disc. This index is now called the Wolf sunspot number.
Wolf's records show the Sun was almost without spots in the late 17th century (the Maunder minimum); during this period Europe was extremely cold. According to Wolf's interpretation, solar activity then switched on around 1700 and continued at a high level, averaged over each cycle, except for a dip in the early 19th century, sometimes called the Dalton minimum.
The snag is that Wolf's interpretation of the old records may not be reliable. Different observers may count sunspots differently, especially when deciding if they form a group. Douglas Hoyt of Research and Data Systems Corporation in Greenbelt, Maryland, Kenneth Schatten of NASA's Goddard Space Flight Center, also in Greenbelt, and Elizabeth Nesmes-Ribes of the Paris Observatory at Meudon have taken a fresh look at the old records, including some that Wolf did not have, and come up with a different picture. Their technique counts only groups of sunspots.
The team used the records from the Royal Greenwich Observatory after 1874 as the benchmark against which other observations were calibrated. Another series of observations, made in Greece, runs from the 1840s to 1883, overlapping with the Greenwich data. In the overlapping years, this record consistently counted fewer sunspot groups than the Greenwich observers did, so a correction factor can be applied to bring the data into line with each other. Older records still overlap with the earlier part of the Greek record, and can be calibrated against it, and so on back into the early 1600s.
The index derived in this way, called the group sunspot number, looks set to become the new standard. The team is still gathering data and improving the index, but believes it has already included about 85 per cent of the available historical information – more than 349 000 observations from more than 350 observers between 1610 and 1993 (Geophysical Research Letters, vol 21, p 2067).
The Maunder and Dalton minima still show up clearly in the group sunspot number. The most important difference from Wolf's analysis is a steady overall increase in solar activity, starting from the end of the Maunder minimum and lasting until today. The team also has also found that the first peak of solar activity in the 19th century occurred in 1801, not 1805 as it is in Wolf's index.
Researchers have often suggested that there is a link between solar activity and climate, the Earth being warmer when the Sun is more active. One of the first things climatologists will do with the new index is to see if it provides a better match with solar and climate cycles from 1700 to 1850 than the Wolf sunspot index. That may result in a revision of the models on which forecasts of global warming are based (see Diagram).FIG-mg19492901.jpg
Magazine issue 1949 , published 29 October 1994
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