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Sunspots more active than for 8000 years
2004 10 28

By Maggie McKee NewScientist.com

The Sun has been more active in the last 70 years than it has for the previous 8000, according to an analysis of tree rings dating back 11,400 years. But researchers say its recent bout of hyperactivity does not account for the rapidly rising temperatures recorded on Earth over the last three decades.

Sunspots are surface concentrations of the star's magnetic field and the more there are, the more energy the Sun is emitting. The dark features have been observed and recorded regularly since 1610.

Scientists have tried to reconstruct previous sunspot activity using ice cores and tree rings. These contain isotopes, such as carbon-14 and beryllium-10, created when high-energy particles from deep space, called cosmic rays, slam into the atmosphere. Fewer cosmic rays reach the Earth when the Sun is very active, because the charged particles from the Sun deflect them.

Now, a team led by Sami Solanki of the Max-Planck-Institut fur Sonnensystemforschung in Katlenburg-Lindau, Germany, has analysed records of trees preserved in riverbeds and bogs that date back 11,400 years to produce the most precise study yet of sunspot history.

Back in time
The team started by using sunspot records to calibrate models of how carbon-14 in tree rings correlate withsolar activity. The models "reproduce the observed record of sunspots extremely well, from almost no sunspots during the seventeenth century to the current high levels", writes Paula Reimer, a paleoclimate expert at Queen's University, Belfast, UK, in an article accompanying the research paper in Nature.

They then extrapolated the tree ring data backwards in time and discovered that no period in the last 8000 years has been as active as the last 70. About 75 sunspots have appeared every year in this period, compared to an annual average of about 30 over the last 11,400 years.

"We are living in extraordinary times as far as solar activity is concerned," says study co-author Manfred Schussler. "Extended periods of high activity seem to be much more rare than we previously thought."

Indeed, the data also showed that high activity periods only occurred for about 10% of the period studied, and tended to last for about three decades. "That's one of the interesting things - this latest cycle has already lasted longer than most do," says Reimer.

Inside the Sun

Models of the Sun can account for the well-known 11-year-long cycle of solar activity but the underlying reason for the 70-year high is unknown. "There is a consensus that the magnetic field underlying the solar activity is generated in the solar interior, but the details of this mechanism are still not understood," Schussler told New Scientist.

Furthermore, previous data from carbon-14 studies of tree rings suggest patterns change on scales of 200 years. "It seems like that periodicity should be driven by the Sun, but people argue back and forth on this all the time," Reimer told says. That is because the total energy emitted by the Sun actually changes by a relatively small amount as the number of sunspots varies.

The new research will allow scientists to see if past climate changes "are too large to be explained by the sunspot cycle alone", Reimer says.

She notes that the current upsurge in sunspots is not enough to account for the approximate 0.5C rise from pre-industrial temperatures over the last 30 years.

Journal reference: Nature (vol 431, p 1047, p 1084)

Maggie McKee

Article From: http://www.newscientist.com/news/news.jsp?id=ns99996591


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