- 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)
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