The sun's
surface has been fairly blank for the last couple of years, and that has some
worried that it may be entering another Maunder minimum, the sun's 50-year
abstinence from sunspots, which some scientists have linked to the Little Ice
Age of the 17th century.
Could a new
sunspot
drought plunge us into another decades-long cold spell?
It's not
very likely, says David Hathaway a solar physicist at NASA's Marshall Space
Flight Center in Huntsville, Ala.
The
question came up after an international solar conference held last week at
Montana State University, where scientists discussed the dearth of solar
activity in the last couple years.
"It
continues to be dead," said Saku Tsuneta with the National Astronomical
Observatory of Japan and program manager for the Hinode
solar mission. "That's a small concern, a very small concern,"
because the period of inactivity seems to be going on longer than normal. Some
scientists think such inactive periods, such as the Maunder minimum, are
responsible for cold spells in the past, such as the Little Ice Age.
The sun's
energy drives all climate and weather on Earth. And Hathaway does agree there
are good indications that fluctuations in solar output related to sunspot
cycles influence the Earth's climate. And the Maunder minimum isn't the only
evidence — scientists have linked two smaller sunspot minimums (periods of time
with very few sunspots) in the early 19th century to cold spells, as well as
periods before the Maunder minimum deduced from tree ring records, he said.
But the sun
isn't the only thing that influences our climate: volcanic eruptions,
large-scale phenomena such as El Nino, and, more recently, the accumulation of
greenhouse gases in the atmosphere also affect the global climate.
Prior to
the industrial revolution, the sun probably accounted for about 10 to 30
percent of climate variability, Hathaway told SPACE.com, but now that
greenhouse gases have started to build up, "the sun's contribution is getting
smaller and smaller," he added.
Short
solar cycle?
Solar
cycles are the ebb and flow of the sun's magnetic activity over a roughly
11-year period, which affects the formation of solar features such as solar
flares and sunspots. Sunspots are cooler, dimmer areas on the sun's surface.
The last
solar cycle, which peaked in 2001, was a particularly intense one, with an
upsurge in solar storms between 2000 and 2002. Such intense activity in the
peak of the solar cycle tends to lead to less activity at the end of the cycle.
Signs of
the current, new solar cycle (which actually overlaps with the last cycle)
showed up in November 2006, and its first sunspots were seen in January of this
year, and again in April, Hathaway said. So already that rules out another
Maunder minimum, Hathaway says, since this solar cycle has already begun
producing spots, even if there haven't been many of them yet.
This cycle
is just simply "off to a slow start," Hathaway said.
The last
three solar cycles were also what Hathaway calls "big cycles," meaning they had
more than the average number of sunspots (the average is around 110 to 120
sunspots on any given day during the cycle's maximum). It's not unusual for
such a spate of prolific
cycles to be followed my more muted solar cycles (such as the cycle that
preceded the last three biggies).
Hathaway
says that solar physicists are divided on their predictions of this new solar
cycle — some say it will be small, others say it will be another doozy.
Predictions have ranged anywhere from 75 to 150 maximum spots during its peak.
"There really are two camps," Hathaway said. Whatever the number ends up being,
though, "it's not zero," he added.
Why the sun
is so fickle in its sunspot production is still something of a quandary. "We
still don't fully understand how the sun does this," Hathaway noted.
Scientists
do know that two processes on the sun influence sunspot activity. The first is
the strength of the shear zone (which lies at the base of the sun's convection
zone, about 30 percent of the way inside the sun).
The shear
zone can stretch out the sun's magnetic fields, which then affect the strength
of the solar cycle, and thus the number of sunspots. The second process, called
the meridional circulation, describes the flow of stellar material from the
equator toward the poles and back again, and it can also influence the cycle's
strength.
Towards the
end of the last solar cycle, for example, "that flow seemed to have slowed down
quite a bit," weakening the cycle and reducing the number of sunspots, Hathaway
said. "It's the slowest we've ever seen," he added.
So based on
the nearly 400-year record of sunspots that scientists have, this slow start
isn't unusual. "It's just taking its merry old time," Hathaway said.
(His
personal prediction is that sunspot activity will start to pick up in the next
few months. "I keep looking everyday," he said.)
No
impending ice age
Though
there is debate about how and whether the Maunder minimum actually caused the Little
Ice Age, scientists have proposed a few hypotheses as to how it could have done
so.
One idea
springs from the fact that the sun emits much more ultraviolet radiation when
it is covered in sunspots, which can affect the chemistry of Earth's
atmosphere. The other is that when the sun
is active, it produces tangled magnetic fields that keep out galactic
cosmic rays. Some scientists have proposed that a lack of sunspots means these cosmic
rays are bombarding Earth and creating clouds, which can help cool the
planet's surface.
But these
ideas aren't yet proven, and anyway, the sun's contribution is small compared
to volcanoes, El Nino and greenhouse gases, Hathaway notes.
Even if
there were another Maunder minimum, he says, we would still suffer the effects
of greenhouse gases and the Earth's climate would remain warm. "It doesn't
overpower them at all," Hathaway said.
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