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Through
dated geological records scientists have known for decades
that variations in the Earth's orbit around the sun –
subtle changes in the distance between the two –
control ice ages. But, for the first 2 million years of
the Northern Hemisphere Ice Age there has always been
a mismatch between the timing of ice sheet changes and
the Earth's orbital parameters.
A new model of ice volume change developed by Boston University
researchers Maureen Raymo and Lorraine Lisiecki proposes
a reason for this discrepancy. Like other models, it is
consistent with traditional Milankovitch theory – which
holds that the three cyclical changes in the Earth's orbit
around the Sun (obliquity, precession, and eccentricity)
influence the severity of seasons and high latitude temperatures
over time. However, the new model differs from earlier ones
in that it allows for a much more dynamic Antarctic ice
sheet.
According to the researchers, from 3 million years ago
to about 0.8 million years ago, Northern Hemisphere ice
volume appears to have varied mostly with the 41,000 year
period of obliquity – the periodic shift in the direction
or tilt of Earth's axis. However, summer insolation (incoming
solar radiation), which is widely believed to be the major
influence on high-latitude climate and ice volume change,
is typically dominated by the 23,000 year precessional
period – the slow "wobble" of the Earth
on its axis.
"Because summer insolation is controlled by precession,
and summer heating controls ice sheet mass balance, it is
difficult to understand why the ice volume record is dominated
by the obliquity frequency," said Dr. Raymo. "It's
not a complete mismatch, but the precession frequency we
think should be strong in geological records is not."
The new model proposes that during this time, ice volume
changes occurred in both the Northern Hemisphere and Antarctica,
each controlled by different amounts of local summer insolation
paced by precession.
"The reason the frequency is not observable in records
is because ice volume change occurred at both poles, but
out of phase with each other. When ice was growing in
the Northern Hemisphere, it was melting in the Southern,"
said Raymo.
The team believes scientists have been operating under
the assumption that Antarctica has been exceptionally
stable for 3 million years and very difficult to change
climatically. "We don't tend to think of ice volume
in that region as varying significantly, even on geologic
time scales," said Raymo. "However, only a modest
change in Antarctic ice mass is required to "cancel"
a much larger Northern ice volume signal."
Records used to measure the ice volume, such as sea levels,
integrate the whole world. According to Raymo, the new
model demonstrates that while the precession frequency
is actually strong in ice volume changes at each pole,
in geologic records Northern and Southern hemisphere ice
volume trends act to cancel each other out at this frequency.
The paper, which was published online today and will
appear in an upcoming issue of the journal Science, proposes
that the Antarctic ice sheet is more dynamic and far more
capable of change than previously believed.
"If our theory holds true, it is a cause for concern
with regard to climate changes not associated with orbital
patterns as well," said Raymo.
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Science
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