- WASHINGTON - An old theory
about why ice ages occur when they do may actually be right, a U.S. researcher
said on Thursday.
- He said variations in the Earth's orbit around the Sun
could explain why ice ages, which occur about every 100,000 years, have
not been more regular.
- "The earth circles the sun every year in an orbit
that becomes either more elliptical or less elliptical over tens of thousands
of years," Jose Rial, a professor of geophysics at the University
of North Carolina at Chapel Hill, said in a statement.
- "Variations in the orbit over time chiefly account
for the ice ages as differing amounts of sunlight warm the planet."
- The shift from circular to elliptical and back is one
of several known long-term changes in the Earth's orbit known as Milankovich
- They are named after Serbian astronomer-mathematician
Milutin Milankovitch, whose theories about ice age timings were accepted
from the 1920s until about 30 years ago.
- The variation in the orbit, known as "eccentricity,"
has two cycles " one every 100,000 years and one every 413,000 years.
But while the 100,000-year cycle shows up in studies of ancient climate
" taken from glacial ice-core samples, old sediments and elsewhere
" there has been no evidence of the 413,000-year cycle.
- Writing in the journal Science, Rial said he found it
and may thus have exonerated Milankovitch.
- He looked at isotopes, or variants, of oxygen found by
drilling into the sea floor. Such "heavy" oxygen is found more
commonly when it is cold.
- Rial found evidence of both the 100,000 and 413,000 year
cycles, but said they were not so easy to spot because the two "interfered"
with each other in much the same way that interfering with radio waves
" a process known as modulation " allows broadcasters to send
information in the form of sound.
- He said the pattern looked like an FM (frequency modulated)
- He does not know the physical mechanism behind this but
says it helps explain why the ice ages seemed to occur at irregular intervals.
- Other experts had suggested that changes in the plane
of the earth's orbit, or periodic dipping into a dust ring that also circles
the sun, might have triggered ice ages. "Understanding the climate
of the recent geological past is important because finding out how Earth's
environment has transformed heat from the sun into climate variations can
help predict future global climate change," Rial said.