Scientists drilling deep into ancient rocks in the Arizona desert say they have documented a gradual shift in Earth's orbit that repeats regularly every 405,000 years, playing a role in natural climate swings. Astrophysicists have long hypothesized that the cycle exists based on calculations of celestial mechanics, but the authors of the new research have found the first verifiable physical evidence.
They showed that the cycle
has been stable for hundreds of millions of years, from before the rise of
dinosaurs, and is still active today. The research may have implications not
only for climate studies, but our understanding of the evolution of life on
Earth, and the evolution of the Solar System.
The new study lends support
to previous studies by others that claim to have observed signs of the
405,000-year cycle even further back, before 250 million years ago, says Linda
Hinnov, a professor at George Mason University who studies the deep past
"Among other things, she said, it "could lead to new insights into
early dinosaur evolution." She called the findings "a significant new
contribution to geology, and to astronomy."
Scientists have for decades
posited that Earth's orbit around the sun goes from nearly circular to about 5
percent elliptical, and back again every 405,000 years. The shift is believed
to result from a complex interplay with the gravitational influences of Venus
and Jupiter, along with other bodies in the Solar System as they all whirl
around the Sun like a set of gyrating hula-hoops, sometimes closer to one
another, sometimes further. Astrophysicists believe the mathematical
calculation of the cycle is reliable back to around 50 million years, but after
that, the problem gets too complex, because too many shifting motions are at
play.
"There are other,
shorter, orbital cycles, but when you look into the past, it's very difficult
to know which one you're dealing with at any one time, because they change over
time," said lead author Dennis Kent, an expert in paleomagnetism at
Columbia University's Lamont-Doherty Earth Observatory and Rutgers University.
"The beauty of this one is that it stands alone. It doesn't change. All
the other ones move over it."
The new evidence lies within
1,500-foot-long cores of rock that Kent and his coauthors drilled from a butte
in Arizona's Petrified Forest National Park in 2013, plus earlier deep cores
from suburban New York and New Jersey. The Arizona rocks in the study formed
during the late Triassic, between 209 million and 215 million years ago, when
the area was covered with meandering rivers that laid down sediments. Around
this time, early dinosaurs started evolving.
The scientists nailed down
the Arizona rocks' ages by analyzing interspersed volcanic ash layers
containing radioisotopes that decay at a predictable rate. Within the sediments,
they also detected repeated reversals in the polarity of the planet's magnetic
field. The team then compared these findings to the New York-New Jersey cores,
which penetrated old lakebeds and soils that hold exquisitely preserved signs
of alternating wet and dry periods during what was believed to be the same
time.
Kent and Olsen have long
argued that the climate changes displayed in the New York-New Jersey rocks were
controlled by the 405,000-year cycle. However, there are no volcanic ash layers
there to provide precise dates. But those cores do contain polarity reversals
similar to those spotted in Arizona. By combining the two sets of data, the
team showed that both sites developed at the same time, and that the
405,000-year interval indeed exerts a kind of master control over climate
swings. Paleontologist Paul Olsen, a coauthor of the study, said that the cycle
does not directly change climate; rather it intensifies or dampens the effects
of shorter-term cycles, which act more directly.
The planetary motions that
spur climate swings are known as Milankovitch cycles, named for the Serbian
mathematician who worked them out in the 1920s. Boiled down to simplest terms,
they consist of a 100,000-year cycle in the eccentricity of Earth's orbit,
similar to the big 405,000-year swing; a 41,000-year cycle in the tilt of
Earth's axis relative to its orbit around the Sun; and a 21,000-year cycle
caused by a wobble of the planet's axis. Together, these shifts change the
proportions of solar energy reaching the Northern Hemisphere, where most of the
planet's land is located, during different parts of the year. This in turn
influences climate.
In the 1970s, scientists
showed that that Milankovitch cycles have driven repeated warming and cooling
of the planet, and thus the waxing and waning of ice ages over the last few
million years. But they are still arguing over inconsistencies in data over that
period, and the cycles' relationships to rising and falling levels of carbon
dioxide, the other apparent master climate control. Understanding how this all
worked in the more distant past is even harder. For one, the frequencies of the
shorter cycles have almost certainly changed over time, but no one can say
exactly by how much. For another, the cycles are all constantly proceeding
against each other. Sometimes some are out of phase with others, and they tend
to cancel each other out; at others, several may line up with each other to
initiate sudden, drastic changes. Making the calculation of how they all might
fit together gets harder the further back you go.
Kent and Olsen say that
every 405,000 years, when orbital eccentricity is at its peak, seasonal
differences caused by shorter cycles will become more intense; summers are
hotter and winters colder; dry times drier, wet times wetter. The opposite will
be true 202,500 years later, when the orbit is at its most circular.
During the late Triassic,
for poorly understood reasons, the Earth was much warmer than it is now through
many cycles, and there was little to no glaciation. Then, the 405,000-year
cycle showed up in strongly alternating wet and dry periods. Precipitation
peaked when the orbit was at its most eccentric, producing deep lakes that left
layers of black shale in eastern North America. When the orbit was most
circular, things dried up, leaving lighter layers of soil exposed to the air.
Jupiter and Venus exert such
strong influences because of size and proximity. Venus is the nearest planet to
us—at its farthest, only about 162 million miles—and roughly similar in mass.
Jupiter is much farther away, but is the Solar System's largest planet, 2.5
times bigger than all others combined.
Kent and Olsen say that
because of all the competing factors at work, there is still much to learn.
"This is truly complicated stuff," said Olsen. "We are using
basically the same kinds of math to send spaceships to Mars, and sure, that
works. But once you start extending interplanetary motions back in time and tie
that to cause and effect in climate, we can't claim that we understand how it
all works." The metronomic beat of the 405,000-year cycle may eventually
help researchers disentangle some of this, he said.
If you were wondering, the
Earth is currently in the nearly circular part of the 405,000-year period. What
does that mean for us? "Probably not anything very perceptible," says
Kent. "It's pretty far down on the list of so many other things that can
affect climate on times scales that matter to us."
Kent points out that
according to the Milankovitch theory, we should be at the peak of a 20,000-some
year warming trend that ended the last glacial period; the Earth may eventually
start cooling again over thousands of years, and possibly head for another
glaciation. "Could happen. Guess we could wait around and see," said
Kent. "On the other hand, all the CO2 we're pouring into the air right now
is the obvious big enchilada. That's having an effect we can measure right now.
The planetary cycle is a little more subtle."
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