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. It
appears this week in the Proceedings of the National Academy of
Sciences.
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.
Linda Hinnov, a professor at George Mason University who studies the
deep past, said 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. 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.”
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.”
THE EARTH INSTITUTE AT COLUMBIA UNIVERSITY
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