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A research team led by Rutgers University’s Professor Dennis Kent has 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, but Professor Kent and co-authors have found the first verifiable physical evidence; they showed that the cycle has been stable for at least 215 million years and is still active today.
Every 405,000 years, gravitational tugs from Jupiter and Venus elongate Earth’s orbit. Image credit: Jenny Mottar / NASA.
Researchers have for decades posited that Earth’s orbit around the Sun goes from nearly circular to about 5% 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.
“It’s an astonishing result because this long cycle, which had been predicted from planetary motions through about 50 million years ago, has been confirmed through at least 215 million years ago,” Professor Kent said.
“Scientists can now link changes in the climate, environment, dinosaurs, mammals and fossils around the world to this 405,000-year cycle in a very precise way.”
The researchers linked reversals in the Earth’s magnetic field to sediments with and without zircons as well as to climate cycles.
“The climate cycles are directly related to how the Earth orbits the Sun and slight variations in sunlight reaching Earth lead to climate and ecological changes. The Earth’s orbit changes from close to perfectly circular to about 5% elongated especially every 405,000 years,” Professor Kent said.
He and his colleagues studied the long-term record of reversals in the Earth’s magnetic field in sediments in the Newark basin, a prehistoric lake that spanned most of New Jersey, and in sediments with volcanic detritus including zircons in the Chinle Formation in Petrified Forest National Park in Arizona.
They collected a core of rock from the Triassic period, some 202 million to 253 million years ago.
The results, published in the Proceedings of the National Academy of Sciences, showed that the 405,000-year cycle is the most regular astronomical pattern linked to the Earth’s annual turn around the Sun.
Prior to this study, dates to accurately time when magnetic fields reversed were unavailable for 30 million years of the Late Triassic.
“The planetary motions that spur climate swings are known as Milankovitch cycles, named for the Serbian mathematician who worked them out in the 1920s,” the scientists explained.
“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.”
“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,” they said.
“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,” the researchers said. “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.”
“This 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,” said George Mason University’s Professor Linda Hinnov, who was not involved in the study.
“Among other things, it could lead to new insights into early dinosaur evolution.”
Because of all the competing factors at work, there is still much to learn.
“This is truly complicated stuff. We are using basically the same kinds of math to send spaceships to Mars, and sure, that works,” said co-author Dr. Paul Olsen, of Columbia University.
“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.
“If you were wondering, the Earth is currently in the nearly circular part of the 405,000-year period,” Professor Kent said.
“What does that mean for us? Probably not anything very perceptible. It’s pretty far down on the list of so many other things that can affect climate on times scales that matter to us.”
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Dennis V. Kent et al. Empirical evidence for stability of the 405-kiloyear Jupiter–Venus eccentricity cycle over hundreds of millions of years. PNAS, published online May 7, 2018; doi: 10.1073/pnas.1800891115
