Better understanding of planetary motions’ role in driving natural climate swings may lead to breakthroughs in the study of evolution, including the rise of the dinosaurs. Kevin Krajick

Scientists Identify 405,000-year Climate Cycle on Earth

Ancient rocks prove theory of overriding orbital behavior that exacerbates shorter-term effects, like those Milankovitch cycles that cause ice ages

Astrophysicists had been saying so for decades, and now geologists have proved it: Earth’s orbit goes through 405,000-year cycles and has done so for hundreds of millions of years. Now, physical proof of the cycle has been found by analyzing ancient rocks in Arizona, New York and New Jersey, explain Dennis Kent and Paul Olsen of Columbia University’s Earth Institute, in the Proceedings of the National Academy of Sciences.

Every 405,000 years, Earth’s orbit around the sun goes from nearly circular to about 5 percent elliptical, and back again.

Sorry, but our climate change problem is still our fault. It has absolutely nothing to do with this mega-cycle, which doesn’t affect Earth’s climate directly, Olsen explains. It does, however, exacerbate shorter-term cycles that do act directly.

This is from Venus too

The mega-cycle is chiefly caused by the gravitational pull of other planets in the solar system.

Despite the graphics in kids’ books, the planets’ orbits around the sun are not precise concentric circles.

Mars, for instance, has a slightly eccentric orbit of 0.0934. Earth’s is eccentric too, but at 0.0167 is much less so than Mars’.

Right now, according to that eccentricity calculation, Earth’s orbit is practically circular.

Kevin Krajick/Lamont-Doherty Earth Observatory

The planets don’t move at the same rate through space around the sun, either. The upshot of differences in their positions and eccentricities is that the planets’ influence on each other’s orbits changes.

The Columbia U. scientists believe the mega-cycle is caused chiefly by Venus and Jupiter.

Venus isn’t big but is nearest to us, and Jupiter is a monster – 2.5 times all the other planets combined, with immense gravitational pull, they explain.

When dinosaurs were young

The evidence was found in 450-meter-long (1,476 feet) rock cores that Kent and his co-authors drilled from a hill in Arizona’s Petrified Forest National Park, from suburban New York and from New Jersey (the site of “exquisitely preserved” massive volcanic spasms). The drills were done in previous years.

The Arizona rock dated to the time of the earliest dinosaurs, the Triassic phase: around 210 million years ago. Meanwhile, cores from New York and New Jersey showed “exquisitely preserved” signs alternating wet and dry cycles. Dating these was a problem, but the scientists observed evidence of reversals in magnetic polarity at all three sites. They showed that all three sites developed at the same time, and that the 405,000-year interval indeed exerts a kind of master control over climate swings.

The shorter-term cycles affecting our weather include 10,000-year Milankovitch cycles, which describe changing eccentricity in Earth’s orbit and are believed to be linked to ice ages. There is a 41,000-year cycle in the tilt of Earth’s axis relative to its orbit around the sun. And there is a 21,000-year cycle caused by a wobble of the planet’s axis.

Kevin Krajick/Lamont-Doherty Earth Observatory

Above them all rides this newly noticed 405,000-year cycle.

“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,” says 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 scientists proved that this long cycle has been governing Earth’s behavior for hundreds of millions of years, from before the first dinosaur hatched from its egg.

If you live long enough

Ultimately, it’s really hard to tease out the cycles: They have almost certainly changed over time, but we don’t know how and they’re all constantly proceeding against each other.

Sometimes some are out of phase with others and will cancel each other out; at other times, they may line up and trigger sudden, drastic changes. 

That said, Kent and Olsen estimate that every 405,000 years, when orbital eccentricity is at its stretched peak, seasonal differences – that are caused by shorter cycles, not the big one – will become more intense. Summers will be hotter, winters colder, dry times will become hyper-arid, and so on.

Exactly the opposite will be true 202,500 years later, when the orbit is at its most circular.

Admittedly, it is true that the longer off something happened, the wider margins of error are likely to be.

“We are using basically the same kinds of math to send spaceships to Mars – and sure, that works,” Olsen says. “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.”

So, where are we now in this mega-cycle? We’re in the nearly circular part. Which means we can expect what? Nothing we'd notice.

“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 timescales that matter to us.” Such as the carbon dioxide we’re putting into our atmosphere. That, says Kent, 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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