May 14, 2018, by Tim Radford
Ancient logjam in the Petrified Forest National Park. Image: By US National Park Service, via Wikimedia Commons
There is such a thing as planetary climate influence which changes the Earth over aeons. Now scientists know just how it happens.
LONDON, 14 May, 2018 – There is now firm evidence in the ancient rocks of planetary climate influence – a climate cycle that lasts for 405,000 years. This confirmation of celestial clockwork – the swing in planetary climate happens because the Earth’s orbit is periodically distorted by the combined tug of Venus and Jupiter – has been tracked back through evidence in the rocks for the last 215 million years.
The 405 kiloyear cycle is only one of a series of cycles that change the levels of sunshine received by Earth on its journey around the sun. Researchers have also known about and documented periodic shifts over cycles of 21,000 years, 41,000 years and 100,000 years, all of them also driven by astronomical change. But the 400,000 year-plus pattern of change, they say, is the most predictable, and most regular.
And although these changes have been known to have affected the pattern of the Ice Ages, the latest study pushes the influence of the longest of them back in time to a date even before the emergence of the dinosaurs.
“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. The beauty of this one is that it stands alone. It doesn’t change. All the other ones move over it,” said Dennis Kent, of the Lamont-Doherty Earth Observatory at Columbia University, and Rutgers University, in the US.
“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.
”All the carbon dioxide we’re pouring into the air right now is the obvious big enchilada. That’s having an effect right now. The planetary cycle is a little more subtle”
“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.”
Such studies are fundamental explorations of the machinery of climate. By understanding how conditions changed in the distant past, researchers can better appreciate the speed and danger of climate change today, driven by profligate combustion of fossil fuels that return to the atmosphere greenhouse gases locked up in the rocks for more than 100 million years.
And the importance of the latest research – published in the Proceedings of the National Academy of Sciences – is that it highlights at least some of the machinery of ancient change and at the same time makes it easier to interpret other evidence in the fossil record.
The planet’s orbit around the sun is an ellipse: sometimes it is almost but not quite a circle, and sometimes the orbit is elongated, which means that the total sunshine that hits the planet changes. This is not the only factor: the planet wobbles on its axis, and its tilt relative to the sun changes over a long cycle as well, and all these things affect climate.
But the most dramatic change occurs every 400,000 years or so, when the elliptical shape is elongated by as much as 5% as a consequence of the gravitation pull of other planets as they loop around the sun, to points where they can tug at Earth.
And Professor Kent and his colleagues have been able to read evidence of this regular cycle in huge cores of rock drilled from a geological feature in the Petrified Forest National Park in Arizona, and rock cores from ancient sediments beneath New York and New Jersey.
The first samples provided the radio-isotopes that geologists use to precisely date rocks. The second confirmed a pattern of wet and dry periods over long cycles of time. And since both sets of rocks contained palaeo-magnetic evidence of the north and south poles’ periodical reversals, researchers were able to calibrate the evidence and arrive at an accurate calendar of changing climate conditions, every 405 millennia.
The study reveals nothing very significant about the effect of long-term astronomical influences on climate change today. Right now Earth’s orbit is near to a circle. In the absence of human interference, Earth should be nearing the end of a long-term warming trend and, according to the astronomical clockwork, should be heading for another Ice Age. Professor Kent is not so sure.
“Could happen. Guess we could wait around and see,” he says. “On the other hand all the carbon dioxide we’re pouring into the air right now is the obvious big enchilada. That’s having an effect right now. The planetary cycle is a little more subtle.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.