Earth’s tilt angle trigger for ending ice ages

International research covering the past million years of global glaciations shows that small changes in the tilt of the Earth’s tilt angle – obliquity – is important for triggering the end of ice ages, or glacial terminations.

The study, published in the journal Science, challenges previous contentions that precession – rotational changes that govern when the Earth is closest to the Sun – is most important.

“Ice ages are the most fundamental feature of Earth’s climate over the last few million years,” says corresponding author Russell Drysdale from the University of Melbourne, Australia.

Nearly 200 years ago, it was proposed that transitions from glacial to interglacial periods – shorter intervals with temperate climates such as the current Holocene that started about 11,600 years ago – are triggered by changes in Earth’s orbital geometry, and this notion ultimately became part of Milankovitch’s astronomical theory of ice ages in the early 1900s.

But it’s been difficult to determine which orbital parameter is most important, as techniques to study ocean sediments, which have been used to understand glacial terminations, have been unable to properly date them past 40,000 years ago.

And timing is important, says Drysdale.

“Refining what exactly drives the ice-age cycles requires an understanding of when exactly they occurred,” he explains.

Advances in geochronology and better understanding of spatial responses to terminations have facilitated the use of speleothems to indirectly date ocean sediments, a technique that a Chinese study used in 2016 to date terminations back to 640,000 years ago – as far as uranium-thorium dating would allow.

That study suggested that precession was most important, which determines the occurrence of high summer insolation intensity thought necessary to melt the ice sheets in the Northern Hemisphere and trigger a glacial termination.

Drysdale’s group used their advances in a new uranium-lead dating technique to peek further back at this “ice-age problem”.

First, they constructed a detailed record of ocean conditions between 980,000 and 800,000 years ago – a key part of Earth’s history that covers two terminations – from ocean sediments sampled near Portugal in the North Atlantic, which lies in the path of meltwaters from collapse of the massive North American and Eurasian ice sheets.

Then they sampled cave mineral deposits from speleothems from the nearby Corchia Cave in Italy, which has a climate closely aligned with the North Atlantic.

“When the North Atlantic coughs, Corchia Cave sneezes,” says Drysdale.

From these speleothems the team compiled a detailed climate record and time scale.

“Because the speleothems and ocean sediments record the same climate signals,” Drysdale explains, “we could marry the two data sets together, and thus tie the detailed ocean record of both terminations to the precise speleothem chronology.”

Their key finding was that obliquity is as important as precession – if not more so – for triggering the glacial terminations. 

“This is because high tilt angles mean stronger summers in both hemispheres,” says Drysdale. “Once triggered, high summer energy in the Northern Hemisphere high latitudes, which requires both high obliquity and strong precession, ensures a termination’s completion.” 

Their analysis also refuted claims, once and for all, that the two terminations bracket the “100,000-year cycle”.

Instead, he says we’ve gone from predominantly single 40,000-year cycles prior to around one million years ago to a cluster of two 40,000-year cycles.

But the rates at which the two terminations were completed were vastly different; the older one, 960,000 years ago, took a few thousand years while the more recent one, about 875,000 years ago, took around 10,000 years.

Comparing their data with previous studies for nine younger terminations they showed that obliquity played a key role in them and their duration.

By using information about the timing of termination events, Drysdale says they’ve been able to refine understanding about the role of astronomical influences on sweeping changes in Earth’s climate.

Does this throw any light on our next ice age?

No, he says. “The closest orbital analogue to the Holocene occurred about 785,000 years ago, which was not covered by our study. This interglacial lasted 10,000 to 15,000 years, depending on where you draw the line.”

They are now continuing their work into the Early Pleistocene (“the so-called 40,000-year world”) to further test their emerging hypothesis that tilt and precession are both important for the pacing of these cycles.

Related reading: Volcanoes nudged the Moon off its axis

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