As levels of the greenhouse gas carbon dioxide rise and warm the globe, Antarctica's ice will become more vulnerable to cycles on an astronomical scale, particularly the tilt of our planet is as it spins around its axis.

New research finds that over 30 million years of history, Antarctica's ice sheets responded most strongly to the angle of Earth's tilt on its axis when the ice extends into the oceans, interacting with currents that can bring warm water lapping at their margins and leading to increased melting. The effect of the tilt peaked when carbon dioxide levels were similar to what scientists predict for the next century, if humans don't get emissions under control. [Collapsing Beauty: Image of Antarctica's Larsen Ice Shelf]

As carbon dioxide levels push past 400 parts per million, the climate will become more sensitive to the Earth’s tilt, or obliquity, researchers reported Jan. 14 in the journal Nature Geoscience.

"Really critical is the amount of carbon dioxide in the atmosphere," said study co-author Stephen Meyers, a paleoclimatologist at the University of Wisconsin, Madison.

A scenario of high carbon dioxide and high tilt angle could be particularly devastating to the miles-thick ice covering Antarctica.

Reconstructing the past

Over about 40,000 years, the Earth's axis tilts back and forth "like a rocking chair," Meyers said. Currently this obliquity is about 23.4 degrees, but it can be as little as 22.1 degrees or as much as 24.5 degrees.

The tilt matters for when and where sunlight hits the globe, and can thus influence climate .

To reconstruct a history of how Antarctica's ice has responded to this tilt, Meyers and his co-authors used a few sources of information on the Earth's climate past. One source was calcium carbonate from the ocean bottom, left behind by single-celled organisms called benthic foraminifera. These organisms excrete a calcium carbonate shell around themselves, locking in a global, continuous record of the chemistry of the oceans and atmosphere.

Sediment records from right around Antarctica provided another source of climate history — a specialty of study co-author and paleoclimatologist Richard Levy of GNS Science and Victoria University of Wellington in New Zealand. These sediments, drilled from the ocean bottom in long, columnar cores, also hold a record of the past. A glacier, for example, dumps a distinctive mixture of mud, sand and gravel where it sits. These cores provide a very detailed picture of where the ice sheets once were, Meyers said, but there are gaps in the record.

Ice cycles

With data from both sources, the researchers pieced together a history of Antarctica from 34 million to 5 million years ago. The first large ice sheets on Antarctica formed 34 million years ago, Levy said, and year-round sea ice became the norm only 3 million years ago, when carbon dioxide levels fell below 400 parts per million.

From about 34 million years ago to about 25 million years ago, carbon dioxide was very high (600 to 800 ppm) and most of Antarctica's ice was land-based, not in contact with the sea. The continent's ice advance and retreat were relatively insensitive to the planet's tilt at this time, the researchers found. Between about 24.5 million and about 14 million years ago, atmospheric carbon dioxide dropped to between 400 and 600 ppm. Ice sheets advanced more often into the sea, but there wasn't very much floating sea ice. At this time, the planet became quite sensitive to the tilt of Earth's axis. [Images of Melt: Earth's Vanishing Ice]

Between 13 million and 5 million years ago, carbon dioxide levels dropped again, going as low as 200 ppm. Floating sea ice became more prominent, forming a crust over open ocean in the winter and thinning only in the summer. Sensitivity to the Earth's tilt declined.

It's not entirely clear why this change in sensitivity to obliquity occurs, Levy told Live Science, but the reason seems to involve the contact between the ice and the ocean. At times of high tilt, the polar regions warm and the temperature differences between the equator and the poles become less extreme. This, in turn, alters wind and current patterns — which are largely driven by this temperature difference — ultimately increasing the flow of warm ocean water to Antarctica's edge.

When ice is mostly land-based, this flow doesn't touch the ice. But when the ice sheets are grounded against ocean bottom, in contact with the currents, the flow of warm water matters a lot. Floating sea ice appears to block some of the flow, decreasing the ice sheet's tendency to melt. But when carbon dioxide levels are high enough that floating sea ice melts, there's nothing stopping those warm currents. That's when Earth's tilt seems to matter the most, as occurred between 24.5 million and 14 million years ago.

This history spells trouble for Antarctica's future. In 2016, the level of carbon dioxide in Earth's atmosphere leapt past 400 ppm , permanently. The last time in Earth's geologic history that carbon dioxide was this high, there was no year-round sea ice in Antarctica, Levy said. If emissions continue as they are, the sea ice will falter, Levy said, "and we will jump back to a world that hasn't existed for millions of years."

"Antarctica's vulnerable marine-based ice sheets will feel the effect of our current relatively high tilt, and ocean warming at Antarctica's margins will be amplified," he said.

On Monday (Jan. 14), another group of researchers reported that the rate of Antarctic melt is already six times faster than it was just a few decades ago. The researchers found that the continent lost about 40 gigatons of ice per year between 1979 and 1990. Between 2009 and 2017, it lost 252 gigatons of ice per year, on average.

The researchers are now looking into the small variations in sensitivity to Earth's tilt that occur across the three broad patterns that they found, but the main message is already clear, Levy said.

"Antarctic sea ice is clearly important," he said. "We need to push on and figure out ways to meet emissions targets."

Originally published on Live Science.