Ice sheets have ebbed and flowed over Earth’s surface for eons. Now scientists have analyzed tiny bits of rock transported by glaciers and gained a better understanding of recent glacial cycles. The team found that precession—gradual changes in the direction of Earth’s axis of rotation—has played an important role in the breakup of Northern Hemisphere ice sheets over the past 1.7 million years. And during the late Pleistocene, that precession-driven collapse coincided with deglaciation, the researchers reported in May in Science.
Here and Gone, Again and Again
Just 30,000 years ago—a blink in geologic time—significant swaths of Earth’s landmasses were covered in glacial ice. That time period was the so-called Last Glacial Maximum, and large ice sheets reigned supreme, said Stephen Barker, a paleoclimatologist at Cardiff University in the United Kingdom. “Where I am here in South Wales, there’d be an ice sheet right next door to me.”
But the majority of those ice sheets have since retreated, and the planet is now in an interglacial period. That shift, from a largely ice covered world to one in which ice is sparser, represents a cycle that has repeated many times, said Barker. “Over the last million years, there have been seven or eight glacial cycles.”
Eyes on the Sun
The question of what has driven the planet’s glacial cycles over the past few million years has long preoccupied scientists. Solar radiation is critically important, researchers have agreed. But the energy received from the Sun at any one point on Earth varies according to two long-term cycles: precession and obliquity. Precession refers to changes in the direction of Earth’s axis of rotation, and obliquity is the tilt of Earth’s rotational axis as the planet orbits the Sun.
These two so-called Milankovitch cycles modulate the amount of solar energy received by Earth’s surface over periods of roughly 23,000 and 41,000 years, respectively. But it’s challenging to determine which of those rhythms correlates most strongly with the planet’s glacial cycles, said Barker. “People have been trying to pick one or the other.”
To help answer that question, Barker and his colleagues analyzed more than 9,000 bits of rock larger than 0.15 millimeter in diameter. The researchers painstakingly picked that material out of a sediment core drilled several hundred kilometers off the southwestern coast of Iceland. These grains of rock reveal the timing of when ancient ice sheets in the Northern Hemisphere grew and ultimately broke up, Barker and his colleagues suggested. That’s because ice moving over Earth’s surface entrains debris, and such material sinks to the seafloor after it’s carried offshore by icebergs.
Barker and his collaborators calculated the rate at which this so-called ice-rafted debris was deposited on the seafloor. “We literally count it,” he said. “We work out how much has been delivered per unit time.” Spikes in the concentration of ice-rafted debris correspond to the breakup of Northern Hemisphere ice sheets, the researchers concluded.
A Hidden Role
The ice-rafted debris the team studied was deposited over the past roughly 1.7 million years. That time span encompasses two important periods, said Barker. There’s the period prior to the Mid-Pleistocene Transition, when glacial cycles were roughly 41,000 years long. And there’s the more recent period, during which glacial cycles have consistently lasted about 100,000 years.
Barker and his colleagues found that glacial cycles before and after the Mid-Pleistocene Transition were correlated with both precession and changes in obliquity. The team showed that minima in precession—meaning that summer in the Northern Hemisphere occurs when the planet is closest to the Sun—were tied to ice sheet breakup. And times of decreasing obliquity were associated with ice sheet growth.
It was particularly surprising to uncover the role of precession prior to the Mid-Pleistocene Transition, said Barker. That’s because the shorter glacial cycles long have been assumed to have been driven solely by changes in obliquity occurring at the same cadence, without any influence from precession, he said. “I nearly fell off my chair when I saw that.”
Furthermore, before the Mid-Pleistocene Transition, ice sheet breakup didn’t always spell the end of an ice age, Barker and his colleagues found. That’s perhaps because ice sheets at that time were limited to higher latitudes, exactly where the effects of obliquity are felt more acutely than those of precession, the researchers suggested. Conversely, after the Mid-Pleistocene Transition, such breakup was often linked to the end of an ice age. One explanation for that difference is that Northern Hemisphere ice sheets might have been larger after the Mid-Pleistocene Transition, and therefore the effects of both obliquity and precession would have been necessary to catapult the planet into a new state. “We need both to help get rid of these larger ice sheets when their time is up,” said Barker.
These results shed light on long-term cycles that affect our planet’s climate, said Tim Naish, a paleoclimatologist at Victoria University of Wellington in New Zealand who was not involved in the research. “Earth’s climate system dances to the beat of these Milankovitch cycles.”
—Katherine Kornei (@KatherineKornei), Science Writer