What Triggered the Ice Ages? The Uncertain Role of CO2

About a million years ago, the earth’s ice ages became colder and longer – with a geologically sudden jump from thinner, smaller glaciers that came and went every 41,000 years to thicker, larger ice sheets that persisted for 100,000 years. Although several hypotheses have been put forward to explain this transition, including a long-term decline in the atmospheric CO₂ level, the phenomenon remains a scientific conundrum.

Two research teams spearheaded by geologists from Princeton University have recently described their attempts to resolve the mystery. A 2019 study measured the CO₂ content in two-million-year-old ice cores extracted from Antarctica, which are by far the oldest cores ever recovered and span the puzzling transition to a 100,000-year ice age cycle that occurred a million years before. A just reported 2020 study utilized seabed sediment cores from the Antarctic Ocean to investigate storing of CO₂ in the ocean depths over the last 150,000 years.

Both studies recognize that the prolonged deep freezes of the ice ages are set off partly by perpetual but regular changes in the earth’s orbit around the sun. That’s the basis of a hypothesis proposed by Serbian engineer and meteorologist Milutin Milankovitch. As shown in the figure below, the earth orbits the sun in an elliptical path and spins on an axis that is tilted. The elliptical orbit stretches and contracts over a 100,000-year cycle (top), while the angle of tilt or obliquity oscillates with a 41,000-year period (bottom), and the planet also wobbles on its axis in a 26,000-year cycle (center).

Milankovitch linked all three cycles to glaciation, but his hypothesis has been dogged by two persistent problems. First, it predicts a dominant 41,000-year cycle governed by obliquity, whereas the current pattern is ruled by the 100,000-year eccentricity cycle as mentioned above. Second, the orbital fluctuations thought to trigger the extended cooling cycles are too subtle to cause on their own the needed large changes in solar radiation reaching the planet – known as insolation. That’s where CO₂ comes in, as one of various feedbacks that amplify the tiny changes that do occur.

Before the 2019 Princeton study, it had been suspected that the transition from 41,000-year to 100,000-year cycles was due to a long-term decline in the atmospheric CO₂ level over both glacial and interglacial epochs. But that belief held when ice-core data went back only about 800,000 years. Armed with their new data from 2 million years in the past, the first Princeton team discovered surprisingly that the average CO₂ level was unchanged over that time span, even though the minimum level dropped after the transition to longer ice age cycles.

This means that the 100,000-year transition can’t be attributed to CO₂, although CO₂ feedback has been invoked to explain the relatively sudden temperature rise at the end of ice ages. Rather, said the study authors, the switch in ice age length was probably caused by enhanced growth of ice sheets or changes in global ocean circulation.

It’s another feedback process involving CO₂ that was investigated by the second Princeton team, who made measurements on tiny fossils embedded in Antarctic Ocean sediments. While it has long been known that the atmospheric CO₂ level and global temperatures varied in tandem over glacial cycles, and that CO₂ lagged temperature, the causes of the CO₂ fluctuations are not well understood.

We know that the oceans can hold more CO₂ than the atmosphere. Because CO₂ is less soluble in warm water than cooler water, CO₂ is absorbed from the atmosphere by cold ocean water at the poles and released by warmer water at the equator. The researchers found that, during ice ages, the Antarctic Ocean stored even more CO₂ than expected. Absorption in the Antarctic is enabled by the sinking of floating algae that carry CO₂ deep into the ocean before becoming fossilized, a process referred to as the "biological carbon pump."

But some of the sequestered CO₂ normally escapes, due to the strong eastward winds encircling Antarctica that drag CO₂-rich deep water up to the surface and vent the CO₂ back to the atmosphere. The new research provides evidence that this wind-driven Antarctic Ocean upwelling slowed down during the ice ages, allowing less CO₂ to be vented and more to remain locked up in the ocean waters.

Apart from any effect this retention of CO₂ may have had on ice-age temperatures, the researchers say their data suggests that the past lag of CO₂ behind temperature may have been caused directly by the effect on Antarctic upwelling of changing obliquity in the earth’s orbit – Milankovitch’s 41,000-year cycle. The study authors believe this explains why the eccentricity and precession cycles now prevail over the obliquity cycle.

Next: Both Greenland and Antarctic Ice Sheets Melting from Below