The threats posed by today's melting glaciers are warned about by ancient sea levels.
Timeline of the ice age sea level
According to a recent study, the global mean sea level changed significantly during the last Ice Age, not just at its conclusion, which is a significant reexamination of Earth's past. The 4.5 million-year-old work reframes scientists' understanding of ice sheets and climate pace.
Ocean levels were roughly 65 feet (20 meters) higher than they are today at certain points in that record, while later lows were on par with the deepest fall of the most recent glacial maximum.
The global average height of the ocean surface, or previous global mean sea level, was recreated by Peter U. Clark of Oregon State University (OSU) and associates.
The scientists came to the conclusion that significant sea level changes were not late comers since huge ice sheets waxed and waned across a major portion of the Pleistocene.
According to Clark, "this is a paradigm shift in our understanding of the history of the ice age." He pointed out that previous models had overlooked the recurring rise and fall of ancient seas well before the final peak of the Ice Age, underestimating how dynamic Earth's ice sheets were.
Sea history is shown by shells.
The team used benthic foraminifera, which are microscopic bottom creatures whose shells preserve historical ocean conditions, to accomplish this. Their oxygen signals measure the amount of water trapped on land as ice as well as the temperature of the ocean.
Using a worldwide temperature reconstruction created in 2024, the researchers initially distinguished between temperature and ice volume. Because of this, they were able to convert the seawater oxygen signal into sea level without relying on antiquated theories.
After that, they contrasted their curve with well-known seabed oxygen record stacks, such the LR04 database, which is frequently regarded as a standard for ice age pacing.
That famous stack combines temperature and ice on its own, with 41,000-year rhythms at the beginning and 100,000-year rhythms at the end.
The novel method circumvents a continuous conversion that might overlook the way that warmer climates alter the isotopic "weight" of ice. In terms of sea level and ice volume, it discovers that many early cycles were already as big as the later ones.
shift in the climate cycle
The transition from predominantly 41,000-year cycles to dominating 100,000-year cycles occurred during the middle Pleistocene transition (MPT), which occurred between approximately 1.2 million and 0.62 million years ago. Previous reconstructions linked the MPT to gradually expanding ice sheets that didn't fully expand until much later.
According to Clark's research, the narrative is distinct. The clock changed, not the fundamental magnitude of the glaciers, because large ice sheets were present long before the MPT. The axial tilt of the Earth, a gradual wobble that modifies the distribution of sunlight, is one suspect. The period of this anomaly is close to 41,000 years.
According to the study, variations in the Southern Ocean carbon cycle during the MPT exacerbated atmospheric CO2 oscillations on a 100,000-year period. When ice sheets surpassed their stability threshold, that would increase temperature cycles and have an impact.
Timeline of the ice age sea level
The oceans frequently rose up to 65 feet (20 meters) above the present between 4.5 and 3 million years ago. This suggests that Greenland and Antarctica were smaller than they are now. As northern ice sheets advanced, lows started to dip below present sea level more frequently after 4 million years ago.
Low stands had descended to depths similar to the Last Glacial Maximum by about 2.5 million years ago, which is about 425 feet (130 meters) below the present. Instead of having thin, wide domes, that depth suggests ice sheets as bulky as the most recent giants.
The record shows a significant threshold. Ice sheets tended to become unstable on the subsequent upturn in obliquity when GMSL dropped about 260 feet (79 meters) below current levels. This is the angle formed between Earth's orbital plane and axis of rotation. Consequently, this aided in initiating a significant deglaciation.
The role of ice height is also indicated by model tests. In situations when shorter ice would be wasted away, higher ice surfaces can retain positive surface mass balance (SMB) and remain colder.
Why this is important
If large, delicate ice sheets were widespread during the Pleistocene, then there are powerful internal feedbacks in the Earth's climate system. When particular thresholds are crossed, the feedbacks have the power to switch ice sheets between growth and disintegration.
The longest Antarctic ice core records are consistent with the theory that CO2 was controlled by southern ocean processes during the MPT. High resolution CO2 from the past 800,000 years shows repeated 100,000 year swings.
When ice sheets reorganise, the new sea-level history also serves as a standard for how quickly oceans might rise or fall. That background is important because, despite the fact that the drivers are different from those of the ice ages' orbital pacing, human warming is speeding up the rate now.
"The presence of those massive ice sheets during that period indicates that internal climate system feedback, rather than external dynamics, most likely influenced their formation and decay," Clark said.
Next things to watch
Researchers would be able to investigate the suggested Southern Ocean carbon cycle link throughout the whole MPT if direct CO2 records were extended beyond 800,000 years. In order to determine how greenhouse gases pulsed prior to that barrier, new drilling initiatives seek to recover older ice.
The way models handle ice physics, ocean heat storage, and feedbacks between clouds, winds, and snowfall will be improved by tighter limitations on previous GMSL. This includes determining if the obliquity threshold behaviour is consistent across temperature and carbon dioxide backgrounds.
Last but not least, the discovery that massive ice sheets formed early indicates that coasts moved frequently and far. To identify discrepancies and enhance both datasets, geological markers from far-off beaches can be rechecked against the updated curve.
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