During the last ice age, global sea levels rose and fell several times in regular cycles, a new study in Science shows. The Earth’s ice cover alternately increased and decreased over a longer period of time. However, in the middle of the Pleistocene, the rhythm of this cold-warm rhythm changed. The reason for this was probably shifts within the ice as well as wind and ocean currents, not primarily changes in the atmosphere and earth’s temperature. This surprising finding could now help to understand the future fate of today’s ice sheets in Greenland and Antarctica.
The Pleistocene began around 2.6 million years ago and ended around 11,700 years ago. This period is widely considered to be “the last ice age” – in contrast to earlier periods in Earth’s history, which were also characterized by a cold climate. During the Pleistocene, the Earth was covered by a thick layer of ice, especially over northern North America and Eurasia, which grew or shrank in phases. Accordingly, global sea levels rose and fell during colder and warmer phases. But how pronounced were these cycles and what rhythm did they follow?
Climate researchers can now reconstruct this using foraminifera in drill cores. These fossil shells of former marine life testify to when and where today’s land was covered by water and how warm the sea was. Researchers led by Peter Clark from Oregon State University have now also followed this lead. Using sophisticated analyzes of foraminiferal shells, they reconstructed the changes in average global sea level over the last 4.5 million years.
How did the climate change during the Ice Age?
The evaluation showed that before the start of the last ice age, around 4.5 to 4.0 million years ago, the sea level was up to 20 meters higher than today. Accordingly, there were significantly smaller ice sheets back then than today. It was not until the beginning of the Ice Age around 3.0 to 2.5 million years ago that the large ice sheets in the northern hemisphere emerged and the sea level fell to approximately its current level. However, throughout the rest of the Ice Age, the annual extent of the ice and the reach of the seas changed several times. At times, the ice grew so much that the sea level sank around 80 meters below its current level, as the team found. At this maximum the ice sheets then became unstable, broke up and the trend reversed.
However, this regular cycle of cold and warm periods did not remain the same throughout the ice age: in the first half of the Pleistocene, glaciation cycles occurred approximately every 41,000 years, whereas in the second half they occurred every 100,000 years. The transition between these two phases occurred in the Middle Pleistocene, about 1.2 million to 620,000 years ago. But how did this transition come about? Why did the cold periods change their rhythm? The researchers were able to clearly refute the previous theory that the ice sheets were simply smaller during the 41 cycles based on their data. “This finding challenges conventional wisdom about the Middle Pleistocene transition and forces us to develop new explanations,” said Clark.
Two main hypotheses are being discussed as the cause of the change. The first states that the Earth’s atmosphere contained less carbon dioxide in the Middle Pleistocene than before – for example due to geochemical processes that increased CO2 binding. As a result, the global climate cooled, ice sheets grew and sea levels fell more than before. Therefore, the cycles between ice formation and melting took longer from now on. The second hypothesis, however, states that the ice sheets began to move differently in the Middle Pleistocene. They grew and shifted because carbon cycles in the ocean and air changed, causing the climate to cool. Unlike the first hypothesis, solar radiation and temperatures only play an indirect, downstream role.
Paradigm shift when looking at the Ice Age
The researchers tested these hypotheses using fossil data and determined that the reason for the changes in ice masses and sea level during the transition period and the second half of the Pleistocene was primarily movements of air, water and ice masses. Wind and weather as well as changed ocean currents promoted CO2 absorption from the atmosphere, which ensured that the climate continued to be cool. This promoted the formation of stable ice sheets and longer cycles than at the beginning of the Ice Age.
This finding therefore tends to support the second hypothesis: “The fact that these large ice sheets have been present all along means that their formation and disintegration were likely influenced by internal feedbacks in the climate system rather than external dynamics,” explains Clark. According to this, not only changes in the Earth’s atmosphere have changed the ice cycles, but also shifts in the ice sheet itself. “This is a paradigm shift in our understanding of the history of the Ice Age,” says Clark.
The knowledge about climate history could now also help to predict the future climate in view of man-made global warming. “Our ability to understand the past gives us a better understanding of ice sheet-climate interactions and provides context for what we might experience in the future,” says Clark. “We have two major ice sheets today, in Antarctica and Greenland. We need to think about what state these ice sheets can exist in under different conditions.”
Source: Peter Clark (Oregon State University) et al.; Science, doi: 10.1126/science.adv8389