According to a study, what climate change could bring us soon also happened during the last interglacial period: In the so-called interglacial period 129,000 to 115,000 years ago, the Arctic Ocean was ice-free in summer. This is substantiated by microfossils of plankton species typically found in areas free of sea ice in sediment cores from the North Pole region. The scientists say that further study of the last interglacial could therefore contribute to a deeper understanding of climate dynamics in a world without Arctic sea ice.
Climate change is making itself felt everywhere on earth - but it is particularly drastic in the far north. The strong Arctic warming is reflected, among other things, in the significant reduction in sea ice cover: The records for summer ice melt in the Arctic Ocean are increasing. According to forecasts, the sea ice could temporarily disappear completely in late summer around 2050. This is not only a drastic sign of global warming, this loss of sea ice would also have significant consequences. Because the Arctic sea ice also plays an important role in the climate system by sealing the water and reflecting light. The dwindling sea ice cover could thus trigger feedback processes that again have a significant impact on the regional and global climate.
When was the Arctic Ocean last ice-free?
For further forecasts and climate models, information on the effects of the last disappearance of the Arctic sea ice would therefore be important. But when was that? The questioning look is directed at the last great interglacial period - the interglacial around 129,000 to 115,000 years ago. According to earlier studies, the temperatures at that time were similar or slightly higher than today. However, it is unclear to what extent this led to a seasonal loss of sea ice cover in the Arctic Ocean at the time. The researchers led by Flor Vermessen from the University of Stockholm have now addressed this question.
Their results are based on examining microfossils in a series of sediment cores. They come from locations close to the poles, which today still lie directly under the thickest parts of the Arctic sea ice. In the datable layers of these samples, the scientists examined the microscopic shells of certain representatives of the zooplankton - so-called foraminifera. There are several species of these organisms that inhabit the open water of certain marine regions. The respective species can be recognized by the characteristics of their tiny calcareous shells.
Freedom from ice in the mirror of microfossils
As the scientists reported, they found the microfossils of the foraminiferal species Turborotalita quinqueloba in the drill core layers from the last interglacial. However, they are not found in younger deposits. As the researchers explain, it is a species that is now only found in the largely ice-free and seasonally productive waters of the Atlantic Ocean. Apparently, there were also corresponding conditions at the locations where the samples were taken during the interglacial period. At that time, the tiny creatures were able to migrate from more southern areas to the polar sea region. In other words: The findings show that the area was not covered by sea ice for a long period of time during the summer months, the scientists conclude.
Accordingly, during the last interglacial period, a process that is also evident today was apparently well advanced: the so-called “Atlantification” of the Arctic Ocean. The finding that the Arctic Ocean was seasonally ice-free at the time is particularly concerning because it was probably only slightly warmer than it is today, the researchers say. According to them, their study now makes the importance of the last interglacial for climate research clear: "Our results show that this epoch is a suitable analogue for studying a seasonally ice-free Arctic Ocean, which is expected to occur in this century," the researchers write .
The study should therefore now be followed by further research, says Vermasse: "In order to understand the conditions in this seasonally sea-ice-free Arctic during the last interglacial, investigations should now be carried out that provide information on the sea surface temperatures and other parameters at that time. In addition, targeted climate and oceanographic model studies are required for this period," says the scientist.
Source: University of Stockholm, specialist article: Nature Geoscience, doi: 10.1038/s41561-023-01227-x