Seasonal sea ice persisted at 80°N during warm conditions in the past

Research notes

Sea ice is declining in the Arctic. But how great is the full range of natural variations? Can we gain perspective on the changes we are seeing now? By analysing seabed sediments, scientists have established records of past sea ice fluctuations. Here is what those record reveal.

By: Katrine Husum, Anna J Pieńkowski and Dmitry V Divine // Norwegian Polar Institute.
Simon T Belt, Denizcan Köseoğlu and Lukas Smik // University of Plymouth, UK.
Ulysses Ninnemann // University of Bergen. Jochen Knies // Geological Survey of Norway and UiT The Arctic University of Norway. Kelly Hogan // British Antarctic Survey, UK.
Riko Noormets // University Centre in Svalbard

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About sixty years ago the world’s first satellite was deployed, and since 1978 earth observation satellites have continuously monitored the extent and concentration of sea ice in the Arctic. The data from these four decades show a decline of sea ice which is thought to be caused by shifting atmospheric winds and increased temperatures, possibly linked to the greenhouse effect. However, they do not show the natural sea ice situation before the world was affected by climate warming.

In order to obtain data on natural variations in sea ice from the time before satellites made instrumental measurements possible, we analyse the chemical composition of marine sediments.

Satellite observations of sea ice in Fram Strait in September since 1979. The declining trend is clear.

Algae and plankton that live near and within sea ice produce “biomarkers”, characteristic compounds called highly branched isoprenoids/HBIs. When the organisms die, they fall to the seabed, and we find HBIs preserved in marine sediments. Marine sediments are deposited over time, like layers in a layer cake, one top of the other. Hence, when we take a sediment core – a vertical sample from the sea floor – we sample all these layers. Ideally, they represent a continuous recording of natural processes from present to the past. We measure HBIs and use 14C radiocarbon dating to determine the age of the layers in the marine sediments.

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Surface sediments from the sea floor obtained by a multicorer. Photo: Katrine Husum / Norwegian Polar Institute

In this study we investigated two marine sediment cores from the northern Barents Sea northeast of Svalbard at about 80°N. The study clearly shows that there has been continuous seasonal sea ice northeast of Svalbard since the last ice age.

The sea ice decreased sharply during the cold climate period Younger Dryas and was at a minimum at the beginning of our current interglacial Holocene warm period. Conditions during the beginning of the Holocene were warmer than they are now; nonetheless, sea ice was present. The data also clearly show continuous presence of sea ice with a coverage of 10-60% in spring northeast of Svalbard since the last ice age.

The youngest data point in this study is from 1905, point in this study is from 1905, so this reconstruction does not show development over the last century. Modern observations suggest that seasons with relatively little sea ice have increased in frequency over the past 10 years. The geographical area of the sea ice cover is shrinking, and sea ice is disappearing earlier in the season than has previously been observed. The long-term historical reference values from this study indicate that the current sea ice situation could resemble the conditions just after the last ice age (approximately 9500-9000 years before our time), when changes in the earth’s orbit and rising greenhouse gas levels drove massive cryosphere retreat.

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Historical long-term reference values based on data on two sediment cores examined for this study show sea ice has been present at 80°N ever since the last ice age, even when natural changes in Earth’s orbit and rising greenhouse gas levels caused glaciers and sea ice to retreat, approximately 11 500-11 000 years ago. The results indicate that current sea ice situation could resemble the conditions in this period; however, the current reduction of sea ice cannot be explained solely by natural forces.

However, unlike that earlier period, orbit changes and other natural driving forces cannot explain the current reduction of sea ice. This suggests that the effects of human activity may already be driving a retreat similar to that seen during the last great cryospheric collapse in Earth’s history.

Further reading

Pieńkowski AJ, Husum K, Belt ST, Ninnemann U, Köseoğlu D, Divine DV, Smik L, Knies J, Hogan K, Noormets R (2021) Seasonal sea ice persisted through the Holocene Thermal Maximum at 80°N. Nature Communications Earth & Environment 2: 124,