Climate History
From White to Blue
Rapid Sea-ice Retreat Triggers Abrupt Climate Change
[10. November 2020]
The reason for the speed and severity of the abrupt climate fluctuations during the last glacial lies in the ocean. These are the findings of a new study by AWI researcher Henrik Sadatzki, which has now been published in the Proceedings of the National Academy of Sciences (PNAS). The study shows that widespread sea-ice retreat occurred within a period of 250 years or less during the last glacial and repeatedly triggered abrupt climate fluctuations. This scientific breakthrough provides a significant new aspect in the long-standing debate concerning the mechanisms of abrupt climate fluctuations.
During the last glacial ca. 10–110,000 years ago, the Northern Hemisphere turned white: massive ice sheets lay over the northern continents and extensive sea ice covered the Nordic Seas between Norway and Greenland. However, the cold glacial climate in the north was interrupted by several abrupt climate-warming events, which involved temperature increases of up to 16.5 °C over ice-covered Greenland.
These historical warming events, known as Dansgaard-Oeschger (D-O) events, were discovered several decades ago in ice-core records from Greenland, but their cause remained a matter of controversy. D-O events are also important for us living today as their speedy warming rate has recently been shown to be similar to the warming rate observed in major parts of the Arctic today, where the sea ice is disappearing.
The new findings published in PNAS provide robust empirical evidence that the previous abrupt climate warming events were closely linked to widespread, rapid sea-ice retreat in the Norwegian Sea. “Our comprehensive and detailed sea ice reconstruction is unprecedented, and documents the significance of rapid sea-ice retreat and the associated feedback mechanisms for abrupt climate change,” states the study’s main author, Henrik Sadatzki from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).
Combining sediment-core and ice-core data
Sadatzki and researchers from various international institutions used two independent methods combining sediment-core and ice-core data to reconstruct the history of the sea ice.
The researchers investigated two sediment cores from the Norwegian Sea and an ice core from East Greenland. The sediment cores were particularly well suited to reconstructing spatial-temporal changes in the sea-ice cover and ocean circulation in the region, where relatively warm Atlantic water flows into the Norwegian Sea. Furthermore, with the help of tephra layers (layers of ash deposits from Icelandic volcanic eruptions), it was possible to link the datasets of these sediment cores with that of the East Greenland ice core.
The sea-ice reconstruction was based on specific organic molecules contained in the marine sediment cores. Some of these were produced by algae living in sea ice, others by algae living in ice-free water. The research team also analysed the bromine content of the East Greenland ice core, which originated from the presence of either seasonal sea ice or open water in the ocean between Greenland and Norway.
The combination of the sediment-core and ice-core data made it possible to estimate the speed of sea-ice retreat in the Nordic Seas, as well as its chronological progression as compared with changes in the ocean circulation and climate, more reliably and precisely than previously possible.
Past sea-ice changes
The data presented by Sadatzki and his colleagues suggests that the Nordic Seas were covered by an extensive sea ice lid during cold periods, while warmer periods were characterised by a reduced seasonal sea-ice cover and more open-water conditions.
“Our data indicate that widespread sea-ice retreat might have happened within 250 years or less, contemporaneously with the onset of a phase in which the ocean was stirred and mixed in the Norwegian Sea, leading into the onset of the abrupt atmospheric warming”, Sadatzki explains.
The data confirms results from model simulations and shows that, as the Nordic Seas turned rapidly from white to blue, heat released from the relatively warmer ocean water to the cold atmosphere led to an amplification of the abrupt climate warming events during the last glacial.
The results of this study document the behaviour of sea-ice changes as a tipping element in the coupled ocean-ice-climate system, providing important constraints for the understanding of abrupt climate changes in the past, and is a warning that ongoing sea-ice retreat in the Arctic might lead to similar changes in the not so distant future.
Original Publication
The study has now been published under the following original title in the online portal of PNAS:Henrik Sadatzki, Niccolò Maffezzoli, M. Dokken, Margit H. Simon, Sarah M. P. Berben, Kirsten Fahl, Helle A. Kjær, Andrea Spolaor, Ruediger Stein, Paul Vallelonga, Bo M. Vinther, and Eystein Jansen: Rapid reductions and millennial-scale variability in Nordic Seas sea ice cover during abrupt glacial climate changes; DOI 10.1073/pnas.2005849117