The cause of the speed and severity of abrupt climate changes during the last cold period lies in the ocean. This is confirmed by a new study by AWI scientist Henrik Sadatzki, now published in Proceedings of the National Academy of Sciences (PNAS). The study shows that a widespread decline in sea ice occurred within 250 years or less, triggering abrupt climate change several times during the last cold period. This scientific breakthrough provides an important new aspect in the long debate on the mechanisms of abrupt climate change.
Heavy sea ice decline triggers abrupt climate change
During the last cold period, 10 to 110,000 years ago, the northern hemisphere turned white. Large ice sheets lay on the northern continents and extensive sea ice covered the North Sea between Norway and Greenland. However, the cold glacial climate in the north was interrupted by several abrupt global warming events, which included temperature increases of up to 16.5 °C above the Greenland ice sheet.
These past warming events, known as Dansgaard-Oeschger (D-O) events were discovered decades ago in Greenlandic ice cores, but their cause remained disputed. The D-O events are also important for our lives today, as it has recently been shown that their rapid warming rate is similar to the rate of warming observed today in large parts of the Arctic, where sea ice is disappearing.
The new results published in PNAS provide robust empirical evidence that past abrupt global warming events have been closely linked to the rapid and widespread decline of sea ice in the North Sea. “Our unparalleled and detailed reconstruction of sea ice documents the importance of a rapid retreat of sea ice and the associated feedback mechanisms for sudden climate change,” says the study’s lead author, Henrik Sadatzki of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).
Combination of sediment core and ice core data
Sadatzki and researchers from various international institutions used two independent methods for reconstructing sea ice, combining sediment core and ice core data.
The scientists studied two sediment cores from the Norwegian Sea and one ice core from eastern Greenland. The sediment cores were excellently suited to reconstruct the spatial-temporal changes in sea ice and ocean circulation in the area where relatively warmer water flows from the Atlantic ocean into the North Sea. In addition, the data sets of these sediment cores could be linked to that of the East Greenland ice core with the help of Tephra layers (ash layers of Icelandic volcanic eruptions).
The reconstruction of sea ice was based on specific organic molecules contained in the marine sediment cores. Some of them were produced by algae in sea ice and others by algae in ice-free water. In addition, the research team analysed the bromine content in the Eastern Greenland ice core caused by either seasonal sea ice or open water in the ocean between Greenland and Norway.
The combination of sediment core and ice core data made it possible to estimate the rate of sea ice decline in the Nordic seas and its timing compared to changes in ocean circulation and climate, more reliable and accurate than previously possible.
Past sea ice changes
The data provided by Sadatzki and his colleagues suggest that the North Sea was covered by an extensive sea ice cover during the cold periods, while the warmer periods were characterized by reduced seasonal sea ice cover and rather open sea conditions.
“Our data indicate that a large-scale sea ice decline within 250 years or less may have occurred at the same time as the beginning of a phase in which the ocean was mixed in the North Sea, leading to the onset of abrupt warming of the atmosphere,” Sadatzki said.
The results of the study illustrate the behaviour of sea ice changes as a tipping element in the coupled ocean-ice-climate system. The data prove findings from simulations using climate models and show that with the rapid change of the North Sea from white to blue, the heat from the relatively warmer seawater was released to the cold atmosphere, which led to an increase in abrupt global warming events during the last cold season.
“Our findings on the link between sea ice decline and abrupt climate change during the recent cold period can also be seen as a warning about the impending consequences of the current sea ice decline and warming in the Arctic,” says Henrik Sadatzki.
Source: Henrik Sadatzki / AWI Bremerhaven