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“Out of the blue” – proof that sea ice triggered the Little Ice Age

Ice snow of the glacier mountains

  • Sea ice can act as a means of climate change on various time and spatial scales ̵
    1; it is not just a passive response to change.
  • The Little Ice Age may have arisen “out of the blue” due to internal variability within the climate system and not in response to external pressures from volcanic eruptions or other factors.
  • A distant pulse of sea ice may have contributed to the demise of the Nordic colonies in Greenland in the 14th and 15th centuries.

A new study finds a trigger for the Little Ice Age, which cooled Europe from 1300 to the mid-19th century, and supports surprising model results that suggest that, under the right conditions, sudden climatic changes can occur spontaneously without being forced from outside .

The study, published in Advances in science, reports on a comprehensive reconstruction of the sea ice that was transported from the Arctic Ocean via the Fram Strait, Greenland and into the North Atlantic over the past 1400 years. The reconstruction suggests that the Little Ice Age – not a real Ice Age, but a regional cooling with a focus on Europe – was triggered in the 13th century by an exceptionally large outflow of sea ice from the Arctic Ocean into the North Atlantic.

Greenland and adjacent ocean currents

The map shows Greenland and adjacent ocean currents. Colored circles show where some of the sediment cores used in the study came from the sea floor. The small historical map from the beginning of the 20th century shows the distribution of storis, or sea ice, from the Arctic Ocean, which flows down the east coast of Greenland. Photo credit: Miles et al., 2020.

While previous experiments with numerical climate models indicated that increased sea ice was necessary to explain long-lasting climate anomalies such as the Little Ice Age, physical evidence was lacking. This study examines the geological data to confirm the model results.

The researchers compiled records of marine sediment cores drilled from the sea floor from the Arctic Ocean to the North Atlantic to get a detailed look at sea ice across the region over the past 1,400 years.

“We decided to put together different strands of evidence to reconstruct spatially and temporally what the sea ice was for the past thousand years and a half and then just see what we found,” said Martin Miles, an INSTAAR researcher who This also did an appointment with the Norwegian research center NORCE and the Bjerknes center for climate research in Norway.

The cores included compounds from algae that live in the sea ice, the shells of unicellular organisms that live at different water temperatures, and debris that the sea ice picks up and transports over great distances. The cores were detailed enough to detect abrupt (decadal) changes in sea ice and ocean conditions over time.

The records show a sudden surge in Arctic sea ice exported to the North Atlantic, which began around 1300, peaked in the middle of the century, and ended abruptly in the late 13th century.

Occurrence of sea ice and polar water

The graphics show the reconstructed time series of changes in the occurrence of sea ice and polar water in the past. The colors of the curves correspond to the positions on the map. The blue shading represents the time of increased sea ice in the 13th century. Photo credit: Miles et al., 2020.

“I’ve always been fascinated by looking at sea ice not just as a passive indicator of climate change, but also how it interacts with the climate system over long periods of time, or can actually lead to change,” said Miles. “And the perfect example of this could be the Little Ice Age.”

“This particular study was inspired by an INSTAAR colleague, Giff Miller, as well as some paleoclimate reconstructions by my INSTAAR colleagues Anne Jennings, John Andrews and Astrid Ogilvie,” added Miles. Miller penned the first paper to suggest that sea ice was an essential part of sustaining the Little Ice Age.

Scientists have argued over the causes of the Little Ice Age for decades, and many suggest that explosive volcanic eruptions must be essential to ushering in the cooling period and making it last for centuries. On the one hand, the new reconstruction provides robust evidence of a massive sea ice anomaly that could have been triggered by increased explosive volcanism. On the other hand, the same evidence supports a fascinating alternative explanation.

Martin Miles

INSTAAR Research Associate Martin Miles in a modern subarctic fjord setting. Photo credit: Martin Miles

Climate models, so-called “control models”, are carried out to understand how the climate system works over time without being influenced by external forces such as volcanic activity or greenhouse gas emissions. A number of recent control model experiments have yielded results representing sudden cold events lasting several decades. The model results seemed too extreme to be realistic – so-called ugly duckling simulations – and the researchers were concerned that they were showing problems with the models.

Miles’ study found that there may be absolutely nothing wrong with these models.

“We’re actually finding number one, we have physical, geological evidence that these ten-year cold sea ice excursions can actually take place in the same region,” he said. In the case of the Little Ice Age, “what we reconstructed in space and time was strikingly similar to the development of a model simulation by Ugly Duckling, in which a spontaneous cold event lasted about a century. It was about unusual winds, sea ice exports and a lot more ice east of Greenland, as we found it here. “The provocative results show that external forcing from volcanoes or other causes may not be required for large climatic fluctuations to occur. Miles continued, “These results strongly suggest that these things can happen out of the blue due to internal variability in the climate system.”

The sea cores also show a sustained, distant pulse of sea ice near the Nordic colonies on Greenland, coinciding with their disappearance in the 15th century. A debate raged on why the colonies disappeared, and usually only agreed that a cool climate weighed heavily on their resilience. Miles and his colleagues want to account for the oceanic changes nearby: very large amounts of sea ice and cold polar water, year after year for almost a century.

“This massive belt of ice, which flows out of the Arctic in the past and still today, runs around Cape Farewell to where those colonies were,” Miles said. He would like to study oceanic conditions in more detail with researchers studying the social sciences related to climate.

Reference: “Evidence of the extreme export of Arctic sea ice that triggered the sudden onset of the Little Ice Age” by Martin W. Miles, Camilla S. Andresen and Christian V. Dylmer, September 16, 2020, Advances in science.
DOI: 10.1126 / sciadv.aba4320

Camilla S. Andresen from the Geological Survey of Denmark and Greenland and Christian V. Dylmer from MMT Sweden AB were co-authors of the study.

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