The Little Ice Age, a period of global cooling that lasted from the early 14th century to the mid-19th century, was triggered by an exceptionally large outflow of sea ice from the Arctic Ocean into the North Atlantic in the 1300s, according to a new paper published in the journal Science Advances.
Bathymetric map of the Fram Strait gateway and downstream region; red circles indicate location of marine sediment cores. Inset: Danish historical ice chart from the early 20th century showing the extension of the Arctic Ocean-origin sea ice observed along Southwest Greenland. Image credit: Miles et al, doi: 10.1126/sciadv.aba4320.
“We decided to put together different strands of evidence to try to reconstruct spatially and temporally what the sea ice was during the past one and a half thousand years, and then just see what we found,” said Dr. Martin Miles, a researcher in the Institute of Arctic and Alpine Research at the University of Colorado, Boulder and the NORCE Norwegian Research Centre and the Bjerknes Centre for Climate Research.
Dr. Miles and colleagues pulled together records from marine sediment cores drilled from the ocean floor from the Arctic Ocean to the North Atlantic to get a detailed look at sea ice throughout the region over the last 1,400 years.
The cores included compounds produced by algae that live in sea ice, the shells of single-celled organisms that live in different water temperatures, and debris that sea ice picks up and transports over long distances. The cores were detailed enough to detect abrupt changes in sea ice and ocean conditions over time.
The records indicate an abrupt increase in Arctic sea ice exported to the North Atlantic starting around 1300, peaking in mid-century, and ending abruptly in the late 1300s.
“I’ve always been fascinated by not just looking at sea ice as a passive indicator of climate change, but how it interacts with or could actually lead to changes in the climate system on long timescales. And the perfect example of that could be the Little Ice Age,” Dr. Miles said.
On one hand, the new reconstruction provides robust evidence of a massive sea-ice anomaly that could have been triggered by increased explosive volcanism.
One the other hand, the same evidence supports an intriguing alternate explanation.
Climate models called ‘control models’ are run to understand how the climate system works through time without being influenced by outside forces like volcanic activity or greenhouse gas emissions.
A set of recent control model experiments included results that portrayed sudden cold events that lasted 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.
The new study found that there may be nothing wrong with those models at all.
“We actually find that number one, we do have physical, geological evidence that these several decade-long cold sea ice excursions in the same region can, in fact do, occur,” Dr. Miles said.
“In the case of the Little Ice Age, what we reconstructed in space and time was strikingly similar to the development in an Ugly Duckling model simulation, in which a spontaneous cold event lasted about a century. It involved unusual winds, sea ice export, and a lot more ice east of Greenland, just as we found in here.”
The provocative results show that external forcing from volcanoes or other causes may not be necessary for large swings in climate to occur.
“These results strongly suggest… that these things can occur out of the blue due to internal variability in the climate system,” Dr. Miles said.
The marine cores also show a sustained, far-flung pulse of sea ice near the Norse colonies on Greenland coincident with their disappearance in the 15th century.
A debate has raged over why the colonies vanished, usually agreeing only that a cooling climate pushed hard on their resilience.
The study authors would like to factor in the oceanic changes nearby: very large amounts of sea ice and cold polar waters, year after year for nearly a century.
“This massive belt of ice that comes streaming out of the Arctic — in the past and even today — goes all the way around Cape Farewell to around where these colonies were,” Dr. Miles said.
“We would like to look more closely into oceanic conditions along with researchers who study the social sciences in relation to climate.”
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Martin W. Miles et al. 2020. Evidence for extreme export of Arctic sea ice leading the abrupt onset of the Little Ice Age. Science Advances 6 (38): eaba4320; doi: 10.1126/sciadv.aba4320
