Sea ice triggered the Little Ice Age

Date:
September 17, 2020
Source:
University of Colorado at Boulder
Summary:
A new study finds a trigger for the Little Ice Age that cooled
Europe from the 1300s through mid-1800s, and supports surprising
model results suggesting that under the right conditions sudden
climate changes can occur spontaneously, without external forcing.



FULL STORY ==========================================================================
A new study finds a trigger for the Little Ice Age that cooled Europe
from the 1300s through mid-1800s, and supports surprising model results suggesting that under the right conditions sudden climate changes can
occur spontaneously, without external forcing.


==========================================================================
The study, published in Science Advances, reports a comprehensive reconstruction of sea ice transported from the Arctic Ocean through the
Fram Strait, by Greenland, and into the North Atlantic Ocean over the
last 1400 years. The reconstruction suggests that the Little Ice Age --
which was not a true ice age but a regional cooling centered on Europe
-- was triggered by an exceptionally large outflow of sea ice from the
Arctic Ocean into the North Atlantic in the 1300s.

While previous experiments using numerical climate models showed that
increased sea ice was necessary to explain long-lasting climate anomalies
like the Little Ice Age, physical evidence was missing. This study digs
into the geological record for confirmation of model results.

Researchers 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 1400 years.

"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 Martin Miles, an INSTAAR researcher who also holds an appointment
with NORCE Norwegian Research Centre and Bjerknes Centre for Climate
Research in Norway.

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 (decadal scale) 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 midcentury, 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," said
Miles. "And the perfect example of that could be the Little Ice Age."
"This specific investigation was inspired by an INSTAAR colleague, Giff
Miller, as well as by some of the paleoclimate reconstructions of my
INSTAAR colleagues Anne Jennings, John Andrews, and Astrid Ogilvie,"
added Miles. Miller authored the first paper to suggest that sea ice
played an essential role in sustaining the Little Ice Age.

Scientists have argued about the causes of the Little Ice Age for
decades, with many suggesting that explosive volcanic eruptions must be essential for initiating the cooling period and allowing it to persist
over centuries. One the 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 researchers
were concerned that they were showing problems with the models.

Miles' 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," he 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. Miles continued, "These results strongly suggest...that
these things can occur out of the blue due to internal variability in
the climate system." 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. Miles and his colleagues 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," Miles said. He would like to look
more closely into oceanic conditions along with researchers who study
the social sciences in relation to climate.


========================================================================== Story Source: Materials provided by
University_of_Colorado_at_Boulder. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Martin W. Miles, Camilla S. Andresen, Christian V. Dylmer. Evidence
for
extreme export of Arctic sea ice leading the abrupt onset of the
Little Ice Age. Science Advances, 2020; 6 (38): eaba4320 DOI:
10.1126/ sciadv.aba4320 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200917105354.htm

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