Cold era, lasting from early 15th to mid-19th centuries, triggered by unusually warm conditions
From:
University of Massachusetts, Amherst
December 15, 2021 -- New research from
the University of Massachusetts Amherst provides a novel answer to one of the
persistent questions in historical climatology, environmental history and the
earth sciences: what caused the Little Ice Age? The answer, we now know, is a
paradox: warming.
The Little Ice Age was one of the
coldest periods of the past 10,000 years, a period of cooling that was
particularly pronounced in the North Atlantic region. This cold spell, whose
precise timeline scholars debate, but which seems to have set in around 600
years ago, was responsible for crop failures, famines and pandemics throughout
Europe, resulting in misery and death for millions. To date, the mechanisms
that led to this harsh climate state have remained inconclusive. However, a new
paper published recently in Science Advances gives an up-to-date picture of the
events that brought about the Little Ice Age. Surprisingly, the cooling appears
to have been triggered by an unusually warm episode.
When lead author Francois Lapointe,
postdoctoral researcher and lecturer in geosciences at UMass Amherst and Raymond
Bradley, distinguished professor in geosciences at UMass Amherst began
carefully examining their 3,000-year reconstruction of North Atlantic sea
surface temperatures, results of which were published in the Proceedings of the
National Academy of Sciences in 2020, they noticed something surprising: a
sudden change from very warm conditions in the late 1300s to unprecedented cold
conditions in the early 1400s, only 20 years later.
Using many detailed marine records,
Lapointe and Bradley discovered that there was an abnormally strong northward
transfer of warm water in the late 1300s which peaked around 1380. As a result,
the waters south of Greenland and the Nordic Seas became much warmer than
usual. “No one has recognized this before,” notes Lapointe.
Normally, there is always a transfer of
warm water from the tropics to the arctic. It’s a well-known process called the
Atlantic Meridional Overturning Circulation (AMOC), which is like a planetary
conveyor belt. Typically, warm water from the tropics flows north along the
coast of Northern Europe, and when it reaches higher latitudes and meets colder
arctic waters, it loses heat and becomes denser, causing the water to sink at
the bottom of the ocean. This deep-water formation then flows south along the
coast of North America and continues on to circulate around the world.
But in the late 1300s, AMOC strengthened
significantly, which meant that far more warm water than usual was moving
north, which in turn cause rapid arctic ice loss. Over the course of a few
decades in the late 1300s and 1400s, vast amounts of ice were flushed out into
the North Atlantic, which not only cooled the North Atlantic waters, but also
diluted their saltiness, ultimately causing AMOC to collapse. It is this
collapse that then triggered a substantial cooling.
Fast-forward to our own time: between
the 1960s and 1980s, we have also seen a rapid strengthening of AMOC, which has
been linked with persistently high pressure in the atmosphere over Greenland.
Lapointe and Bradley think the same atmospheric situation occurred just prior
to the Little Ice Age—but what could have set off that persistent high-pressure
event in the 1380s?
The answer, Lapointe discovered, is to
be found in trees. Once the researchers compared their findings to a new record
of solar activity revealed by radiocarbon isotopes preserved in tree rings, they
discovered that unusually high solar activity was recorded in the late 1300s.
Such solar activity tends to lead to high atmospheric pressure over Greenland.
At the same time, fewer volcanic
eruptions were happening on earth, which means that there was less ash in the
air. A “cleaner” atmosphere meant that the planet was more responsive to
changes in solar output. “Hence the effect of high solar activity on the
atmospheric circulation in the North-Atlantic was particularly strong,” said
Lapointe.
Lapointe and Bradley have been wondering
whether such an abrupt cooling event could happen again in our age of global
climate change. They note that there is now much less arctic sea ice due to
global warming, so an event like that in the early 1400s, involving sea ice
transport, is unlikely. “However, we do have to keep an eye on the build-up of
freshwater in the Beaufort Sea (north of Alaska) which has increased by 40% in
the past two decades. Its export to the subpolar North Atlantic could have a
strong impact on oceanic circulation”, said Lapointe. “Also, persistent periods
of high pressure over Greenland in summer have been much more frequent over the
past decade and are linked with record-breaking ice melt. Climate models do not
capture these events reliably and so we may be underestimating future ice loss
from the ice sheet, with more freshwater entering the North Atlantic,
potentially leading to a weakening or collapse of the AMOC.” The authors
conclude that there is an urgent need to address these uncertainties.
This research was supported by funding
from the National Science Foundation.
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