1200-year mini Ice Age was
caused by global warming
Isotope analysis reveals detailed pattern of ancient
weather event, and prompts warnings of a repeat. Richard A Lovett reports.
And the ice
came back: Isotope analysis suggests global warming was responsible for a
1200-year cold snap.
Geologists
studying boulders left behind by the melting of ancient Canadian ice sheets are
confirming a theory that a 1200-year-long Northern Hemisphere cold snap called
the Younger Dryas was, ironically, caused by global warming.
The Younger
Dryas lasted from about 12,900 until 11,700 years ago and saw temperatures in
northeastern North America, Europe, and Greenland drop by a whopping 10 degrees
Celsius.
“[It was] a
sharp reversal back toward nearly full Ice Age conditions,” says Andy Wickert,
a geologist at the University of Minnesota in the US, who was not part of the
new study.
One theory is that the downturn was
caused by an asteroid or comet impact that filled the atmosphere with haze,
reducing the amount of sunlight reaching the surface. But it’s not clear how
that could have caused such a prolonged cooling. A more popular theory is that
it was caused by the influx of large amounts of fresh water into the North
Atlantic.
At issue is
something called the North Atlantic thermohaline circulation, in
which currents such as the Gulf Stream carry warm water from the topics as far
north as Canada, Iceland, the UK, and even Norway. Along the way, the water
chills, becoming denser, until it sinks and is replaced by more warm water from
the south. The process is an important mechanism for drawing heat northward
from the tropics, and a major reason why Norway is warmer than Siberia.
But fresh
water is about 2.5% less dense than seawater. A large enough influx of it into
the North Atlantic would throw a major spanner into the works by preventing the
surface water from sinking into higher-saline depths as it chills. Richard
Alley, a geoscientist at Pennsylvania State University, University Park,
compares it to jamming a screwdriver into a grocery-store conveyor belt at the
point where it goes down, bringing it to a sudden halt.
“It isn’t a
perfect analogy,” he says, “but if you dump enough water into [the North
Atlantic] in the summer, the water will cap the ocean the next winter, freeze,
and you will have a cold event,” he says.
(This, in fact,
was the idea behind the 2004 movie The Day After Tomorrow, although
the movie sufficiently exaggerated the science that most scientists in the
field joined the critics in calling it dreadful.)
In the new
study, published online in the journal Geology,
the researchers found a route by which a large river could have dumped massive
amounts of water into the North Atlantic at just the right time to trigger the
Younger Dryas cold snap.
The water
would have come from an enormous lake, called Lake Agassiz, which at various
times sprawled across large swaths of the Canadian provinces of Manitoba,
Saskatchewan, and Ontario. Prior to the Younger Dryas, it drained south, into
America’s Mississippi river and from there into the Gulf of Mexico, where its
effect on climate would have been minimal.
But based on
trace isotopes in granite boulders, the scientists showed that a new channel
opened to the eastward at just the right time to trigger the Younger Dryas,
with the water running through the modern-day Great Lakes and out to the North
Atlantic via Canada’s Saint Lawrence River.
The find,
says study author Anders Carlson, a glacial geologist at Oregon State
University, Corvallis, came from using beryllium-10 to measure how long ago
these boulders first emerged from beneath the ice.
Beryllium-10
is formed by cosmic ray bombardment, which creates it in quartz grains, at a
rate of about four to five atoms per gram, per year, Carlson says. By measuring
the amount in boulders that had been plucked out of bedrock by the ice sheet,
dragged south by the ice, then shielded by thousands of metres of it until it
melted, he says, it’s possible to determine when these rocks
first emerged into the daylight.
He compares it to measuring the amount of sunburn accumulated by these
rocks since they first came to light – although, since most cosmic rays come
from sources other than the sun, “spaceburn” is probably a more accurate term.
Either way, “we can directly date the ice retreat,” he says.
Figuring out the cause of a 12,000-year-old climate change may not seem all
that important, but the reality is that we’re in another era of warming, in
which ice is again rapidly melting. To get an equivalent freshwater discharge
into the North Atlantic, Carlson says, the Greenland ice sheet would need to
entirely melt in about 1300 years.
“That is plausible,” Carlson says. “It’s a lot of warming, but it’s not out
of the realm of possibility.”
And while the Younger Dryas cold snap was a Northern Hemisphere phenomenon,
the same factors that kept heat from moving north caused it to build up in the
south. “Glaciers in the New Zealand Alps retreated,” Carlson says.
Not to mention that cooling in the far north could shift the monsoon.
“Its normal position is slightly north of the Equator,” Carlson says, “but
if you cool the Northern Hemisphere, you could shift the average monsoon belt
south.”
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