CCNet, 82/2000 - 4September 2000
ON LITTLE ICE AGES & THE THREAT OF GLOBAL COOLING
From NEW SCIENTIST, 2 September 2000, pp. 30-33
FEEL THE PULSE
Europe frozen, China starving .... The Little Ice Age crippled the
world for hundreds of years. Was it an aberration or one in a series of
cold spells? FRED PEARCE explains why there could be more to come.
ARCTIC PACK ICE extended so far south that Eskimos landed their kayaks
on the north coast of Scotland. Further south, hungry Highlanders
raided the Scottish lowlands. King James VI set up colonies of
Presbyterian refugees from famine in Catholic northern Ireland. Wolves
raided the villages of England for what was probably the last time,
while Londoners frolicked at Frost Fairs on the frozen River Thames.
This was the little ice age, which began in the 13th century and peaked
in the 17th before finally releasing its grip some 200 years ago. At
its height, there was widespread famine across northern Europe. Some
suggest that half the populations of Norway and Sweden perished.
Meanwhile, icy fingers stretched across the globe: snow blanketed parts
of Ethiopia, crops failed in China and ice appeared on Lake Superior.
For some, the little ice age is just a historical curiosity-a random
blip in a balmy world of climatic certainty. For others, it heralds the
perils of today's climate change. But in the past three years a third
interpretation has emerged. Gerard Bond from Lamont Doherty Earth
Observatory of Columbia University in Palisades, New York, believes that
the little ice age is the most recent sign of a pervasive "pulse" in the
world's climatic system. Appearing once every 1500 years or so, this
pulse seems largely unaffected by glaciations-the "big ice ages" that
come round every 100 000 years or so. It could even have been around a
Tantalisingly, the pulse's origin remains a mystery. Wobbles in Earth's
orbit are generally agreed to be the trigger for glaciations, but there
are no accepted astronomical events that explain this newly discovered
cycle. Yet there are a few intriguing suggestions that could help us to
understand how our climate is really likely to change in the future.
Researchers have unwittingly been on the trail of the planetary pulse
for some years, but didn't discern the pattern because they only heard
occasional beats. In the early 1980s a graduate student in Germany made
the first breakthrough, discovering what are now known as Heinrich
While at the University of Goettingen, Hartmut Heinrich found a number
of curious layers of rock fragments buried in sediment on the floor of
the North Atlantic. These layers appeared to continue all the way from
the coast of Canada to the waters west of the British Isles, even
showing up in sediment cores drilled as far south as Bermuda.
Radiocarbon dating revealed that they were laid down in bands at
intervals of roughly 8000 years all the way through to the last
glaciation, which ended 10 000 years ago.
The odd thing about them is that the rock fragments came from the
Hudson Bay area of northern Canada. How did they get so far south? The
only explanation, Heinrich concluded, was that they had been carried by
icebergs breaking off from the main North American ice sheet. Huge
armadas of icebergs must have floated south for thousands of kilometres,
he said, before melting and depositing their cargo on the ocean floor.
In the early 1990s there was another important finding: the Dansgaard /
Oeschger or D/0 cycles, a series of large and sudden temperature
changes that occurred across Greenland, again during the last
glaciation. Willi Dansgaard of the University of
Copenhagen discovered the cycles when he analysed the isotopic
composition of oxygen in layers of Greenland ice and found fluctuations
in temperature averaging at least 2 'C.
The importance of the two cycles wasn't recognised immediately: both
seemed at first to be minor local curiosities confined to the last
glaciation. The Heinrich events were put down to inherent instabilities
in the ice sheets, and the D/ 0 cycles to local changes in ocean
currents off Greenland.
It took two further discoveries to change this view. Oceanographer
Wallace Broecker, also from Lamont~Doherty, realised that the changes
in ocean currents associated with the D/ 0 cycles were a vital element
in a global ocean circulation known as the "conveyor belt". This is a
fundamentally important mechanism for distributing heat around the
globe-a kind of planetary thermostat-so its effects are unlikely to be
Then Bond began to suspect that D/ 0 cycles and Heinrich events were
linked and occurred at the same time as other climate change
elsewhere-the advances and retreat of glaciers in Europe and North
America, for instance. They might even, he speculated, have a common
To find out if they did, Bond began to re-examine sediment cores from
the bed of the North Atlantic. Over the past five years he has analysed
cores taken from three parts of the Atlantic. Some are old taken years
ago by the Lamont-Doherty research vessel Vema from beneath the waters
off Ireland and the channel between Greenland and Iceland. Others are
new cores Bond himself drilled off Newfoundland. He combed the cores
for climate signals over the past 30 000 years, covering both the end of
the last glaciation and our own postglacial, supposedly warm and
tranquil epoch, the Holocene.
Sure enough, Bond found Heinrich's rock fragments every 8000 years.
But he also found other much more frequent signs of sudden climate
change (see Diagram). The cores all revealed layers, occurring roughly
every 1500 years, that contained thick deposits of two other materials
normally alien to the seabed of the North Atlantic. There were grains
of quartz and feldspar rock stained red with haematite from rocks in
east Greenland and the Arctic islands of Svalbard. These, concluded
Bond, were scraped up by ice and rode south with icebergs that broke
off from the ice sheets. And there was also glass from the volcanic
eruptions that have convulsed Iceland throughout its history. He
surmised that these darkbrown shards, which normally fell into the
ocean around the island, periodically hitched a ride on the great
iceberg fleets into the warmer waters of the Atlantic before being
Heinrich has said that these ice armadas must be the result of the
expansion of continental ice sheets that reached some point of
instability and then began to collapse. Bond has proved that this is
unlikely to explain his 1500-year pulse. For one thing, the cold spells
occurred at similar intervals through both glacial and interglacial
eras-whether there were large ice sheets in place or not. For another,
the volumes of rock tracers from both Greenland and Iceland produce
peaks at the same time which is unlikely if they were dependent on local
So what did cause the cold pulses? Bond returned to the seabed cores
once more. The same layers that yielded the rock tracers also contained
unusually large numbers of skeletons from cold water plankton and
remarkably few warm-water species-showing that the surface waters of the
ocean were around 2 'C colder during the cold pulses than they are
today. Crucially, the oceans appear to have begun to cool about 500
years before the armadas set sail. It looks as if the cooling triggered
the iceberg flows, not the other way round.
The conclusions seem inescapable. Rapid climate change has punctuated
both the glacial era and the period since-which scientists had until
recently believed was drama-free. Moreover, those rapid changes
occurred as pulses unaltered by the end of the last glaciation. The
pulse is nearregular, but not quite like a musical metronome. The
interval varies, generally between 1300 and 1800 years, says Bond. But
it is recognisably a pulse, just as a human heartbeat that races and
then slows is recognisable. It is, he says, a .pacemaker of rapid
In recent years, climatologists have sought to explain climate change
largely in terms of solar cycles, wobbles in the Earth's orbit,
occasional volcanic eruptions and the rise and fall of concentrations
of greenhouse gases. Suddenly this is not enough. Something else is
going on. And it seems to tie together disparate climatic events,
including droughts as well as temperature changes, round the world. 1
Research published earlier this year by climate scientists Charles
Keeling and Tom Whorf from the Scripps Institution of Oceanography in
La Jolla, California, links the cycles with other findings. These
include recurrent events such as are shown by consecutive layers of
dust in the sediments of lakes in the American Midwest, and single
events such as a major drought in the Amazon rainforest around 2200 BC,
and another at roughly the same time in Mesopotamia that researchers
think caused the collapse of Akkadia, one of the world's first empires.
Late last year Bond's colleague Peter deMenocal reported a temperature
switchback in tropical Africa, with recurrent swings of 5 'C.
DeMenocal examined seabed sediments from off Africa's west coast. He
found that every 1500 years or so there were huge increases in dust
particles in the sediments, suggesting big dust storms on land. The
sediments also revealed dramatic increases in the remains of
temperature-sensitive marine plankton, suggesting significant shifts in
temperature. Importantly, the timing and direction of these cycles
match those that Bond found. "The transitions were sharp. Climate
changes that we thought should take thousands of years to happen
occurred within a generation or two," deMenocal says.
Is the pulse now history, or is it part of our continuing climate
drama? New cores drilled by Bond off Newfoundland in 1998 show for the
first time that the ocean sediment record of the little ice age bears
all the hallmarks of the pulse. The layers of volcanic glass,
haematite-stained rock and cold-water plankton are all in place. As
the little ice age lasted up to the start of the 19th century, some
argue that much of the warming in the 20th century is evidence of
Earth's gradual recovery from this cool spell. No wonder Broecker says
that the key to understanding humanity's role in the global warming
trend "may lie in unravelling the demise of the little ice age".
But this still leaves unanswered the big question: what is causing the
mysterious pulse? Most researchers believe the essential clue lies in
ocean circulation and the intimate relationship between the oceans and
the atmosphere. And the crux of that relationship seems to be in the
North Atlantic, where the ice armadas appear. This region is where the
northbound Gulf Stream comes to a halt and its water cools and freezes.
The cold and increasingly salty water sinks to the bottom of the ocean,
maintaining the conveyor belt's slow global circulation system, which
Broecker calls the "Achilles heel" of our climate.
This process of deep-water formation only occurs in one other place,
Broecker says - off Antartica. And the balance between these two
locations - and perhaps the overall rate of formation - seems to change
over time. Currently, the North Atlantic site dominates, while the rate
of deep-water formation in the Antartic has decline dramatically over
the past 800 years. He believes that this sea-sawing may be linked to
the 1500-year cycle, though the evidence is scarce and relies on
computer models of how the ocean conveyor may trigger climate shifts.
Bond says spontaneous internal oscillation of the conveyor belt could
cause pulses in the atmosphere. But speculation is growing that there
may be an external force at work. It could be changes in the solar
radiation reaching the Earth. Or it could be extraterrestrial
gravitational forces acting on Earth's tides, as the most recent and
tantalising theory suggests.
Earlier this year, Keeling pointed out something intriguing about
marine tides. He noticed that the changing alignment of the Earth, Moon
and Sun alters the strength of these tiodes, and that these changes
occur with roughly the same frequency as Bond's climatic cycles.
Keeling also suggested why. Stronger tides, he says, increase the
vertical mixing of water to the surface, where it cools the atmosphere
above. Weak tides, meanwhile, reduce mixing, keep the cold water at the
ocean floor and allow the world to warm.
Keeling calculates that tides last reached a maximum strength in 1425
or thereabouts - coincidentally, the depths of the little ice age. This
startling idea has caught the attention of Bond, who says it is "at
least as good" as the others.
Whatever the cause of this pulse, it seems likely that we will be in
for another cold spell somewhere along the line. Working ahead from the
little ice age, that would set the date for the next Thames Frost Fair
at around the year 3000. Any advance bookings?
Copyright 2000, New Scientist
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CCNet ESSAY, 4 September 2000
EVERYTHING IS CONNECTED: SEARCHING FOR HISTORICAL IMPACTS IN NORTH
ARMERICA AND A SURVEY OF SOUTHERN AND EASTERN NATIVE AMERICAN SITES
* PART 1
By E.P. Grondine <email@example.com>
I used the opportunity provided by my trip to the Mars planning
conference in Houston to conduct a survey of a number of Native
American archaeological sites. As the survey results are of more
lasting importance than the outcome of the Mars planning session, I'm
sending them off first, and my report on the Mars planning session will
be along shortly.
Earlier posts by CCNet participants had brought up the possibility of
an impact event producing the "Great Raft", a log jam which blocked
the Atchafalaya and Red Rivers; I believe I may now be able to throw
some light on the time when this possible impact event may have
occurred, and its far reaching consequences. I have also begun
collecting materials relating to a massive impact event which occurred
in the Bald Mountains on the Tennessee-North Carolina border around AD
1200, and parts of this material will be presented here. Finally,
there have been multiple speculations on the construction of the
Serpent Mound in Southern Ohio; I will review some of the most recent
information, and explain its symbolism in detail.
CCCMENU CCC for 2000