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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 lot longer.

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 events.

While at the University of Göttingen, 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/O 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/O 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/O 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 merely local.

Then Bond began to suspect that D/O 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 cause.

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 dark-brown 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 dropped off.

Cold trigger

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 events.

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 near-regular, 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
climatic change”.

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.

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 Antarctica. 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 Antarctic has declined dramatically over the past 800 years. He
believes that this see-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 tides, 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 in the oceans and draw cold ocean
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 the 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?