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October 2001

Glaciers That Speak in Tongues
and other tales of global warming

By Wallace S. Broecker

A century and a half ago, the world?s mountainous regions were somewhat
colder than they are today. We know this because historical
records?writings, paintings, early photographs?show that glaciers were
larger then. In the Swiss Alps, the area for which the most detailed
documentation is available, glaciers expanded during the Little Ice Age,
a cold episode that ran from about 1300 to 1860. (This ice age is called
?little? because, even at its worst, the cooling required to produce it
was only one-tenth the cooling that induced the ?big? ice age that
peaked 20,000 years ago.)

Though records for glaciers in other parts of the world are less
detailed, we know that in the high peaks of the tropical Andes, in the
temperate-zone Andes of Chile and Argentina, and in the Southern Alps of
New Zealand?s South Island, glaciers were substantially larger in 1850
than they are now. So we suspect that the Little Ice Age cooled not just
Europe but the world.

The Little Ice Age ended abruptly. Starting in 1860, the world?s
glaciers began a retreat that has continued right up to the present.
Without a doubt, therefore, planet Earth has gotten warmer over the past
century and a half. But humanity?s exact contribution to the warming is
still under debate. Along with most atmospheric scientists, I take very
seriously the results of computer simulations showing that
human-produced forces are very likely driving the rise in temperature
that we have seen over the past quarter century. Yet roughly half the
overall warming since 1860 occurred before carbon dioxide (CO_2 )
emissions from human activities had reached significant levels. Some
take this as evidence that most of the current upswing in temperature is
merely a continuation of the natural events that brought the Little Ice
Age to a close.

To truly understand the scenario of global warming, we need to know how
much Earth?s temperatures would have fluctuated in the absence of the
Industrial Revolution and whether we are now exacerbating or
counteracting these fluctuations. And we can know these things only if
it can be shown that Earth has had predictable temperature cycles during
the past several thousand years. Many scientists doubt that such
large-scale regularities exist, but assuming for the moment that they
do, how would we detect them? Even year-to-year changes in average
global temperature are difficult to chart, despite the help we get from
the worldwide network of thermometers and from the scores of satellites
orbiting high above Earth. Getting a sense of relevant natural
fluctuations is especially difficult because during the period that
geologists call the Holocene Epoch (the 11,000 years since the end of
the last ice age), Earth?s temperatures have been remarkably stable.
Unlike the previous 100,000 years, when the climate underwent numerous
large jumps and drifts, measured in many degrees Fahrenheit, the changes
during the entire Holocene have been only 1° or 2° F?too small to
ascertain with the natural climate indicators we have been using until
now (such as tree rings and fossil pollen), whose accuracy is no better
than 2° F.

This is why climatologists have turned to mountain glaciers. The record
created by these glaciers is an excellent proxy for climate, standing in
for hundreds of years of thermometer readings. Not only does this proxy
tell us about past temperatures, its margin of error is less than 0.4° F.

Everywhere on Earth, the higher you climb, the colder it gets. The
reason is that as air rises, it expands and therefore cools. On average,
air temperature changes at a rate of 1° F for every 300 feet of
elevation. At some point on the way up, air temperature reaches the
freezing point. Glaciers can?t form or endure, of course, unless the air
temperature remains low, and how far down a mountain they reach depends
on what glaciologists call the equilibrium snowline?the boundary between
an upper zone where accumulation of snow outpaces melting and a lower
zone where the reverse is true. This boundary corresponds fairly closely
to the altitude at which air temperature reaches the freezing point. In
the Alps, large glaciers have tongues of ice streaming down past the
equilibrium snowline and on into valleys in the lower zone. The melting
in such tongues below the snowline is balanced by the downward flow of
ice from above the snowline. When the air temperature shifts, such
tongues can shrink or enlarge dramatically. For example, since 1860 the
Rhône Glacier, in the Alps of southwestern Switzerland, has retreated
about three miles in response to a rise in the equilibrium snowline of
about 300 feet. This rise corresponds to an atmospheric warming of
slightly more than 1° F.

Records of glacial advances and retreats provide an accurate gauge of
temperature changes.

Such astounding sensitivity suggests that a record of glacial advances
and retreats during the Holocene Epoch would yield the information we
seek about recent climate cycles. But this is easier said than done. The
problem is that glaciers act like giant erasers. Each advance eradicates
almost all traces of what?s come before. At their greatest size during
the Little Ice Age, Europe?s glaciers covered an area at least as large
as was covered in any previous Holocene advance, so the record of
earlier Holocene advances is mostly obscured.

We do, however, have evidence that well before the Little Ice
Age?several times during the past 11,000 years, in fact?Alpine glaciers
pushed out to roughly the same position they occupied in 1850. One such
indication is the size of the moraines?the looping walls of debris that
mark the edge of each major Alpine ice tongue. These piles of rocky
rubble are huge, some standing 300 feet above the valley floor. It is
difficult to imagine that they could have been formed during a single
forward push of ice; instead they appear to be the result of a great
many such pushes in the course of thousands of years. Within these huge
debris piles are layers representing ancient soils, indicating that many
earlier advances occurred. But carbon 14 dates derived from soil
materials are often misleading and so cannot be relied on to provide the
precise chronology we seek.

During the last decade, however, a major breakthrough occurred, owing to
the appearance of a new climate proxy: wood and peat that have been
washing out from beneath the retreating Alpine glaciers. For this kind
of material, carbon dating is quite reliable and allows us to determine
precisely the warm periods when trees and other plants were able to grow
in places that are now covered by glaciers.

In September 2000, I had the opportunity to witness a harvest of this
ancient wood and peat when Christian Schluechter, a University of Bern
geologist who pioneered such studies, led a small group of interested
scientists to the terminus of the Unteraare Glacier in the northern
Swiss Alps. He explained to us that once each year, toward the end of
summer, the meltwater that has accumulated beneath the ice suddenly
breaks out of its confinement and sweeps over an apron of large cobbles
lying at the foot of the glacier. To our amazement, we saw pieces of
wood and peat wedged here and there among the cobbles. During the single
hour our helicopter taxi service allotted us for exploring the apron,
our group found fifty separate pieces. Many of them showed evidence of
compression, shearing, and twisting caused by the weight and motion of
the overlying ice.

Our finds were not the first ?warm artifacts? to be harvested from the
forelands of retreating Swiss glaciers. Schluechter and his colleagues
had been collecting wood and peat fragments for several years and have
obtained carbon dates for nearly a hundred of them. Some of the wood
dates all the way back to the earliest Holocene. Even more important,
rather than being spread evenly through time, the dates for these pieces
of wood and peat fall into distinct groups, with each group presumably
representing a warm episode when Alpine glaciers were even smaller than
they are today.

Geologists are now investigating whether these groupings correspond to
another new source of evidence of cyclic patterns in Earth?s recent
history. This evidence comes from studies of sediment in the deep waters
of the North Atlantic. The rock fragments in these sediments are much
too large to have been transported there by ocean currents; they could
have reached their present location only by having been frozen into
large icebergs that floated long distances from their point of origin
before melting. During the past decade, Gerard Bond, my colleague at
Columbia University?s Lamont-Doherty Earth Observatory, has studied the
makeup of such ice-rafted debris. Noticing that some of the sediment
grains were stained with iron oxide, he reasoned that they must have
come from locales where glaciers had overrun outcrops of red sandstone.
Bond concluded that a detailed analysis of deep sediment cores would
reveal changes in the mix of sediment sources over time. This proved to
be an excellent strategy, for Bond found something so unexpected that it
stunned all of us who study climate history. The proportion of these
red-stained grains fluctuated back and forth over time from lows of 5
percent to highs of about 17 percent, and these fluctuations had a
pattern: a nearly regular, 1,500-year cycle. Even more amazing, he found
that the cycles ran virtually unchanged, in both amplitude and duration,
through both ice-age and non-ice-age periods during the last 100,000 years.

Bond puzzled over what might be pacing this cycle. As a geologist, he
knew that the sources of the red-stained grains were generally closer to
the North Pole than were the places yielding a high proportion of
?clean? grains. At certain times, apparently, more icebergs from the far
north were making their way well to the south before finally melting and
shedding their sediment. Bond hypothesized that the alternating cycles
might be evidence of changes in ocean-water circulation.

Ocean waters are constantly on the move, and water temperature is both a
cause and an effect. As water cools, it gets denser and sinks to the
bottom. In one part of what I like to call the ?bipolar seesaw,? the
bottom layer of the world?s oceans comes from cold, dense water sinking
in the far North Atlantic. This causes the warm surface waters of the
Gulf Stream to be pulled northward, as they are today. Bond realized
that during this part of the ocean cycle, a large proportion of the
icebergs that bear red grains would melt while still fairly far north.
But sometimes the ocean reorganizes itself, and the Southern Hemisphere
holds sway in driving ocean circulation. At such times, surface waters
in the North Atlantic would generally be colder, permitting icebergs
bearing red-stained grains to travel farther south before melting and
depositing their sediment.

The onset of the Little Ice Age in about 1300, which followed the
so-called Medieval Warm Period of the eighth through tenth centuries,
may represent the most recent time that such a switchover occurred. The
contrast in the North Atlantic is apparent if we consider that in the
tenth century, Erik the Red and his band of Vikings colonized the lands
surrounding the fjords in southwestern Greenland. Not only did the
Vikings navigate their wooden vessels back and forth between Scandinavia
and Greenland without being thwarted by sea ice, but they were also able
to grow enough grass to support sizable flocks of sheep. As time went
on, however, conditions deteriorated. The last recorded communication
from the colonists occurred in the early fourteenth century?just at the
onset of the Little Ice Age in the Alps?and eventually the colony died
out (see ?The Vikings? Silent Saga,? /Natural History/, November 2000).

Further evidence of the impact of the Little Ice Age comes from records
kept by Icelanders, whose writings indicate that between 1650 and 1850,
their island was icebound for several months each year?a great hardship,
since fishing was a main source of sustenance. They reported with
pleasure that the ice began to wane in 1880, permitting them to extend
the fishing season. Readings from their thermometers (which they began
to use in about 1870) also suggest that the mean annual temperature was
rising.

Could it be that these ocean oscillations, because of their effect on
air temperature, also explain the snowline fluctuations seen in the
Swiss Alps? So far, none of the wood or peat fragments sluiced from
beneath the ice have yielded carbon 14 dates from the eighth through the
thirteenth centuries, which would correspond to the Medieval Warm Period
and the interval leading up to the Little Ice Age. But another source of
evidence demonstrates that Alpine glaciers were smaller during this
time. Medieval farmers living below the huge Aletsch Glacier, in what is
now south-central Switzerland, constructed a crude aqueduct of
hollowed-out larch tree trunks to carry water from a small mountain lake
down to a village. We know from written records that parts of this
aqueduct had to be rebuilt after being overrun by the 1350 advance of
the Aletsch Glacier.

If future dating of wood and peat expelled from beneath retreating
glaciers in the Alps and other mountainous regions worldwide supports
Bond?s 1,500-year cycles, and if the Medieval Warm Period/Little Ice Age
oscillation can be shown to be part of the most recent of these cycles,
we will have taken an important step toward establishing a major natural
rhythm in Holocene Epoch temperatures. This rhythm could then be
extrapolated into the future. Because the midpoint of the Medieval Warm
Period was about A.D. 850, an extension of Bond?s cycles would place the
midpoint of the next warm interval in the twenty-fourth century.

While offering a useful basic framework, this pattern alone does not
account for all aspects of past fluctuations and thus is not a
sufficient predictor for the future. For example, we usually think of
cycles as having a regular, bell-like shape. But during the millennia
corresponding to the last ice age, Bond?s 1,500-year cycles were closer
to rectilinear, indicating sudden starts and stops. As is clearly
recorded within the deep layers of Greenland?s ice, the transitions
often took just a few decades. This abruptness was especially pronounced
as the climate warmed. Has this also been the case during our own epoch,
the Holocene? If the Holocene?s ?Bondian? cycles, too, have been
rectilinear, one would expect the post-Little Ice Age warming to have
been completed within a few decades. One would also expect that in the
absence of the Industrial Revolution, global temperature would have
stabilized for a warm plateau of several hundred years. But in fact,
global temperatures in the decades immediately after the Little Ice Age
did not simply jump to a new plateau.

Also, studies of Alpine glaciers show that the Little Ice Age had three
cold peaks, in about 1350, 1650, and 1850. Do we have any clues about
the causes of these additional, smaller fluctuations? I pondered this
problem in the early 1970s. At the time, Holocene climate records were
few and far between. In fact, only one?a 70,000-year record obtained
from a one-mile-long, four-inch-diameter ice core drilled in northern
Greenland?had enough length and detail to provide any clues. Danish
paleoclimatologist Willy Dansgaard and his colleagues had managed to
obtain paleotemperature results from samples of this core and had
concluded that much of the variation in climate could be accounted for
by a combination of 80-year and 180-year cycles, which they thought
reflected periodic fluctuations in the Sun?s energy output. When I
merged Dansgaard?s pattern with that of the warming expected from the
steady increase in man-made greenhouse gases, I obtained a composite
that matched the major features of the actual global thermometric
record?namely, a warming phase extending from 1860 to the time of World
War II and followed by a thirty-year pause. I predicted that when, in
the near future, Dansgaard?s natural cycle turned from a cooling into a
warming phase, the natural and the man-made factors would join forces
and produce a prominent renewal of the warming trend. My warning was
published in the journal /Science/ in 1975. In 1976 the thirty-year
plateau came to an end, and the warming that then began has continued
right up to the present.

My prediction was correct, but was it soundly based? Ten new ice-core
records?from Greenland, from Antarctica, and from high-mountain sites
elsewhere on the planet?are now available. None show Dansgaard?s
combined 80-year and 180-year cycles. I thus have been inclined to write
off the success of my prediction as just a happy accident. Still, the
changes we are attempting to document have a magnitude of only a few
tenths of a degree, and perhaps in most records they are masked by
regional climate change.

What drives Bond?s 1,500-year cycle? High on the list of possibilities
must be the Sun. But sufficiently accurate, satellite-based measurements
of solar activity cover only the past twenty years?not long enough to
warrant our drawing firm conclusions. We do, however, have a longer-term
record of the number of dark spots resulting from magnetic storms on the
Sun?s surface. These spots usually wax and wane in an eleven-year cycle.
During an interval known as the Maunder Minimum (A.D. 1650-1710),
however, no sunspots were observed. Minze Stuiver, an isotope geologist
at the University of Washington, has shown that more carbon 14 atoms
were created in our atmosphere during the Maunder Minimum than either
before or after it. The reason, Stuiver postulated, is that the
electrically charged particles streaming out of sunspots generate a
magnetic field that deflects incoming cosmic rays from our solar system.
During periods of low sunspot activity, such as the Maunder Minimum,
however, the magnetic shield is turned off and more cosmic rays bombard
Earth?s atmosphere, manufacturing extra carbon 14 atoms. Having
demonstrated the link between sunspots and carbon 14 production, Stuiver
was then able to use carbon 14 measurements on tree-ring-dated wood to
deduce sunspot minimums prior to the invention of the telescope. His
data indicate that these occurred at roughly two-century intervals and
might be the cause of Dansgaard?s 180-year cycle. Stuiver?s record
contains no hint of Dansgaard?s 80-year cycle or of Bond?s 1,500-year
cycle, however.

If not the Sun, then what might be the driver of Bond?s cycle? For me,
the top candidate remains the ocean?s bipolar seesaw. In the global
ocean circulation pattern we are accustomed to seeing, sinking cold
surface water in the North Atlantic is replaced by warm Gulf Stream
waters, which are drawn northward. What may underlie this pattern is
actually salt, since an extra gram of salt per liter makes seawater
denser by an amount equivalent to a cooling of 8° F. The North Atlantic
is unusually salty, because the location of mountains and the direction
of prevailing winds lead to the export of water vapor from the Atlantic
to the Pacific Ocean. The salt left behind makes that surface water not
only saltier but denser, and the water sinks in the familiar pattern.

Meanwhile, acting like a giant conveyor belt, the normal circulation of
the world?s oceans transports excess salt out of the Atlantic Ocean.
This process may not occur at a steady rate. Rather, it may oscillate,
leading to Bond?s 1,500-year cycle. As I see it, at times the conveyor
mechanism carries salt away faster than it is being replaced. This may
lower the water density in the North Atlantic enough to make the
conveyor shut down. At this point, the warm Gulf Stream weakens, and the
circulation pattern reorganizes itself under the domination of the
Southern (Antarctic) Ocean. With the new regime, salt ceases its rapid
departure from the North Atlantic, and salt levels there begin to rise
again. When the northern waters are again dense enough to rejuvenate
deep-water formation in the North Atlantic, the cycle is complete.

Are underlying natural cycles retarding or reinforcing human-induced
global warming?

The details of this mechanism are still not clear, but what is clear is
why such a cycle might lead to warmings and coolings of the lands
surrounding the North Atlantic. When the north rules, an enormous amount
of heat?as much as would be generated by a million large power plants?is
carried north of the Strait of Gibraltar by the Gulf Stream. This heat
is released into the atmosphere in winter and is carried to northern
Europe by the prevailing westerly winds. When the south rules instead,
this source of heat is lost and the Alps cool.

But how does any of this explain the cooling in the Southern
Hemisphere?the Little Ice Age in New Zealand?s Southern Alps and in
South America?s Andes? Instead of being limited to the land area
adjacent to the North Atlantic, this cold episode appears to have
affected much of the planet. There must have been some link between
patterns of ocean circulation and conditions in the atmosphere that
affected both hemispheres. Although the nature of this link has not been
discovered, it probably involves Earth?s ?tropical heat engine??the way
air rising from the equator fuels the atmosphere with heat and water vapor.

So where do we stand? For a start, certain regular fluctuations in the
Holocene climate seem to occur, but scientists are still left with many
uncertainties about them. This prevents us from making a meaningful
prediction concerning how the climate would have changed in the absence
of the Industrial Revolution. We cannot prove the existence of either
the 1,500-year cycle?which the available Greenland ice cores fail to
record?or the combined 80- and 180-year cycles. However, we can state
with some confidence that natural Holocene temperature fluctuations have
been on the same scale as the human-caused effects estimated to result
from greenhouse gases. Hence, we cannot assume that in the absence of
human intervention, Earth?s temperatures would have remained stable.

Unfortunately, we cannot even say whether natural changes are at this
point retarding or reinforcing human-induced greenhouse warming. The
situation will be much clearer two decades from now, however, as
computer simulations predict an additional 1.5° F warming by the year
2020. If such an increase in global temperatures occurs, there will not
be any doubt: natural causes alone would not have been sufficient to
account for it.

We may have to remove CO_2 from power-plant exhausts and even from the
atmosphere itself.

Does this mean we can all sit back, do nothing, and wait for the results
to roll in? Certainly not. In twenty years, we may well conclude that we
must stem the rise of CO_2 , and if so, we?ve got a lot of preparation
to do. Very likely, fossil fuels will remain our primary source of
energy. With more people and a higher standard of living in the less
developed countries?and with even a sustained per capita level of demand
in the United States?energy use will at least double by the year 2050.
We must learn how to remove CO_2 from power-plant exhausts and probably
from the atmosphere itself. (Estimates of the cost of CO_2 removal
suggest that it would add a few cents per kilowatt-hour to the cost of
electricity or about forty cents per gallon to the cost of gasoline.)
Plus, of course, the CO_2 we remove must be put somewhere: stored in the
deep ocean or in deep saline aquifers or, if we want to be sure it never
comes back to haunt us, converted into magnesite (MgCO_3 ), a
geologically inert mineral. More difficult than the technical aspects
are the political ones. Ready or not, we will have to face them all.

To strengthen their case, corporate spokespersons, avid consumers, and
plenty of other people and institutions inclined to dismiss the ongoing
rise in atmospheric CO_2 as inconsequential may be happy to latch on to
the paleoclimatic reconstruction presented here. This would be
unfortunate. Unless all the work done on climate simulations and
fossil-fuel-use projections is seriously flawed, one thing is certain:
our planet will indeed experience a major human-induced warming during
this century.

We have learned that Holocene temperatures have undergone natural
fluctuations, but the causes of these changes are so subtle that we have
yet to figure them out. Apparently, our climate system responds to even
tiny nudges. This being the case, the potential effects of human
activities should not be underestimated. If we continue along a
business-as-usual energy course, we?ll be giving the climate a large shove.

/A recent visit to the Alps provided Wallace S. Broecker with a
firsthand glimpse of new evidence of past climate changes. Guided by
Swiss glaciologists, he and the others in the visiting field team found
fifty samples of ancient peat and wood that had washed out from beneath
the glacial ice?signs that vegetation once grew in areas now frozen
over. The Newberry Professor of Earth and Environmental Sciences at
Columbia University's Lamont-Doherty Earth Observatory, Broecker is the
author of a text on the evolution of Earth,/ How to Build a Habitable
Planet /(Columbia University Press, 1998). Among his previous articles
for/ Natural History /is ?Global Warming on Trial? (April 1992)./

Copyright © 2001 American Museum of Natural History

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