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By the 20th century, scientists had rejected old tales of world
catastrophe, and were convinced that global climate could change only
gradually over many tens of thousands of years. But in the 1950s, a few
scientists found evidence that some changes in the past had taken only
a few thousand years. During the 1960s and 1970s other data, supported
by new theories and new attitudes about human influences, reduced the
time a change might require to hundreds of years. Many doubted that such
a rapid shift could have befallen the planet as a whole. The 1980s and
1990s brought proof (chiefly from studies of ancient ice) that the global
climate could indeed shift, radically and catastrophically, within a century
perhaps even within a decade.
This essay covers large one-way jumps of climate. For short-term
cyclical changes, see the essay on Changing Sun, Changing
Climate. For the main discussion of rapid changes in glaical ice,
see the essay on Ice Sheets and Rising Seas,
and for changes in ocean circulation, see the essay on Ocean
Currents and Climate. A shorter article on "The Discovery of
Rapid Climate Change" is online here.
abrupt climate change, sudden change, global
warming, greenhouse effect, sea level rise, ice sheet collapse
"A small forcing can cause a small [climate]
change or a huge one." National Academy of Sciences, 2002.(1)
|Climate, if it changes
at all, evolves so slowly that the difference cannot be seen in a
human lifetime. That was the opinion of most people, and nearly all
scientists, through the first half of the 20th century. To be sure,
there were regional excursions, such as long spells of drought in
one place or another. But people expected that after a few years "the
weather" would automatically drift back to its "normal" state, the
conditions they were used to. The planet's atmosphere was surely so
vast and stable that outside forces, ranging from human activity to
volcanic eruptions, could have no more than a local and temporary
- LINKS -
Full discussion in
| Looking to times long past, scientists recognized that massive
ice sheets had once covered a good part of the Northern Hemisphere.
The Ice Age was tens of thousands of years in the past, however, and
it had been an aberration. During most of the geological record, the
Earth had been bathed in uniform warmth — such was the fixed
opinion of geologists. As one meteorologist complained, geology textbooks
in 1990 were still copying down from their predecessors the venerable
tradition that the age of the dinosaurs (and nearly all other past
ages) had enjoyed an "equable climate."(2)
The glacial epoch itself seemed to have been a relatively stable condition
that lasted millions of years. It was a surprise when evidence turned
up, around the end of the 19th century, that the recent glacial epoch
had been made up of several cycles of advance and retreat of ice sheets
not a uniform Ice Age but a series of ice ages.
| Some geologists denied the whole idea, arguing
that every glaciation had been regional, a mere local variation while
"the mean climate of the world has been fairly constant."(3)
But most accepted the evidence that the Earth's northern latitudes,
at least, had repeatedly cooled and warmed as a whole. The global
climate could change rapidly that is, over the course of only
a few tens of thousands of years. Probably the ice could come again.
That gave no cause to worry, for it surely lay many thousands of years
in the future.
| A very few meteorologists
speculated about possibilities for more rapid change, perhaps even
the sudden onset of an ice age. The Earth's climate system might be
in an unstable equilibrium, W.J. Humphreys warned in 1932. Although
another ice age might not happen for millions of years, "we are not
wholly safe from such a world catastrophe."(4)
The respected climate expert C.E.P. Brooks offered the worst scenario.
He suggested that a slight change of conditions might set off a self-sustaining
shift between climate states. Suppose, he said, some random decrease
of snow cover in northern latitudes exposed dark ground. Then the
ground would absorb more sunlight, which would warm the air, which
would melt still more snow: a vicious feedback cycle. An abrupt and
catastrophic rise of tens of degrees was conceivable, "perhaps in
the course of a single season."(5) Run the cycle backward, and an ice age might suddenly descend.
| Most scientists dismissed Brooks's speculations
as preposterous. Talk of sudden change was liable to remind them of
notions popularized by religious fundamentalists, who had confronted
the scientific community in open conflict for generations. Believers
in the literal truth of the Bible insisted that the Earth was only
a few thousand years old, and defended their faith by claiming that
ice sheets could form and disintegrate in mere decades. Hadn't mammoths
been discovered as intact mummies with grass in their stomachs, evidently
frozen in a shockingly abrupt change of climate? Scientists scorned
such notions. Among other arguments, they pointed out that ice sheets
kilometers thick must require at least several thousand years to build
up or melt away. The physics of ice, at least, was simple and undeniable.
| The conviction that climate changed only slowly was not affected
by the detailed climate records that oceanographers recovered, with
increasing frequency from the 1920s through the 1950s, from layers
of silt and clay pulled up from the ocean floor. Analysis showed no
changes in less than several thousand years. The scientists failed
to notice that most cores drilled from the seabed could not in fact
record a rapid change. For in many places the mud was constantly stirred
by burrowing worms, or by sea floor currents and slumping, which blurred
any abrupt differences between layers.
| Lakes and peat bogs
retained a more detailed record. Most telling were studies in the
1930s and 1940s of Scandinavian lakes and bogs, using ancient pollen
to find what plants had lived in the region when the layers of clay
("varves") were laid down. Major changes in the mix of plants suggested
that the last ice age had not ended with a uniformly steady warming,
but with some peculiar oscillations of temperature.(6)
The most prominent oscillation already noticed in glacial moraines
in Scandinavia around the turn of the century had begun with
a rise in temperature, named the Allerød warm period. This
was followed by a spell of bitterly cold weather, first identified
in the 1930s using Swedish data. It was dubbed the "Younger Dryas"
period after Dryas octopetala, a graceful but hardy Arctic
flower whose pollen gave witness to frigid tundra. (The glacial period
that preceded the Allerød was the "Older Dryas.") The Younger
Dryas cold spell was followed by a more gradual warming, ending at
temperatures even higher than the present. In 1955 the timing was
pinned down in a study that used a new technique for dating, measuring
the radioactive isotope carbon-14. The study revealed that the chief
oscillation of temperatures had come around 12,000 years ago. The
changes had been rapid where "rapid," for climate scientists
at mid-century, meant a change that progressed over as little as one
or two thousand years. Most scientists believed such a shift had to
be a local circumstance, not a world-wide phenomenon. There were no
data to drive them to any other conclusion, for it was impossible
to correlate sequences of varves (or anything else) between different
continents. That would only become possible when radiocarbon dating
overcame the many inaccuracies and uncertainties that beset the technique
in its early years.(7*)
| Even swifter changes could show up in the clay varves derived from
the layers in the mud of lake beds laid down each year by the spring
runoff. But there were countless ways that the spring floods and even
the vegetation recorded in the layers could have changed in ways that
had nothing to do with climate a shift of stream drainages,
a forest fire, the arrival of a tribe of farmers who cleared the land.
Abrupt changes in varves, peat beds, and other geological records
were easily attributed to such circumstances. Scientists could win
a reputation by unraveling causes of kinks in the data, but for climatology
it all looked like nothing but local "noise."(8)
| Thus it was easy to dismiss the large climate
swings that an Arizona astronomer, Andrew Ellicott Douglass, reported
from his studies of tree rings recovered from ancient buildings and
Sequoias. Other scientists supposed these were at most regional occurrences.
Even regional climate changes scarcely seemed to affect the trees
that most scientists looked at (the American Southwest was exceptional
in its radically varying climate and precariously surviving trees).
It didn't help that Douglass tried to correlate his weather patterns
with sunspots, an approach most meteorologists thought hopelessly
| If researchers had found simultaneous changes at widely different
locations, they might have detected a broad climate shift. Carbon-14
dating remained fraught with uncertainties, however, and matching
up the chronologies of different places was difficult and controversial.
Morevover, even a massive and global climate change could bring rains
in one locale, cold in another, and little shift at all of vegetation
in a third. So each study remained isolated from the others.(9)
| Some pointed out that the overall rate of
advance and retreat of the great ice sheets had been no faster than
present-day mountain glaciers were seen to move.(10) That was compatible with "the uniformitarian
principle." This geological tenet held that the fundamental forces
that molded ice, rock, sea, and air did not vary over time. Some further
insisted that nothing could change otherwise than the way things are
seen to change in the present. Geologists cherished the uniformitarian
principle as the very foundation of their science, for how could you
study anything scientifically unless the rules stayed the same? The
idea had become central to their training and theories during a century
of disputes. Scientists had painfully given up traditions that explained
certain geological features by Noah's Flood or other one-time supernatural
interventions. Although many of the theories of catastrophic geological
change were argued on fully scientific grounds, by the end of the
nineteenth century scientists had come to lump all such theories with
religious dogmatism. The passionate debates between "uniformitarian"
and "catastrophist" viewpoints had only partly brought science into
conflict with religion, however. Many pious scientists and rational
preachers could agree that everything happened by gradual natural
processes in a world governed by a reliable God-given order.(11)
| Nowadays temperatures apparently could not
rise or fall radically in less than millennia, so the uniformitarian
principle declared that such changes could not have happened in the
past. The principle thus went hand-in-glove with a prevailing "gradualist"
approach to all things geological. Alongside physical arguments that
the great masses of ice, rock and water could not change quickly,
paleontologists subscribed to a neo-Darwinian model of the evolution
of species which argued that here too change must be continuous and
gradual. All that seemed to apply to climate. Textbooks pointed out,
for example, that there were plausible reasons to believe that tropical
rain forests had scarcely changed over millions of years, so the climates
that sustained the orchids and parrots must have been equally stable.
There was no reason to worry about the fact that old carbon-14 dates
were accurate only within about a thousand years plus or minus, so
that a faster change could hardly have been detected. If there were
unmistakable fluctuations like the Younger Dryas, presumably those
had regional rather than global scope restricted to the vicinity
of the North Atlantic or an even narrower area (few studies had been
done anywhere else).
the carbon-14 expert Hans Suess, studying the shells of plankton
embedded in cores of clay pulled from the seabed by Columbia University’s
Lamont Geological Observatory, discovered a change at the fastest
speed that anyone expected. Suess reported that the last glacial
period had ended with a "relatively rapid" rise of temperature
about 1°C (roughly 2°F) per thousand years.(12)
The rise looked even more abrupt when David Ericson and collaborators
inspected the way fossil foraminifera shells varied from layer to
layer in the Lamont cores. They reported a "rather sudden change
from more or less stable glacial conditions" about 11,000 years
ago, a change from fully glacial conditions to modern warmth within
as little as a thousand years. They acknowledged this was "opposed
to the usual view of a gradual change."(13) Indeed Cesare Emiliani, who often disagreed with Lamont
scientists, published an argument that the temperature rise of some
8°C had been the expected gradual kind, stretching over some
More was at stake than simple dating. A graduate student in the
Lamont group, Wallace Broecker, put a bold idea in his doctoral
thesis. Looking at this and other data, he found "a far different
picture of glacial oscillations than the usual sinusoidal pattern."
Like Brooks, he suggested that "two stable states exist, the glacial
state and the interglacial state, and that the system changes quite
rapidly from one to the other." (A revision of Brooks's 1926 book
on Climate through the Ages was published in 1949, and
it was popular enough to be reprinted in 1970.)(15)
This was only one passage in a thick doctoral thesis that few people
read, and sounded much like Brooks's speculations on cataclysmic
changes, long since dismissed by scientists as altogether implausible.
<=Uses of shells
|After considerable debate,
Emiliani won his point. The rapid shift that Ericson had reported
was not really to be found in the data. Like some other sudden changes
reported in natural records, it reflected peculiarities in the method
of analyzing samples, not the real world itself. Yet mistakes can
be valuable, if they set someone like Broecker to thinking about overlooked
possibilities. Sometimes the mistake even turns out to reflect a valid
understanding, when, as Broecker later remarked, "...you go back around
and actually the discovery itself was valid, even though the thing
that led to it was wrong."(16)
By 1960, three Lamont scientists Broecker, Maurice Ewing, and
Bruce Heezen were reporting a variety of evidence, from deep-sea
and lake deposits, that a radical global climate shift of as much
as 5-10°C had in fact taken place in less than a thousand years.(17) While it would necessarily take many
thousands of years to melt the great ice sheets, they had realized
that meanwhile the atmosphere and the ocean surface waters, which
were less massive, could be fluctuating on their own. Broecker speculated
that the climate shifts might reflect some kind of rapid turnover
of North Atlantic ocean waters a natural place for an oceanographer
|A few scientists responded with more specific
models. Most important was a widely noted paper by Ewing and William
Donn, who were "stimulated by the observation that the change in climate
which occurred at the close of the [most recent] glacial period was
extremely abrupt." Their model proposed ways that feedbacks involving
Arctic ice cover could promote change on a surprisingly rapid scale.(18) Following up, J.D. Ives drew on his detailed field studies
of Labrador to assert that the topography there could support what
he called "instantaneous glacierization of a large area." By "instantaneous"
he meant an advance of ice sheets over the course of a mere few thousand
years, which was roughly ten times faster than most scientists had
imagined.(19) However, the Ewing-Donn theory
turned out to have fatal errors, and most scientists continued to
doubt that such swift changes were possible.
| Further information came from studies of fossil pollen recovered
from layers of peat laid down in bogs. The scientists who undertook
such work had not set out to study the speed of climate change. Their
inquiry was mostly a routine, plodding counting of hundreds of specks
under the microscope, assembling data on vegetation shifts to catalog
the way ice sheets came and went. But the carbon-14 dates offered
surprises for an attentive eye. For example, a 1961 study mentioned
in passing that at one location in Wisconsin, the transition from
glacial-period pines to oak trees had taken at most 200 years.(20)
Earth scientists had to be careful in describing such results, for rapid change
remained a touchy question. During the 1950s, Immanuel Velikovsky
and others had excited the public with popular books describing
abrupt and marvelous upheavals in the Earth's history. Frozen mammoths
were brought forth again as proof that the world’s climate
could change catastrophicall
y overnight (although every arctic hiker knows how swiftly a freeze
can come even in summer, or how a misstep in a shifting riverbed
could bury the careless in permafrost). Experts grew weary of explaining
to students and newspaper reporters that the scenarios were sheer
fantasy. The battle against Velikovsky and his ilk only reinforced
geologists' insistence on the uniformitarian principle, which they
took as a denial of any change radically unlike changes seen in
the present. Ideas of catastrophic change were also tainted by the
way zealots used the ideas, persistently and increasingly, as they
sought "scientific" proof for their fundamentalist interpretation
of passages in the Bible. (Typical was the complaint of a paleontologist
who prefaced his 1992 book with a disclaimer: "in view of the misuse
that my words have been put to in the past, I wish to say that nothing
in this book should be taken out of context and thought in any way
to support the views of the 'creationists'...")(21) If pollen types did shift abruptly
in some bog, scientists could account for that as an artifact of
a purely local change. There seemed to be no good evidence, nor
plausible physical cause, for any swift global upset.
rise & ice
| Hints to
the contrary came unexpectedly from entirely different fields. In
the late 1950s, a group in Chicago carried out tabletop "dishpan"
experiments using a rotating fluid to simulate the circulation of
the atmosphere. They found that a circulation pattern could flip between
distinct modes. If the actual atmospheric circulation did that, weather
patterns in many regions would change almost instantly. On a still
larger scale, in the early 1960s a few scientists created crude but
robust mathematical models that demonstrated that global climate really
could change to an enormous extent in a relatively short time, thanks
to feedbacks in the amount of snow cover and the like.(22)
<=Sea rise & ice
| Probably it was no coincidence that this
new readiness of scientists to consider rapid and disastrous global
change spread in the early 1960s. That was exactly when the world
public was becoming anxious over the possibility of sudden global
catastrophe. Alongside the fantasies of Velikovsky, and increasingly
shrill warnings from Bible fundamentalists, there were sober possibilities
of disaster brought on by nuclear war, not to mention threats to the
entire planet from chemical pollution and other human industrial ills.
|Now that theoretical ideas and the general trend of opinion alike made
it easier for climate scientists to envision sharp change, they were
increasingly able to notice it in their data. Broecker in particular,
looking at deep-sea cores, in 1966 pointed to an "abrupt transition
between two stable modes of operation of the ocean-atmosphere system,"
especially a "sharp unidirectional change" around 11,000 years ago.(23) It proved possible to build simple fluid-flow models that
showed how a switch in the pattern of ocean currents could promote
such a change. Improved deep-sea records, going back hundreds of millennia,
brought additional information. By comparing the irregular curves
from a number of cores, Broecker noticed that the general pattern
of glacial cycles was not a simple symmetric wave. It looked more
like a sawtooth where "gradual glacial buildups over periods averaging
90,000 years in length are terminated by deglaciations completed in
less than one tenth this time."(24)
=>Sea rise & ice
| The view was supported
by data gathered independently at the University of Wisconsin-Madison,
where Reid Bryson was already interested in rapid climate changes.
In the late 1950s, supported by an Air Force contract to study weather
anomalies, he had been struck by the wide variability of climates
as recorded in the varying width of tree rings. And he was familiar
with the Chicago "dishpan" experiments that showed how a circulation
pattern might change almost instantaneously. Bryson brought together
a group to take a new, interdisciplinary look at climate, including
even an anthropologist who studied the ancient native American cultures
of the Midwest. From bones and pollen they deduced that a disastrous
drought had struck the region in the 1200s the very period
when the flourishing towns of the Mound Builders had gone into decline.
It was already known that around that time a great drought had ravaged
the Anasazi culture in the Southwest (the evidence was constricted
tree rings in ancient logs from their dwellings). Compared with this
drought of the 1200s, the ruinous Dust Bowl of the 1930s had been
mild and temporary. A variety of historical evidence hinted that the
climate shift had been world-wide. And there seemed to have been distinct
starting and ending points. By the mid 1960s, Bryson concluded that
"climatic changes do not come about by slow, gradual change, but rather
by apparently discrete 'jumps' from one [atmospheric] circulation
regime to another."(25*)
| Next the Wisconsin team reviewed carbon-14 dates of pollen from
around the end of the last ice age. In 1968, they reported evidence
for a rapid shift around 10,500 years ago, and by "rapid" they meant
a change in the mix of tree species within less than a century (they
quoted a "half-life" as short as 55 years). That was about as fast
as a forest could adjust, so the climate itself could have changed
even faster. Perhaps the Younger Dryas was not just a local Scandinavian
| Bryson and
his collaborators were developing a systematic technique for translating
their counts of different kinds of pollens into a record of rainfall
and temperature. It was a technique "built on a foundation of debatable
assumptions," as one reviewer observed, yet still "a major step forward."
They produced for the American Midwest the most accurate, detailed,
and comprehensive climate record available anywhere.(26)
Looking at hundreds of carbon-14 dates spanning the past dozen millennia
dates that improvements had made accurate enough to give a
reasonable correlation among widely dispersed sites they believed
they could confirm Bryson's disturbing conclusion. Climate change
generally did not come smoothly, but in a steplike pattern; periods
of "quasi-stable" climate ended in swift transitions.(27)
In a 1974 followup, they spoke more boldly of stable periods interrupted
by catastrophic "discontinuities," when "dramatic climate change occurred
in a century or two at most."(28) The "at most" was a confession that
the power of pollen studies was limited. For even if the climate changed
overnight, it could take a century or more for the mix of trees in
a forest to evolve until it accurately reflected the new conditions.
| Moreover, it did not take a global climate change to transform
any particular forest. Strictly local events could do that. There
was no way to correlate climate changes in different parts of the
world precisely, since radiocarbon measurements had a wide range of
error and other dating techniques were still worse. This limitation
of the data did not worry most experts, for they felt it was sheer
speculation to propose any physical mechanism that could change the
entire world's climate in less than a thousand years or so.
| Yet confirmation of
changes at that rate, at least, was coming from a variety of other
work. An example was George Kukla's study of snail shells and pollen
in layers of loess (wind-blown dust) in Czechoslovakia another
study that was designed to investigate gradual shifts, but in which
a close look at the data revealed unexpectedly
abrupt transitions.(29) The emerging picture of severe instability was reinforced
by studies of cores drilled from the Greenland and Antarctic ice caps,
and by deep-sea cores that covered much longer times. Evidently the
hundred-thousand-year glacial cycles did follow a sawtooth pattern:
each cycle showed a slow descent into a long-lasting cold state that
ended with a mysteriously abrupt rise of temperature. As Emiliani
put it in 1974, "We used to think intervals as warm as the present
lasted 100,000 years or so. Instead, they appear to be short, infrequent
episodes."(30) Another respected
climatologist explained that the old view of "a grand, rhythmic cycle"
must be replaced by a "much more rapid and irregular succession,"
in which the Earth "can swing between glacial and interglacial conditions
in a surprisingly short span of millennia (some would say centuries)."(31)
| Within these larger transitions, even quicker secondary oscillations
showed up in various data, such as carbon-14 studies of ancient glacier
moraines and lake levels.(32*) Above all there was the Younger Dryas.
Evidence from shells in a few excellent deep-sea cores showed a geographically
widespread temperature oscillation. Many scientists found this evidence
of little interest, however. Sea-floor slumping or various chemical
and biological effects effects could easily have confused the data.(33*) Up through the early 1970s, few of
the scientists who studied ancient climates paid much attention to
putative short-term changes. Their energies continued to focus on
pinning down the grand multi-millennial rhythm of the ice ages and
the famous puzzle of its causes.
|It was the pursuit of
these long cycles, more than any expectation of finding abrupt changes,
that attracted scientists to a high-altitude frozen plateau. A Danish
group headed by Willi Dansgaard drilled a long core of ice at Camp
Century, Greenland in cooperation with Americans led by Chester Langway,
Jr. The proportions of different oxygen isotopes in the layers of
ice gave a fairly direct record of temperature. But mixed in with
the expected gradual cycles, the group was surprised to notice what
they called "spectacular" shorter-term shifts including, once
again, an oscillation around 12,000 years ago. Some of the shifts
seemed to have taken as little as a century or two. Nobody could
be sure of that, however, for the odd wiggles in the data might represent
not a world climate shift, but only local accidents in the ice.(34)
graph of temperatures
of glacial-epoch experts, meeting at Brown University in 1972, reached
something close to a consensus. Reviewing the Camp Century ice cores,
new foraminifera studies by Emiliani, and other field evidence, the
scientists agreed that interglacial periods tended to be short and
to end more abruptly than had been supposed. In view of the cooling
reported in the Arctic since the 1940s, they suspected we might right
now be near the end of the present interglacial period. The majority
concluded that the current warm period might possibly end in rapid
cooling within the next few hundred years "a first order environmental
| Bryson, Stephen Schneider, and a few others
took the concern to the public. They insisted that the climate we
had experienced in the past century or so, mild and equable, was not
the only sort of climate the planet knew. For all anyone could say,
the next decade might start a plunge into a cataclysmic freeze, drought,
or other change unprecedented in recent memory, but not without precedent
in the archeological and geological record. (Link from below) While Bryson warned
that the increasing pollution of the atmosphere would shade the Earth
and bring rapid cooling, this was not the only possibility. The growing
realization that small perturbations could trigger sudden climate
change also impressed scientists who were growing concerned about
the rising level of the greenhouse gas carbon dioxide ( CO2).
Perhaps that might bring serious global warming and other weather
changes within as little as a century or two.
|As abrupt changes became more credible, scientists noticed them
in still more kinds of evidence. One example was the shells of beetles,
which are abundant in peat bogs, and so remarkably durable that they
can be identified even 50,000 years back. Beetles swiftly invade or
abandon a region as conditions shift, so the species you find give
a sensitive measure of the climate. Russell Coope, studying bog beetles
in England, turned up rapid fluctuations from cold to warm and back
again, a matter of perhaps 3°C, around 13,000 years ago. It all
happened within a thousand years at most, he reported (if the change
had been even faster his data could not have shown it).(36)
This singular approach got a skeptical response from other scientists
who pursued the well-established study of pollens, for they were accustomed
to seeing more gradual transformations of forests and grasslands.
They easily dismissed the fluctuations in Coope’s records as
local peculiarities of English beetles.
| The Camp Century cores, too, might tell little about change on
a global scale. The data might be sensitive to changes of ice cover
in the seas near Greenland, or to a local shift of the ice cap's glacial
flow. Other evidence, especially oxygen isotopes in shells from deep-sea
cores that reflected conditions in the entire North Atlantic, showed
changes only over several thousand years.
| Nevertheless, as pieces
of evidence accumulated, a growing number of scientists found it plausible
that the climate over large regions, if not the entire world, had
sometimes changed markedly in a thousand years or even less. Perhaps
one reason was that the early 1970s meanwhile saw further development
of global energy-balance models in which a few simple equations produced
radical instability. In particular, Mikhail Budyko
in Leningrad pursued calculations about feedbacks involving ice cover,
and suggested that at the rate we were pumping CO2
into the atmosphere, the ice covering the Arctic Ocean might melt
entirely by 2050 (link from below). Conversely,
a buildup of snow and ice might reflect enough sunlight to flip the
Earth into a glaciated state.(37)
These ideas prompted George Kukla and his wife Helena to inspect satellite
photos of Arctic snow cover, and they found surprisingly large variations
from year to year. If the large buildup seen in 1971 were repeated
for only another seven years, the snow and ice would reflect as much
sunlight as during a glacial period. "The potential for fast changes
of climate," they warned, "evidently does exist on the Earth."(38)
| Meanwhile glacier experts
developed ingenious models that suggested that global warming might
provoke the ice sheets of Antarctica to break up swiftly, shocking
the climate system with a huge surge of icewater.(39) (link from below) Bryson and other scientists worked harder
than ever to bring their concerns to the attention of the scientific
community and the public. As Broecker put it, any decade now a severe
"climatic surprise" could hit the world.(40)
<=>Sea rise & ice
| Most scientists spoke more cautiously. When
leading experts had to state a consensus opinion they were circumspect,
as in a 1975 National Academy of Sciences report about plans for international
cooperation in atmospheric research. Evaluating past statistics, the
panel concluded that predictable influences on climate made for only
relatively small changes. These changes, they said, would take centuries
or longer to develop. Any big jerks that might matter for current
human affairs were likely to be just "noise," the usual irregularities
of climate. The panel agreed that there was a significant "likelihood
of a major deterioration of global climate in the years ahead," but
they could not say how rapidly that might happen. Scientists of the
time disagreed on whether the greatest global risk was cooling by
atmospheric pollution or greenhouse effect warming. No doubt the present
warm interglacial period would end eventually, but that might be thousands
of years away. About the only thing the scientists fully agreed on
was that they were largely ignorant.(41)
| As a landscape that looks smooth from a distance
may display jagged gullies when seen through binoculars, so sharper
and sharper changes appeared as measuring techniques got better. An
example was an analysis that Emiliani published in 1975 of some deep-sea
cores from the Gulf of Mexico. Thanks to unusually clear and distinct
layers of silt, he found evidence of a remarkable event around 11,600
years ago: a rise of sea level at a rate of meters per decade.(42)
Another compelling example was a 1981 study of a few sediment cores
that had accumulated very rapidly, giving excellent time resolution.
They showed a startling cooling around 11-12,000 years ago
as much as 7-10°C in less than a thousand years before
the warming resumed. One expert warned that temperatures in the past
had sometimes jumped 5°C in as little as 50 years.(43*)
=>Sea rise & ice
| Was there really any mechanism that could
have caused such leaps of temperature? The known cosmic causes, for
example a modulation of sunlight, seemed unlikely to be strong enough
to push truly rapid world-wide changes.An expert noted that most of
his colleagues "take the European late-glacial chronology as standard
for the whole world, in the belief that climatic changes must have
been broadly synchronous because they were cosmically caused."(44) A close look at the best evidence, however, found only
events affecting the North Atlantic region (where most of the experts
did their work). A local trigger for the Younger Dryas, in particular,
was suggested by the fossil shorelines of a gigantic lake of fresh
water that had been dammed up behind the North American ice sheet.
Evidence suggested that as the sheet melted back, an ice dam had suddenly
broken up and released the entire lake to flood down the St. Lawrence
River. By adding fresh water to the North Atlantic, that could have
shut down the "thermohaline" circulation in which warm water
from the tropics moves north, then sinks as it grows denser from cold
and salt. Without this injection of tropical heat, the region's temperature
would drop until the circulation resumed. ( (The cause of the Younger
Dryas is still in dispute, but a sudden release of water from an ice
sheet remains the most plausible theory.)(45*)
| Other mechanisms that scientists thought up
were more global in scope. Had an eruption of icebergs following the
sudden disintegration of Arctic Ocean ice sheets cooled the entire
North Atlantic Ocean? Perhaps a cluster of volcanic eruptions had
affected the whole Northern Hemisphere?(46) Or a catastrophic disintegration of
Antarctic ice sheets might have sent forth masses of ice to cool all
the Earth's oceans? (See above.) Then again, the changes
might be purely chaotic, autonomous and unpredictable stutterings
between different quasi-stable modes of the planet's climate system?
| There were all too
many feedback forces that might turn a slow local temperature change
into an abrupt global one. The more traditional candidates included
changes in ice and snow cover, ocean currents, or the pattern of wind
circulation and storms. During the 1980s, additional speculations
lengthened the list. Perhaps a rise in global temperature would cause
microbial life to burgeon in the vast expanses of peat bogs and tundra,
emitting more “marsh gas”? That was methane, a greenhouse
gas that blocks heat radiation even more effectively than CO2,
so it could cause more warming still in a vicious feedback circle..
Or what about clathrates peculiar ices that locked up huge
volumes of methane in the muck of cold seabeds perhaps these
would disintegrate and release greenhouse gases?
| It was getting easier for scientists to consider
such colossal transformations, for uniformitarian thinking was under
attack. By the early 1980s, some geologists were stressing the importance
of rare events like the enormous floods that had drained temporary
lakes during the melting of the continental ice sheets. In biology,
Stephen Jay Gould and a few others were arguing that some species
had evolved in "punctuated" bursts.(47)
Other scientists were offering plausible scenarios of cosmic catastrophes
that might happen only once in tens of millions of years. Had a stunning
climate change, following the fall of a giant asteroid, exterminated
the dinosaurs in a single frozen year? Could something like that befall
| Many scientists continued to look on such speculations as little
better than science fiction. The evidence of abrupt shifts that turned
up in occasional studies may seem strong in retrospect, but at the
time it was not particularly convincing. Any single record could be
subject to all kinds of accidental errors. The best example was in
the best data on climate shifts, the wiggles in measurements from
the Camp Century core. These data came from near the bottom of the
hole, where the ice layers were squeezed tissue-thin and probably
folded and distorted as they flowed over the bedrock.
| Broecker later remarked that the relatively smooth temperature
record of oxygen isotopes in deep-sea sediments "tended to lull scientists
into concluding that the Earth's climate responds gradually when pushed."
Many continued to believe that the oceans could only vary gradually
over thousands of years, with a thermal inertia that must moderate
any climate changes. These scientists should have realized that the
top few meters of ocean exchange heat only slowly with the rest. And
they should have recalled that at most places in the deep sea, sediments
accumulate at only a few centimeters per thousand years. Churning
by burrowing worms and other creatures within the mud (“bioturbation”)
smears the layers, blurring any record of change.(48*)
Ice did not have these problems. Thus further progress would depend
on getting more and better ice cores.
Ice drilling was becoming a little world of its own,
inhabited by people of many nations (Dansgaard's "Danish" team spoke
eight different languages). Their divergent interests made for long
and occasionally painful negotiations. But the trouble of cooperation
was worth it for bringing in a variety of expertise, plus (what
was also essential) a variety of agencies that might grant funds.Drilling
teams hunted ancient ice in places barely possible to reach
eventually they penetrated not only the polar ice caps, but mountain
icefields from Peru to Tibet and the teams had to somehow
get there with tons of equipment and supplies. The outcome was a
series of engineering triumphs, which could turn into maddening
fiascos when a costly drill head got irretrievably stuck a mile
down. Engineers went back to their drawing-boards, team leaders
contrived to get more funds, and the work slowly pushed on.(49) There is a supplementary experimental site on the History of Greenland Ice Drilling,
with glimpses into the inner workings of the U.S. "GISP" projects
of the 1980s.
| A breakthrough came after the ice drillers went
to a second location, a military radar station named "Dye 3" some
1,400 kilometers distant from Camp Century. By 1981, after a decade
of tenacious labor and the invention of an ingenious new drill, they
had extracted gleaming cylinders of ice ten centimeters in diameter
and in total more than two kilometers long. Dansgaard's group cut
out 67,000 samples, and in each sample analyzed the ratios of oxygen
isotopes. The temperature record showed what they called "violent"
changes which corresponded closely to the jumps at Camp Century.
Moreover, the most prominent of the changes in their record corresponded
to the Younger Dryas oscillation seen in pollen shifts all over Europe.
It showed up in the ice as a swift warming interrupted by "a dramatic
cooling of rather short duration, perhaps only a few hundred years."(50*)
| A particularly good correlation came from
a group under Hans Oeschger. An ice drilling pioneer, Oeschger was
now measuring oxygen isotopes in glacial-era lake deposits near his
home in Bern, Switzerland. That was far from Greenland, but his group
found "drastic climatic changes" that neatly matched the ice record.
The severe cold spells became known as "Dansgaard-Oeschger events."
They seemed to be restricted to the North Atlantic and Europe.(51*)
| As ice drillers improved
their techniques, making ever better measurements along their layered
cores, they found a variety of large steps not only in temperature
but also in the CO2 concentration.(52*) This was a great surprise to everyone. Since the gas circulates
through the atmosphere in a matter of months, the steps seemed to
reflect world-wide changes. Other scientists promptly pointed out
that the observations might be a mere artifact the amount of
gas absorbed might change with the local temperature in Greenland
because of the physical chemistry of ice. Yet clearlysomething
had made spectacular jumps. A variety of other evidence for very abrupt
climate changes was accumulating, and some began to entertain the
notion of such change on a global scale.
| Most of these scientists, after presenting
their data, could not resist adding a few suggestive words about possible
causes. Dansgaard's group was typical in speculating about "shifts
between two different quasi-stationary modes of atmospheric circulation."(53)
That was the most common idea about how climate might change rapidly,
harking back to the "dishpan" experiments of the 1950s. It implied
transient variations of wind patterns within broad limits, and mostly
concerned how weather might change in a particular region. The new
thinking about grand global shifts urged a broader view. It was hard
to see how the atmosphere could settle into an entirely new state
unless something drastic happened in the oceans. For it is sea water,
not air, that holds most of the heat energy and most of the moisture
and CO2 of the climate system. The question of
century-scale shifts, now a main topic in climatology, came to rest
on the desks of ocean scientists.
| Their response was prompt. Experts mooted
various hypotheses about how changes in the surface waters might affect
CO2 levels. There were complex links among temperature,
sea water chemistry, biological activity, and the chemical nutrients
that currents brought to the surface. Oceanographers also had reasons
to believe that the pattern of North Atlantic Ocean circulation could
change on a short timescale. Since the circulating waters carry tremendous
quantities of heat northward from the tropics, if the circulation
ground to a halt, temperatures in many regions of the Northern Hemisphere
would immediately plunge.
|Broecker began to warn that the ocean-atmosphere climate system did not necessarily
respond smoothly when it was pushed it might jerk. In 1987,
he wrote that scientists had been "lulled into complacency." People
were increasingly taking their cue from elaborate supercomputer simulations
of the general circulation of the atmosphere. They failed to realize
that these models, in the very way they were constructed, allowed
only smooth and gradual changes. The authors of an "unstable" model
would rework it until it yielded more consistent results. Broecker
strongly suspected that "changes in climate come in leaps rather than
gradually" — posing a drastic threat to human society and the
natural world. As computer modelers labored to incorporate interactions
between air and sea, their new simulations hinted
that he was right.(54*)
| After 1988
| Early in the 1990s, further revelations startled climate scientists.
The quantity, variety, and accuracy of measurements of ancient climates
were increasing at a breakneck pace compared with the data
available in the 1970s, orders of magnitude more were now in hand.
The first shock came from the summit of the Greenland ice plateau,
a white wasteland so high that altitude sickness was a problem. From
this location all ice flowed outward, so glacier experts hoped that
even at the bottom, three kilometers (two miles) down, the layers
would be relatively undisturbed by movement. Early hopes for a new
cooperative program joining Americans and Europeans had broken down,
and each team drilled its own hole. An ingenious decision transmuted
competition into cooperation. The two holes were drilled just far
enough apart (30 kilometers) so that anything that showed up in both
cores must represent a real climate effect, not an accident due to
bedrock conditions. The match turned out to be remarkably exact for
most of the way down. A comparison of variations in the cores showed
convincingly that climate could change more rapidly than almost any
scientist had imagined.(55)
For more on ice drilling, see Joel Genuth's Greenland
Ice Sheet Project (GISP) site and the official
GISP Web site.
|Swings of temperature that scientists in the 1950s believed to take
tens of thousands of years, in the 1970s to take thousands of years,
and in the 1980s to take hundreds of years, were now found to take
only decades. Ice core analysis by Dansgaard's group, confirmed by
the Americans' parallel hole, showed rapid oscillations of temperature
repeatedly at irregular intervals throughout the last glacial period.
Greenland had sometimes warmed a shocking 7°C within a span of
less than 50 years. For one group of American scientists on the ice
in Greenland, the "moment of truth” struck on a single
day in midsummer 1992 as they analyzed a cylinder of ice, recently
emerged from the drill hole, that came from the last years of the
Younger Dryas. They saw an obvious change in the ice, visible within
three snow layers, that is, scarcely three years! The team analyzing
the ice was first excited, then sobered — their view of how
climate could change had shifted irrevocably. The European team reported
seeing a similar step within at most five years. "The general
circulation [of the atmosphere] in the Northern Hemisphere must have
shifted dramatically," Dansgaard’s group eventually concluded.(56*)
| The record of dust found in the
ice certainly suggested that a wide region had been involved, but
might the change have been restricted to parts of the world near Greenland?
The first hints of the answer came from oceanographers, who had been
hunting out seabed zones where bioturbation by burrowing worms did
not smear any record of rapid change. In some places the sediments
accumulated very rapidly, while in others, the sea water lacked enough
oxygen to sustain life. The first results, from the Norwegian Sea
in 1992, confirmed that the abrupt changes seen in Greenland ice cores
were not confined to Greenland alone. Later work on seabed cores from
the California coast to the Arabian Sea, and on chemical changes recorded
in cave stalagmites from Switzerland to China, confirmed that the
oscillations found in the Greenland ice had been felt throughout the
Northern Hemisphere. Meanwhile, in the late 1980s and early 1990s,
improved carbon-14 techniques gave the first accurate dates for sediments
containing pollen and other carbon-bearing materials at locations
ranging from Japan to Tierra del Fuego. Good dates finally allowed
correlation of many geological records with the Greenland ice. The
results suggested that the Younger Dryas events had affected climates
around the world. The extent and nature of the perturbation was controversial.
But scientists were increasingly persuaded that abrupt climate shifts
could have global scope, even if they affected different places differently
— colder here and warmer there, wetter here and drier there.(57)
|Could such drastic variations happen not only during glacial times,
but also in warm periods like the present? That was the most interesting
question in 1992, as the European drillers penetrated clear through
the last glacial epoch to the preceding "Eemian" period,
more than 100,000 years back — a time similar to our own, or
even warmer. There, too, they saw dramatic shifts. However, further
analysis cast that into doubt. The layers from the Eemian warm period
were down near bedrock, distorted by ice flow. Comparison of the two
groups' cores gave divergent results. Again scientists had benefitted
from drilling parallel cores. But this time the lesson, valuable if
unwelcome, was that they must do more work. (Another grand multinational
project to drill Greenland ice from the previous warm period was completed
in 2004. The team’s three-kilometer core showed a reassuringly
stable climate for the last part of the Eemian warm period. But they
failed again to get reliable results for most of the period, including
the crucial early time most similar to our own when the atmosphere
had been warming up.)
Antarctic cores could not help. Little snow falls there, and the
layers of ice were too thin and squashed together to reveal rapid
variations. Certainly no climate variation of Younger Dryas magnitude
had been seen recently. So there was reason to hope that our present
climate was relatively stable, at least for the moment. The Europeans
and Americans nevertheless agreed that through most of the last
100,000 years the global climate had oscillated "on a scale that
human cultural and industrial activities have not yet faced."(58)
doubt even the best set of data if they have no theory to explain
it, but at least one plausible explanation was at hand. A flip-flop
of the entire North Atlantic Ocean’s circulation pattern might
have been involved in the Dansgaard-Oeschger events. People came
up with various proposals for things that might have triggered a
switch, such as the surge of an ice sheet that released a flotilla
That was not easy to swallow. As one scientist remarked, many of
his colleagues "do not believe that the small, energy-starved polar
'tail' can wag the large, energy-rich tropical 'dog'." But the evidence
of iceberg surges was strong, and computer models suggested that
such a surge could indeed have caused a drastic global circulation
shift. Oceanographers began to work out how the tropical oceans
could take part in a sudden global change. Or perhaps set it off
— if the system was so unstable, the trigger might not lie
in the well-studied North Atlantic. The tropical Pacific and Atlantic
ocean and wind systems seemed to have feedbacks that, once perturbed,
might reorganize the entire system of clouds, rainfall and currents.
"Taken to a logical extreme," one expert insisted in 2004,
"a competing 'tropical driver' hypothesis for abrupt climate
changes could easily explain the observed global synchronization."
For example, a "permanent El Niño" might move the
Earth into a state not seen since several million years ago, when
so much ice had been melted that the sea level stood roughly 25
meters above the present level.(59)
Did the same instability exist today? There
was suggestive evidence that abrupt flips of circulation had in
fact happened in previous times
of warmth.(60) "There
is surely a possibility," Broecker wrote, "that the ongoing
buildup of greenhouse gases might trigger yet another of these ocean
reorganizations." The media picked up the dramatic image of
Europe returning to the frigid conditions of the Younger Dryas —
global warming could bring on a new Ice Age almost instantly! When
an international panel of experts made their best guess on the matter
in 2001, they concluded that a shutdown of the Atlantic circulation
in the coming century was "unlikely" but "cannot
be ruled out." If the shutdown did come, Broecker warned, it
could mean "widespread starvation" within decades. In
the next few years, scientists reported that the Atlantic waters
were indeed growing less salty, thanks to fresh water from increased
rainfall and the melting of ice. Still more troubling was a 2005
announcement that the amount of heat carried southward by the North
Atlantic circulation had decreased by as much as 30% since the 1950s.
<=> The oceans
However, the observational record was so skimpy, and the system
so noisy, that this could be just a normal and temporary fluctuation.
A replay of the catastrophic Younger Dryas glacial scenario was
not likely under the very different conditions of the present. Computer
modelers redoubled their attention to the question, and their simulations
showed only gradual, centuries-long changes in the ocean circulation.
Broecker admitted that he had overestimated the danger, and in 2004
he publicly cautioned against the "exaggerated scenarios"
that had recently appeared in a Hollywood summer spectacle. However,
the models were deliberately constructed to give stable solutions,
and they could never include all the complex feedbacks that might
conceivably cause a sudden shift. As one group that studied the
Younger Dryas remarked, "the geological understanding of past
abrupt climate changes is only preliminary. This does not bode well
for predicting future, abrupt climate changes."(61).
|Other mechanisms for drastic shifts also came under detailed scrutiny.
An example was the clathrate ices, frozen in layers spread through
sea floor muds. Clathrates might hold more carbon compounds than all
the world's coal and oil. New studies made it plausible that warming
of the oceans could cause some of the deposits to disintegrate in
a landslide-like chain reaction, which would vent enough methane and
CO2 into the atmosphere to redouble global warming.
The idea sounded like science fiction (indeed some science fiction
writers used it), and it seemed highly unlikely to happen anytime
|In the 1990s, geologists
found that such titanic greenhouse gas outbursts had probably caused
a spectacular warming 55 million years ago. At any rate something
back then had radically changed climate, with global heating and an
abrupt change of the deep ocean circulation, bringing mass extinctions
so sweeping that geologists called it the start of a new geological
era. Clathrates were the leading suspect (although evidence was offered
in 2007 that the change might have been caused, or at least set in
motion, by massive volcanic releases of CO2).
Whatever caused it, the total carbon release that wrought such havoc
was roughly comparable to the amount of carbon that humanity would
emit if we burned all available coal and oil. Back then, the rise
in temperature had apparently stretched over tens of thousands of
years, "rapid" only to a geologist, and the same seemed
likely for any future replay. But the past emissions had come stepwise,
and in some future century too, clathrates might pump gases into the
atmosphere at a rate fast enough to bring serious change within a
human lifetime. There seemed little risk of truly catastrophic eruptions
in the foreseeable future, but methane from the seabed could make
global warming more rapid and severe..(63)
|Ominously, data showed
that sudden climate shifts did not happen only during a glacial period.
In 1993, Dansgaard and his colleagues reported that rapid oscillations
had been common during the last interglacial warm period enormous
spikes of cooling, like a 14-degree cold snap that had struck in the
span of a decade and lasted 70 years. The instability was unlike anything
the ice record showed for our current interglacial period. The announcement,
Science magazine reported, "shattered" the standard picture
of benign, equable interglacials.(64)
| Others soon showed that these
measurements, made near the bottom of the core, were distorted by
ice flow that stirred together layers from warm and cold periods.
Interglacials were perhaps not so horrendously variable.(65) Yet in terms of how scientists thought about the present
climate system, one might say that the ice had been broken. People
recalled that the present system was certainly subject to abrupt but
harrowing droughts, like the one revealed by Bryson that had devastated
native North American cultures. Persuasive new geological evidence
blamed extreme prolonged droughts for the downfall of ancient Mayan
and Mesopotamian civilizations as well.(66)
|An altogether different type of evidence for
rapid change came from improved observations of Arctic and Antarctic
regions. New views from satellites, plus vigorous programs of precise
measurements from airplanes and on the ground, showed that enormous
glaciers could quickly change their speed of travel, while entire
ice sheets could break up within a matter of months. As one expert
remarked, this "ran counter to much of the accepted wisdom regarding
ice sheets." The accepted wisdom, he explained, "lacking
modern observational capabilities, was largely based on 'steady-state'
assumptions."(67) Now the
plausible possibility that a swift alteration of land or sea ice could
transform climate had to be added to all the other potential feedbacks
from global warming.
<=Sea rise & ice
|The new view of climate was reinforced by one of the last great
achievements of the Soviet Union, an ice core drilled with French
collaboration at Vostok in Antarctica. The record reached back through
nearly four complete glacial-interglacial cycles and drastic
temperature changes peppered almost every stretch of data. This Antarctic
record was too fuzzy to say whether any of these changes had come
and gone on the decade-size timescale of the Younger Dryas. But warm
interglacial periods had certainly been subject to big swings of temperature
lasting for centuries. Especially striking to the researchers, by
contrast, was our own era, the ten thousand years since the last glaciation.
It was, "by far, the longest stable warm period recorded in Antarctica
during the past 420 [thousand years]." When
Bryson, Schneider, and others had warned that the century or so of
stability in recent memory did not reflect "normal" long-term variations,
they had touched on an instability grander than they guessed (see
above). The entire rise of human civilization since the end of
the Younger Dryas had taken place during a period of warm, stable
climate that was unique in the long record. The climate known to history
seemed to be a lucky anomaly. (Paleoclimatologist William Ruddiman
suggested that this was no coincidence. Perhaps the rise of agriculture,
with its deforestation and rice paddies, had added enough methane
and CO2 to the atmosphere to dampen the normal
ice-age cycle?) The well-recorded history of the most recent century
or so happened to show even more unusual stability, compared with
what new evidence was revealing about severe variations in earlier
| The accumulation of evidence, reinforced by at least one reasonable
explanation (the reorganization of ocean circulation) destroyed long-held
assumptions. Most experts now accepted that abrupt climate change,
huge change, global change, was possible at any time. A report written
by a National Academy of Sciences committee in 2001 said that the
recognition, during the 1990s, of the possibility of abrupt global
climate change constituted a fundamental reorientation of thinking,
a "paradigm shift for the research community."(69)
| The first strong consensus
statement had come in 1995 from the Intergovernmental Panel on Climate
Change, representing the considered views of nearly all the world's
climate scientists. The report included a notice that climate "surprises"
were possible "Future unexpected, large, and rapid climate
system changes (as have occurred in the past)."(70) The report’s authors did not emphasize the point,
however, and the press seldom mentioned it.
Despite the profound
implications of this new viewpoint, hardly anyone rose to dispute
it. Yet while they did not deny the facts head-on, many denied them
more subtly, by failing to revise their accustomed ways of thinking
about climate. For example, few of the scientists studying pollen
in bogs went back to their data and took on the difficult task of
looking for catastrophically rapid shifts in the past. "Geoscientists
are just beginning to accept and adapt to the new paradigm of highly
variable climate systems," said the Academy committee in 2001. Beyond
geoscientists, "this new paradigm has not yet penetrated the impacts
community," that is, the economists and other specialists who tried
to calculate the consequences of climate change.(71) Policy-makers and the public lagged even farther behind
in grasping what the new scientific view could mean.
few years that changed. Scientists' attention focused on Antarctic
and Arctic ice changes. New evidence and theories suggested that
the Greenland and West Antarctic ice sheets might melt much faster
than had been suspected, bringing a serious sea level rise within
the next few centuries, and possibly sooner. Even more alarming,
to scientists and the public alike, were startling changes in "before
and after" satellite pictures of sea ice in the Arctic. The
ice pack was getting thinner and shrinking. "At the present
rate," one group of scientists reported, "a summer ice-free
Arctic Ocean within a century is a real possibility." That
had last been seen millions of years ago, an epoch with a sea level
roughly 25 meters above the present. As the
Arctic Ocean opened to the winds, it would have profound effects
on weather far to southward, and perhaps hasten global warming.
The changes foreseen by Budyko, a generation earlier, were underway
(see above). But the sea ice was seen to
be dwindling faster than modern computer models predicted. Reviewing
the familiar feedback where less ice and snow meant more sunlight
absorbed, one group commented dryly that "Thresholds may produce
unexpected system responses."(71a*)
Media ranging from science magazines to movies reflected the new
science in their distorted mirrors, offering scenarios of a climate
that could change within a few decades or even, according to Hollywood,
a few weeks. Around 2005 the phrase "tipping point" appeared
in both scientific and popular climate reports, an admission that
change could be not only rapid but irreversible.
<=Sea rise & ice
“tipping point” that research could not dismiss came
from feedbacks in the carbon cycle. Although data were spotty (mostly
short-term studies of only a few of the countless biological systems),
results in the early 2000s were discouraging. More likely than not,
as plants and soils got warmer they were releasing additional CO2,
methane and other greenhouse gases. Possibly the worst problem was
tundra, where researchers saw greenhouse gases bubbling out of the
sodden ground before their eyes. A Russian researcher said it was
an "ecological landslide that is probably irreversible and
is undoubtedly connected to climatic warming."(71b*)
news was, nobody had yet found a mechanism that, outside Ice Age conditions,
could plausibly bring a massive global climate change in a decade
or so. Unlesss scientists had completely overlooked something essential
(which was possible but far from likely), whatever changes happened
would be spread out over several decades or longer.The bad news was...
well, there were several items. The feedbacks long anticipated were
kicking in, so that large changes were already seen to be happening
within the timescale of a human lifetime. By 2005 climatologists had
established, against passionate opposition from a small band of contrarians,
that the decades since 1980 had seen rapid global warming on a scale
unlike anything known in human history.
And scientists kept turning up more possible mechanisms for feedbacks
that could accelerate warming. For example, the system of carbon
uptake and release by forests was still so poorly understood
that scientists admitted there was a "potential for major abrupt
change."(71c*) In any case the known
feedbacks were so strong that it seemed likely that — unless
human civilization rose to the challenge very soon — global
warming would become self-sustaining and irreversible. The world
at the end of the 21st century would be seriously different from
the world of the 20th, and changes would continue for centuries
See the summary of expected Impacts
of Global Warming.
|A lesson about
how science proceeds can be learned from this
history. Asked about the discovery of abrupt climate change, many
climate experts today would put their finger on one moment: the day
they read the 1993 report of the analysis of Greenland ice cores.
Before that, nobody confidently believed that the climate could change
massively within a decade or two; after the report, nobody felt sure
that it could not. So wasn't the preceding half-century of research
a waste of effort? If only scientists had enough foresight, couldn't
we have waited until we were able to get good ice cores, and settle
the matter once and for all with a single unimpeachable study?
| The actual history shows that even the best scientific data are
never that definitive. People can see only what they find believable.
Over the decades, many scientists who looked at tree rings, varves,
ice layers, and so forth had held evidence of rapid climate shifts
before their eyes. They easily dismissed it. There were plausible
reasons to believe that global cataclysm was a fantasy of crackpots
and Bible fundamentalists. Records of the past were mostly too fuzzy
to show rapid changes, and where such a change did plainly appear,
scientists readily attributed it (usually correctly) to something
other than climate. Sometimes the scientists' assumptions were actually
built into their procedures. When pollen specialists routinely analyzed
their clay cores in 10-centimeter slices, they could not possibly
see changes that took place within a centimeter's worth
of layers.(72*) If the conventional beliefs had
been the same in 1993 as in 1953 namely, that significant climate
change always takes many thousands of years scientists would
have passed over the decade-scale fluctuations in ice cores as meaningless
|First scientists had to convince themselves, by shuttling back and
forth between historical data and studies of possible mechanisms,
that it made sense to propose shifts as "rapid" as a thousand years.
Only then could they come around to seeing that shifts as "rapid"
as a hundred years could be plausible. And only after that could they
credit still swifter changes. Without this gradual shift of understanding,
the Greenland cores would never have been drilled. The funds required
for these heroic projects came to hand only after scientists reported
that climate could change in damaging ways on a timescale meaningful
to governments. In an area as difficult as climate science, where
all is complex and befogged, it is hard to see what one is not prepared
to look for.
Ocean Currents and Climate
Simple Models of Climate
Ice Sheets & Rising Seas
1. National Academy of Sciences
(2002), p. 7. BACK
2. Crowley and North (1991),
pp. 234-35; steady climate was still a "paradigm" in various geosciences into the 1980s,
according to Schimel and Sulzman (1995).
3. Gregory (1908), p. 340.
4. Humphreys (1932).
5. Brooks (1925), pp. 90-91.
6. Work of Knud Jessen, Johannes Iversen (both Danes) and
others, reviewed in Manten (1966).
7. Outside Scandinavia, carbon-14 dating of trees overridden by
the North American ice sheet showed the front had advanced and retreated by up to a kilometer a
year between 13,600 and 12,200 years ago. Flint (1955).
8. Classic work included that of Johannes Iversen on the arrival
of agriculture in Denmark and Leonard Wilson on forest fire and other rapid glacial-era changes
9. For discussion on the above points I am grateful to Ken
Brown, Daniel A. Livingstone and other respondents from the QUATERNARY and PALEOLIM
10. Flint (1955), p. 249.
11. Palmer (1999); also Huggett (1990), pp. 119-21 and passim.
12. Suess (1956), p. 357; other
scientists called this an "abrupt increase:" Ewing and Donn
(1956), p. 1061.
13. Ericson et al. (1955); Ericson et al. (1956), quotes p. 388.
14. Emiliani (1957).
15. Broecker (1957), p. V-9;
16. Broecker, interview by Weart, Nov. 1977, AIP.
17. Broecker et al. (1960).
18. "Stimulated:" Broecker et al.
(1960), p. 442; Ewing with Heezen had collected some of the crucial cores and noticed the
rapid change, Ewing and Donn (1956).
19. Ives (1957), quote p. 87; see
also Ives (1958); Ives (1962).
20. West (1961); the abruptness
of the transition was noted later by Lamb (1977), p. 80.
21. Ager (1993), p. xi.
22. Budyko (1962); Wilson (1964).
23. Broecker (1966), pp. 299,
24. Broecker and Donk (1970).
25. Bryson, personal communications, 2002. Anthropologist:
David Barreis. Barreis and Bryson (1965), p. 204; see Bryson and Barreis (1968), chs. 2, 3; Bryson (1968). The causes of the collapse of the great urban center
Cahokia and other elements of the Mississippian culture remain controversial today, with climate
change a strong contender.
26. Webb and Bryson (1972);
reviewer: Bradley (1985), pp. 322-329, quote p. 327; for a
general review of "transfer functions" for deducing temperature, see Sachs et al. (1977).
27. Bryson et al. (1970), p. 72.
28. Discontinuities: Wendland and
Bryson (1974); a century or two: Bryson (1974).
29. Kukla and Kocí
(1972), p. 383.
30. Quoted in Alexander
(1974), p. 94.
31. Mitchell (1972), pp.
32. One author, speculating about the coming of a new ice age,
pointed to "evidence of (at least) five rapid hemispheric coolings of about 5°C... each
event spread over not more than about a century," Flohn (1974),
quote p. 385; one line of evidence was carbon-14 studies of tree stumps in glacial deposits: Denton and Karlén (1973). But their fluctuations lasted
several centuries, and the authors predicted not a new ice age but a shift to a mild climate.
33. Broecker and Donk (1970);
Ruddiman & McIntyre too found evidence in deep-sea cores of faunal
change (including one core where the warming was interrupted by a cold
spell). They called the change "abrupt" although they thought it was spread
over a few thousand years: Ruddiman and McIntyre (1973), p. 129; a few years later
they realized the spread was due to bioturbation, and the changes were
actually "very abrupt." See their review of relevant studies from 1941
to 1977, Ruddiman and McIntyre (1981), pp. 146-50.
34. Dansgaard et al. (1971);
"spectacular": Dansgaard et al. (1972), p. 396;
Dansgaard et al. (1973). The Camp Century and later work
is discussed in Dansgaard (2004) and interviews
on GISP tape-recorded 1992-1994, records of Study of Multi-Institutional
Collaborations, AIP. BACK
35. Kukla and Matthews
36. Coope (1977); already in
1970 a cooling within a thousand years or so was seen, although not remarked upon, Coope et al. (1971).
37. Budyko (1972).
38. Kukla and Kukla (1974),
quote p. 713; this was brought to the public, e.g. in Time
39. E.g., Flohn (1974), with
reference to work by Lorenz, Budyko, and Sellers on instability; Dansgaard et al. (1972), p. 396, speculating on cooling.
40. Broecker (1975).
41. GARP (1975), from App.
A (pp. 186-90) by J. Imbrie, W.S. Broecker, J.M. Mitchell, Jr., J.E. Kutzbach.
42. Emiliani et al. (1975),
for criticism, see Science 193 (24 Sept. 1976):
43. Ruddiman and McIntyre
(1981); another example: century-scale changes in carbon-dated peat bog pollen, including
a clear oscillation 11,000-9,000 years ago, Woillard and Mook
(1982); in 50 years: Flohn (1979).
44. Mercer (1969), p. 227.
45. Diversion of glacial meltwater from the Mississippi
to the St. Lawrence was suggested by Kennett and
Shackleton (1975) and Johnson and McClure (1976);
Rooth (1982) suggested this disrupted the North
Atlantic circulation. Ruddiman and McIntyre (1981), p. 204, dismissed this since
they saw no decrease in North Atlantic biological productivity, but some
later data supported the idea; in 1985 Broecker suspected the meltwater
pulse was the entire cause of the Younger Dryas, but later he suggested
it was only the trigger that set the timing for a switch between thermohaline
circulation modes, Broecker et al. (1989) (whose
"synthesis of palaeoclimatic observations invigorated the community
over the next decade," according to Somerville
et al. (2007), p. 106; Broecker et al. (1990). See "Ocean
currents" essay. A computer model by Bryan
(1986) supported this, but another model found that a cold North Atlantic
surface sufficed to bring a Younger Dryas-like climate, Rind
et al. (1986). In later years other triggers were proposed, see e.g.,
Broecker (2006), including an asteroid strike
on the North American ice sheet. BACK
46. Mercer (1969) considered
breakup of an Arctic Ocean ice sheet; this is cited as a likely explanation
by Ruddiman and McIntyre (1981), pp. 204ff.; see Ruddiman and McIntyre (1981); eruptions: Flohn (1974). BACK
methane hydrate, a type of clathrate, exists in huge quantities beneath
the sea floor. When the pressure is removed and it warms, this ice-like
substance will emit the trapped gas. Photo: Gary Klinkhammer, Oregon State
University. More info: realclimate.org.
47. Gould (2002), pp.
1006-21 gives one version of the history, with his characteristically polemical approach.
48. Also, the sluggish response of the massive polar icecaps to
change smoothed the oxygen-isotope record. Broecker (1987);
for these issues in general, see Palmer (1999); Huggett (1990).
49. For Greenland drilling, see interviews on GISP tape-recorded
1992-1994, records of Study of Multi-Institutional Collaborations, AIP;
Mayewski and White (2002); Alley
(2000). For this and especially Lonnie Thompson's high-altitude work
see Bowen (2005). BACK
50. Dye 3: Dansgaard et al.
(1982), "violent," "dramatic" (also "drastic"), p. 1273; see also
Oeschger et al. (1984); Camp Century CO2:
Neftel et al. (1982); note that they do not
discuss a jump that is evident in their data. BACK
51. Siegenthaler et al. (1984),
"drastic" p. 149. They found nothing similar in North American records; Barnola et al. (1987) found nothing like it in their Antarctic ice
core, but admitted their methods would not detect very rapid changes.
52. A century-scale shift closely correlated with temperature
change was found by Oeschger et al. (1984); see also Dansgaard et al. (1984); decade-scale shifts are visible in the data,
although not specially remarked upon, in Hammer et al. (1986).
53. Dansgaard et al. (1982), p.
54. "The basic architecture of the models denies the possibility
of key interactions that occur in the real system. The reason is that we do not yet know how to
incorporate such interactions into the models." Broecker (1987),
pp. 123, 126; new models: Bryan and Spelman (1985); Manabe and Stouffer (1988).
55. GISP interviews, records of Study of Multi-Institutional
Collaborations, AIP. Firsthand accounts are Mayewski
and White (2002); Alley (2000); Dansgaard
56. Dansgaard et al. (1989);
increasingly abrupt changes were seen on further study, Johnsen
et al. (1992); Grootes et al. (1993); jumps
of Greenland snow accumulation "possibly in one to three years" were reported
by Alley et al. (1993), see also Mayewski
(1993); five-year steps: Taylor et al. (1997);
changes in dust had been noted, indicating at least continental scope
for the change, and a Younger Dryas temperature step in less than a decade
was found to be hemisphere-wide since methane gas changed as well: Severinghaus
et al. (1998). Good histories are Alley (2000)
and Cox, (2005), ch. 8. BACK
57. First ocean results: Karpuz
et al. (1992), Lehman and Keigwin (1992).
For references 1987-94 (including also Alaska, Ohio, New Zealand, etc.)
see Broecker (1995), pp. 306-08; for later
developments, National Academy of Sciences (2002)
and Lynch-Stieglitz (2004), also Cox
(2005), ch. 8. BACK
58. Divergence in cores: Taylor
et al. (1993), Grootes et al. (1993). 2004
work: NGRIP (2004) (North Greenland Ice Core
Project members, K.K. Andersen et al.); see report by Cuffey
(2004) and also Cox (2005), ch. 8. Hammer
et al. (1997), Preface, "not yet faced," p. 26,315.
59. Wag the dog: Alley (1998);
"easily explain": Chiang and Koutavas
(2004). El Niños: Cane and Evans (2000);
Federov et al. (2006). BACK
60. Barber et al. (1999).
61. Broecker et al. (1992);
quotes: Broecker (1997), p. 1588; IPCC (2001), p. 420; Atlantic freshening: Hansen
et al. (2001); Dickson et al. (2002); Curry
et al.(2003); Curry and Mauritzen (2005);
slowed circulation: Bryden et al. (2005). "Exaggerated:"
Broecker (2004); see Weaver
and Hillaire-Marcel (2004); "does not bode well": Lowell
et al. (2005). BACK
62. Science fiction: notably the award-winning Robinson (1994).
63. The "Paleocene-Eocene Thermal Maximum."
Appenzeller (1991); Kennett
and Stott (1991); Koch et al. (1992); Dickens et
al. (1995); Norris and Röhl (1999);
Katz et al. (1999); Nunes
and Norris (2006); an overview is Kunzig (2004).
Harvey and Huang (1995) estimated clathrates
could bring at worst a 10-25% increase in warming. Volcanic outburst:
Storey et al. (2007). BACK
64. Dansgaard et al. (1993);
65. Alley et al. (1995); Chappellaz et al. (1997), comparing with Vostok cores.
66. Maya: Hodell et al.
(1995); Mesopotamia: Weiss et al. (1993); for global
climate shifts throughout the postglacial period, see also deMenocal
et al. (2000).
67. Rignot and Thomas (2002),
p. 1505. BACK
68. "longest stable...": Petit
et al. (1999), p. 434. Ruddiman and Thomson
(2001); on Ruddiman see Kerr (2004a).
69. National Academy of Sciences
(2002), p. 16, see also pp. 1, 119, 121.
70. IPCC (1996), p. 7.
71. National Academy of Sciences
(2002), p. 121. BACK
71a. Old data from submarines (some
of it released on the initiative of Vice President Gore) revealed thinning
of ice, Rothrock et al. (1999); for sea ice
extent, Overpeck et al. (2005), )Lindsay
and Zhang (2005), Stroeve et al. (2005).
Weather effects: Francis and Hunter (2006).
Computer projection of an ice-free Arctic by Holland
et al. (2006) were widely publicized. Ice more sensitive to warming
than models predicted: Stroeve et al. (2007).
71b. "Tipping point" suggested
i.a. by Foley (2005). Carbon cycle:
e.g., Bellamy et al. (2005), Heath
et al. (2005), Govindasamy et al. (2005).
Russian (Sergei Kirpotin) quoted by Pearce (2005),
along with a report that "the permafrost of western Siberia is turning
into a mass of shallow lakes as the ground melts," lakes were expanding
on the North Slope of Alaska, Katey Walter had found methane hotspots
in eastern Siberia where the bubbling gas kept the surface from freezing
in winter, etc. See also Lawrence and Slater (2005).
Also, melting of permafrost allowed dark shrubs to spread, which would
increase local heating, Chapin et al. (2005);
ocean biological systems in general had instabilities, Hsieh
et al. (2005), etc. BACK
71c. For example, Best
(2006) speculated that increased organic sediments sent down rivers
due to deforestation, factory farming, etc., once buried in the seabed,
will be converted to methane by bacteria, perhaps seriously adding to
greenhouse gas emissions. Another potential threat was a dieback of the
Amazon rain forest. It was the IPCC's conclusion that little was understood
about the "potential for major abrupt change... in the uptake and
storage of carbon by terrestrial systems,"
Randall et al. (2007), p. 642.. BACK
72. There is a famous comparable case in another
field of science. In the 1930s, physicists used thin screens to block
extraneous large particles from their instruments as they measured the
tiny particles resulting from nuclear reactions. Since they never imagined
that an atom could split into two large chunks, they automatically prevented
themselves from discovering uranium fission. For discussion on the difficulties
of detecting rapid change, I am grateful to Ken Brown, Daniel A. Livingstone
and other respondents from the QUATERNARY and PALEOLIM listservs.
© 2003-2007 Spencer Weart & American Institute of Physics