| It was an incredible claim, yet the evidence
was eloquent. The scraped-down rock beds, the boulders perched wildly
out of place, the bizarre deposits of gravel found all around northern
Europe and the northern United States, all these looked exactly like
the effects of Alpine glaciers only far, far larger. By the
late 19th century, after passionate debate, most scientists accepted
the incredible. Long ago (although not very long as geological time
went, for Stone Age humans had lived through it), northern regions
had been buried kilometers deep in continental sheets of ice. This
Ice Age stood as evidence of a prodigious climate change.
- LINKS -
| Toward the end of the 19th century, field studies by geologists
turned up another fact, almost as surprising and controversial. There
had been not one Ice Age but several. The stupendous ice sheets had
slowly ground south and retreated, time and again. The series of glacial
periods had alternated with times of warmer climate, each cycle lasting
many tens of thousands of years. German geologists, meticulously studying
the scars left by ancient rivers on what were now hillsides in the
Alps, worked out a scheme of four major cycles.(1a)
| Geologists turned up
evidence that the past few million years, during which the ice sheets
cycled back and forth, was an unusual time in the Earth's history.
They gave it a name of its own, the Pleistocene epoch. Before that
there had been long eras of less turbulent climate, when fossils of
tropical plants and animals had been deposited in regions that were
now frigid. Much farther back there had been a few other relatively
brief epochs of glaciation, revealed by very ancient ice-scraped rocks
and gravel deposits. Most geologists concluded that the planet’s
climate had at least two possible states. The most common condition
was long temperate epochs, like the balmy times of the dinosaurs.
Much rarer were glacial epochs like our own, lasting a few millions
of years, in which glacial ice ages alternated with warmer "interglacial"
periods like the present. This essay does not cover studies of
the very remote past, before the Pleistocene.
What could explain the change from a warm to a glacial epoch, and the cycling of ice ages within
a glacial epoch? A solution to the puzzle would bring deep satisfaction and eternal fame to
whoever solved it. Perhaps the solution would also tell when the next ice age might descend
upon humanity — and would reveal mechanisms that could produce other kinds of climate change..
|Evidence and Speculations (to 1954)
From the mid-19th to the
mid-20th century, many theories were offered to explain the ice ages and other climate changes. None amounted to more than plausible hand-waving. Most favored were ideas
about how the uplift of mountain ranges, or other reconfigurations of the Earth's surface, would
alter the circulation of ocean currents and the pattern of winds. Other theories ranged from the
extraterrestrial, such as a long-term cyclical variation of solar energy, to the deep Earth, such as
massive volcanic eruptions. All these theories shared a problem. Given that something had put
the Earth into a state conducive to glaciation, what made the ice sheets grow and then retreat,
over and over again? None of the theories could readily explain the cycles. ||
Full discussion in
| Many things in the natural world come and
go in cycles, so it was natural for people to suppose that there was
a regular pattern to the ebb and flow of ice sheets. After all, there
was evidence convincing to many meteorologists, although doubted
by as many more that temperature and rainfall varied in regular
cycles on human timescales of decades or centuries. The epochs of repeated ice ages likewise seemed to follow a cyclical pattern, on a
far grander timescale. A series of repeated advances and retreats
of the ice was visible in channels carved by glacial streams and in
the fossil shorelines of lakes in regions that were now dry. If the
pattern of advance and retreat could be measured and understood, it
would give a crucial clue to the mystery of ice ages.
Simple observations of surface features were joined by inventive methods for measuring what a
region's climate had been like thousands or even millions of years ago. In particular, from the
early 20th century forward, a few scientists in Sweden and elsewhere developed the study of
ancient pollens ("palynology"). The tiny but amazingly durable pollen grains are as various as sea
shells, with baroque lumps and apertures characteristic of the type of plant that produced them.
One could dig up soil from lake beds or peat deposits, dissolve away in acids everything but the
sturdy pollen, and after some hours at a microscope know what kinds of flowers, grasses or trees
had lived in the neighborhood at the time the layer of lake-bed or peat was formed. That told
scientists much about the ancient climate. We had no readings from rain gauges and
thermometers 50,000 years ago, but pollen served as an accurate "proxy."
| Studying ancient pollens, scientists found
again a sequence of colder and warmer spells, glacial and interglacial
periods. The most recent ice age had ended ten thousand years or so
ago. Other ingenious studies showed that a particularly warm period
had followed. For example, fossil hoards of nuts collected by squirrels
revealed that five thousand or so years ago, hazel trees had grown
farther north in Sweden than at present. Were we drifting toward another
The problem that researchers set themselves was to find a pattern in
the timing of the changes. Unfortunately, there were no tools to accurately determine dates so far
in the past; any figure might be wrong by thousands of years. That did not stop people from
seeing regular patterns. An example was a 1933 study of ancient beach deposits by W.M. Davis.
As the continental ice sheets formed and then melted, they had locked up and then released so
much water that the oceans had dropped and risen many tens of meters. Wave-carved fossil
shores stood as testimony of the different sea levels. Davis believed he saw a pattern, in which
the warm interglacial periods were long. Our own time seemed near to the preceding ice age, so
he concluded that the Earth ought to get warmer for a while before it cooled again. When this
was added to reports that the climate of the 1930s was measurably getting warmer, predictions
appeared in Science magazine and in the public press that "The poles may become
useful and inhabited places."(3)
| The pattern of past changes, no matter how accurately geologists
might measure it, would always be suspect until a plausible theory
explained it. Of all the proposed theories, only one was bound by
its very nature to give regular cycles of change. This theory promised,
moreover, to give the timing of past changes precisely from basic
physical principles, and to predict future ice ages. The history of
the measurement of ancient climates is inseparable from the history
of this "astronomical" theory.
| In the mid 19th century,
James Croll, a self-taught British amateur scientist, published calculations
of how the gravitational pulls of the Sun, Moon, and planets subtly
affect the Earth's motion and orientation. The inclination of the
Earth's axis and the shape of its orbit around the Sun oscillate gently
in cycles lasting tens of thousands or hundreds of thousands of years.
During some periods the Northern Hemisphere would get slightly less
sunlight during the winter than it would get during other centuries.
Snow would accumulate. Croll argued that this would change the pattern
of trade winds, leading to the deflection of warming currents like
the Gulf Stream, and finally a self-sustaining ice age. The timing
of such changes could be calculated exactly using classical mechanics
(at least in principle, for the mathematics were thorny). Croll believed
that the timing of the astronomical cycles, tens to hundreds of thousands
of years long, roughly matched the timing of ice ages.(4*)
|Most scientists found
Croll's ideas unconvincing, and his timing of the ice ages wholly
wrong.(5) Nevertheless a few enthusiasts pursued his astronomical theory.
It became almost plausible in the hands of the Serbian engineer Milutin
Milankovitch. Working in the 1920s and 1930s, he not only improved
the tedious calculations of the varying distances and angles of the
Sun's radiation, but also applied an important new idea. Suppose there
was a particular season when the sunlight falling in a given hemisphere
was so weak, even in the summer, that the snow that fell in high latitudes
in winter did not all melt away? It would build up, year after year.
As others had pointed out, a covering of snow would reflect away enough
sunlight to help keep a region cold, giving an amplifying feedback.
Under such circumstances, a snowfield could grow over the centuries
into a continental ice sheet. Milankovitch was encouraged by Wladimir
Köppen, an eminent climatologist, who pointed out that the sensitive
zone would lie between the latitudes of 55 and 65 degrees north (where
moraines marked the edge of past continental ice sheets). Milankovitch
ground through calculations for the slow variations of the angle that
sunlight fell in that particular zone, especially in summer. Comparing
the calculations with geological evidence for the timing of past ice
ages, Köppen pronounced a good match.(6)
"The possibility of dating the varying episodes of the Pleistocene
ice ages by correlating them with the [Milankovitch] radiation curve appealed to a number of
workers," a meteorologist reported in 1940. "Correlations with the radiation curve were found
everywhere."(7) It was also encouraging
that even the tiny changes in solar radiation that came with the eleven-year sunspot cycle had
some effect on weather at least according to some studies. By the 1940s, some climate
textbooks were teaching that Milankovitch's theory gave a plausible solution to the problem of
timing the ice ages.(8)
| Supporting evidence came from "varves," a Swedish word for the
pairs of layers seen in the mud covering the bottom of northern lakes.
Each year the spring runoff laid down a thin layer of silt followed
by a settling of finer particles. From bogs and outcrops where the
beds of fossil lakes were exposed, or cores of slick clay drilled
out of living lakes, researchers painstakingly counted and measured
the layers. Some reported finding a 21,000-year cycle of changes.
That approximately matched the timing for a wobbling of the Earth's
axis which Milankovitch had calculated as a crucial element (namely, the cycle of seasons determined by the 26,000-year precession of the equinoxes combined with a longer precession of the entire elliptical orbit).(9)
|Most geologists, however, dismissed the astronomical theory. For
contrary to the optimistic Köppen, they could not fit Milankovitch’s
timing to the accepted sequence of four ice ages. A generation of
geologists had laboriously constructed this sequence from studies
around the world of surface features, such as the gravel deposits
(moraines) that marked where glaciers had halted. The Milankovitch theory,
wrote one authority condescendingly in 1957, had served a useful function
as "a dogma of faith" that had stimulated research, but compared with
the actual glacial record, the orbital chronology "must be stamped
as illusory." Another problem lay in the fact that ice sheets had
spread at the same time in the Northern and Southern Hemispheres.
Since the astronomical theory relied upon an increase in the sunlight
falling on one hemisphere along with a decrease on the other hemisphere,
many experts considered the world-wide pattern of ice ages a devastating refutation.(10) Finally, there was a basic
physical argument against the theory which seemed insurmountable.
One reviewer who had himself seen 21,000-year variations
in lake deposits explained at a 1952 conference that it was a problem of magnitudes. The
computed variations in the angle and intensity of incoming sunlight were only tiny changes,
"insufficient to explain the periods of glaciation."(11) Meanwhile, the studies that had found correlations between sunspot
cycles and weather had all turned out wrong, giving an air of cranky unreliability to every
connection between solar radiation variations and climate. That same year, a leading American
planetary scientist wrote a European colleague to ask how the astronomical theory stood over
there, remarking that "People I have consulted in this country... are not impressed by this work."
His correspondent replied, "I have discussed the question of the appraisal of Milankovitch's
theory with colleagues here. They are of the opinion that the theory cannot account for past
changes. The effects are too small and the chronology of the occurrence of glaciation is so
uncertain that any correspondence... appears fortuitous."(12) So what had caused the ice ages? That was still anybody's guess.
| Reliable Dates and Temperatures
| The tool that would unlock the secret was
constructed in the 1950s, although it took scientists a decade to
make full use of it. This tool was radiocarbon dating. It could tell
with surprising precision the age of features like a glacial moraine.
You only needed to dig out fragments of trees or other organic material
that had been buried thousands of years ago, and measure the fraction
of the radioactive isotope carbon-14 in them. Of course researchers
had to devise and test a number of laboratory techniques before they
could get trustworthy results. Once that was done, they could assign
a timescale to the climate fluctuations that had previously
been sketched out by various traditional means. The best of these
means, in the 1950s, was pollen science. The study of ancient climates
had turned out to be invaluable
for identifying strata as an aid to oil exploration, and that had paid
for specialists who brought the technique to a high degree of refinement.(13) Carbon-14 measurements could now assign accurate dates
to the palynologists’ tables of cool and warm periods in northern
regions. For example, dating of lake deposits in the Western United
States showed surprisingly regular cycles of drought and flood
which seemed to match the 21,000-year cycle predicted by Milankovitch.
But other carbon-14 dates seemed altogether out of step with the Milankovitch
|The swift postwar development of nuclear
science meanwhile fostered another highly promising technique.
In 1947, the nuclear chemist Harold Urey discovered a way to measure
ancient temperatures. The key was in the oxygen built into fossil
sea shells. The amount of heavier or lighter
oxygen isotopes that an organism took up from sea water varied according
to the water's temperature when it was alive, so the ratio (O18/O16 ) served
as a proxy thermometer.(14) This ingenious method was taken up by Cesare Emiliani,
a geology student from Italy working in Urey's laboratory at the University
of Chicago. Emiliani measured the oxygen isotopes in the microscopic
shells of foraminifera, a kind of ocean plankton. Tracking the shells
layer by layer in long cores of clay extracted from the seabed, he
found a record of temperature variations. Emiliani's 1955 paper, a
landmark of paleoclimatology, provided the world's first high-quality
record of ice age temperatures.(15)
<=Uses of shells
| Historians usually treat techniques as a stodgy foundation,
unseen beneath the more exciting story of scientific ideas. Yet techniques
are often crucial, and controversial. The stories of two especially
important cases are explored in short essays on Uses
of Radiocarbon Dating and Temperatures from
| Emiliani tentatively identified the rises and dips of temperatures
with the geologists' traditional chronology of the past three ice ages. His efforts were motivated largely by a desire to learn something
about the evolution of the human race, which had surely been powerfully
influenced by the climate shocks of the ice ages. But his results
turned out to tell less about the causes of human evolution than about
the causes of climate change. To get a timescale connecting the temperature
changes with depth down the core, he made carbon-14 measurements covering
the top few tens of thousands of years (farther back there was too
little of the isotope to measure). That gave him an estimate
for how fast sediments accumulated on the seabed at that point. Emiliani
now found a rough correlation with the varying amount of sunlight
that, according to Milankovitch's astronomical calculations, struck
high northern latitudes in summer. To get the match he had to figure
in a lag of about five thousand years. That seemed reasonable, considering
how long it would take a mass of ice to react. "A causal connection
is suggested but not proved," Emiliani concluded.(16*)
|The chemist Hans
Suess, another graduate of Urey's lab, took the lead in improving
the carbon-14 chronology. He reported, among other things, that the last ice
age had come to a surprisingly abrupt end, starting sometime around
15,000 years ago. Looking farther back, Suess found hints of a roughly
40,000-year cycle, which sounded like the 41,000-year cycle that Milankovitch
had computed for slight variations in the inclination of the Earth's
axis.(17) Emiliani too, reporting a cycle of
roughly 50,000 years, was increasingly confident that orbital changes
set the timing of ice ages.(18)
His curves, however, did not match up with the canonical four ice
To resolve the issue, Emiliani began urging colleagues to launch a major program and pull up
truly long cores, a hundred-meter record covering many hundreds of thousands of years. But for a
long time the drillers' crude techniques were incapable of extracting long, undisturbed cores from
the slimy ooze. As one of them remarked ruefully, "one does not make wood carvings with a
| In the early 1960s suggestive new evidence was dug up (literally) by the geochemist Wallace
Broecker and collaborators. Ancient coral reefs were perched at various
elevations above the present sea level on islands that geological
forces were gradually uplifting. The fossil reefs gave witness to
how sea level had risen and fallen as ice sheets built up on the continents
and melted away. The coral could be dated by hacking out samples and
measuring their uranium and other radioactive isotopes. These isotopes
decayed over millennia on a timescale that had been accurately measured
in nuclear laboratories. Unlike carbon-14, the decay was slow enough
so there was still enough left to measure after hundreds of thousands
of years. As a check, the sea level changes could be set alongside
the oxygen-isotope temperature changes measured in deep-sea cores.
Again the orbital cycles emerged, plainer than ever. At a conference
on climate change held in Boulder, Colorado in 1965, Broecker announced
that "The Milankovitch hypothesis can no longer be considered just
an interesting curiosity."(20) People at the conference began to speculate on how the
calculated changes in sunlight, although they seemed insignificantly
small, might somehow trigger ice ages. That could happen if the climate
system were so delicately balanced that a small push could prompt
it to switch between different states.
| Meanwhile oceanographers managed to extract a fine set of cores that reached
back more than 400,000 years. Analyzing the cores, Emiliani announced he could not make the data
fit the traditional ice ages timetable at all. He rejected the entire
scheme, painstakingly worked out around the end of the 19th century
in Europe and accepted by generations of geologists, of a Pleistocene
epoch comprising four major glacial advances alternating with long
and equable interglacial periods. Emiliani said the interglacials
had been briefer, and had been complicated by irregular rises and
falls of temperature, making dozens of ice ages.(21) Many other scientists found his chronology dubious, but
he defended his position tenaciously. Most significantly, he believed
the sequence correlated rather well with the complex Milankovitch
curve of summer sunlight at high northern latitude. Calculating how
the cycle should continue in the future, in 1966 Emiliani predicted
that "a new glaciation will begin within a few thousand years."(22) It was a step toward what would soon become widespread
public concern about future cooling.
CLICK FOR FULL IMAGE
| Seldom was such work straightforward. Geologists defended their
traditional chronology passionately and skillfully. For a few years
they held their ground, for it turned out that Emiliani's data on
oxygen isotopes taken up in plankton shells did not directly measure
ocean temperatures after all. Emiliani fiercely defended his position,
but other workers in the late 1960s convinced the scientific community
that he was mistaken. When water was withdrawn from the oceans to
form continental ice sheets, the heavier and lighter isotopes evaporated
and fell as rain or snow in different proportions. The way plankton
absorbed oxygen at a given temperature mattered less than what
proportion of each isotope was available in the sea water as ice sheets
came and went.
Yet in a deeper sense Emiliani was vindicated. Whatever the forces that changed the isotope
ratio, its rise and fall did represent the coming and going of ice ages. "Emiliani's
'paleotemperature' curve," the new findings revealed, "...may be renamed a 'paleoglaciation'
| These changes did turn
out to correlate with ocean surface temperatures. New evidence for
that came from scientists who took a census of the particular species
of foraminifera, recognizing that the assemblage of different species
varied with the temperature of the water where the animals had lived.
The data confirmed that there had been dozens of major glaciations
during the past couple of million years, not the four or so enshrined
in textbooks. Corroborating evidence came from a wholly different
type of record. In a brick-clay quarry in Czechoslovakia, George Kukla
noticed how wind-blown dust had built up into deep layers of soil
(what geologists call "loess"). Although Kukla could not get dates
that matched Emiliani's, the multiple repetitions of advance and retreat
of ice sheets were immediately visible in the colored bands of different
types of loess. It was one of the few cases in this story where traditional
field geology, tramping around with your eyes open, paid a big dividend.
<=Uses of shells
| In 1968, still more complete and convincing evidence came from
an expedition that Broecker and a few others took to Barbados. Terraces
of ancient coral covered much of the island, rising to hundreds of
meters above the present sea level. The dates for when the coral reefs
had been living (125,000, 105,000, and 82,000 years ago) closely matched
dates from Milankovitch cycles for times when the ice sheets should
have been melted and the seas at their highest (127,000, 106,000,
and 82,000 years ago). The dating matched, that is, so long as one
looked for the times when the maximum amount of sunlight struck a particular band of mid-northern
latitudes during the summer. "The often-discredited hypothesis of
Milankovitch," declared Broecker and his collaborators, "must be recognized
as the number-one contender in the climatic sweepstakes."(24*)
| Since the Milankovitch cycles could be computed directly from celestial
mechanics, one could project them forward in time, as Emiliani had
done in 1966. In 1972, presenting more Caribbean cores, he again advised
that "the present episode of amiable climate is coming to an end."
Thus "we may soon be confronted with... a runaway glaciation." (He
meant "soon" as geologists reckoned time, in centuries or
millennia.) However, he added, greenhouse effect warming caused by
human emissions might overwhelm the orbital shifts, so we might instead
face "a runaway deglaciation."(25)
| Some other
scientists agreed that the current interglacial warm period had peaked
6,000 or so years ago, and should be approaching its natural end.
A prominent example was Kukla, continuing his study of loess layers
in Czechoslovakia. He could now date the layers thanks to a new technique
provided by other scientists. Geological and oceanographic studies
had shown that over the course of millions of years, from time to time
the Earth's entire magnetic field flipped: the North magnetic pole
became the South magnetic pole and vice-versa. These reverses were
recorded where layers of sediment or volcanic lava had entombed the
direction of the magnetic field at the time. Geologists had worked
out a chronology in lava flows, dated by the faint radioactivity of
an isotope of potassium that decayed very slowly.(26)
If even one magnetic-field reversal could be identified in any set
of layers, it pinned down the timing of the entire sequence. When
the loess layers were dated in this fashion, Milankovitch cycles turned
up. Extrapolating the cycles into the future, Kukla thought the next
shift to an ice age "is due very soon." (To geologists of the time,
"very soon" meant within a few thousand years, although
Kukla and a few others thought cooling might possibly become severe
within a century or two.)(Link
from below) (27)
|If the climate experts of the time seem to have been a bit preoccupied
with ice ages, that fitted their training and interests. For a hundred
years their field had concerned itself above all with the ice ages.
Their techniques, from pollen studies to sea floor drilling, were
devoted to measuring the swings between interglacial and glacial periods.
Home at their desks, they occupied themselves with figuring how
ice age climates had differed from the present, and attacking
the grand challenge of explaining what might cause the swings. Now
that they were beginning to turn their attention from the past to
the future, the most natural meaning to attach to "climate change"
was the next swing into cold.(28)
| In 1972, a group of leading ice-age
experts met at Brown University to discuss how and when the present
warm interglacial period might end. A large majority agreed that "the
natural end of our warm epoch is undoubtedly near." Near, that is,
as geologists reckoned time. Unless there were impacts from future
human activity, they thought that serious cooling "must be expected
within the next few millennia or even centuries."(29) But many other scientists doubted these conclusions. They
hesitated to accept the Milankovitch theory at all unless they could
get definitive proof from some entirely different kind of evidence.
| Theories Confirmed (1971-1980)
The Greenland ice sheet is a daunting sight. Most investigators first
come to it by air, past colossal bare cliffs where unimaginable quantities of ice pour down to the
sea in a slow-motion flood. Beyond that the landscape rises and rises, over entire mountain
ranges hidden under ice, to a limitless plain of gently undulating white. Greenland had played an
important role in the 19th-century controversy over the ice ages. A few geologists had dared to
postulate the existence, in the distant past, of seas of solid ice kilometers thick. Then astonished
explorers of Greenland found just such a thing beneath their skis.
| In the
late 1950s scientists came back to Greenland, hoping to find the
key to the history of climate change. The logistics were arduous.
But there was good support thanks to the International Geophysical
Year backed up by the United States government's concern to
master the Arctic regions that lay on the shortest air routes to the
Soviet Union. At Camp Century, Greenland, workers drilled short cores
from the ice to demonstrate that it could be done. An improved drill,
brought onto the ice in 1961, produced glistening cores 5 inches in
diameter in segments several feet long. This was no small feat in
a land where removing your gloves for a few minutes to adjust something
might cost you the skin on your fingertips, if not entire fingers.
After another five years of difficult work, organized by the U.S.
Army's Cold Regions Research and Engineering Laboratory, the drill
at Camp Century reached bedrock. The hole reached down some 1.4 kilometers
(7/8 of a mile), bringing up ice as much as 100,000 years old.(30*)
Two years later, in 1968, another long core of ancient ice was retrieved
from a site even colder and more remote: Byrd Station in West Antarctica.(31)
Much could be read from these cores.
For example, individual layers with a lot of acidic dust pointed to past volcanic eruptions.
Individual eruptions could be assigned dates simply by counting the annual layers of ice.(32) (Known eruptions like the destruction
of Pompeii in the year 79 gave a check on the counts.) Farther down the layers became blurred,
but approximate dates could still be assigned. Deep in the ice there were large amounts of
mineral dust, evidence that during the last ice age the world had been windier, with storms
carrying dust clear from China. Still better, ancient air had been trapped and preserved as bubbles
in the ice, a million tiny time capsules packed with information about past climates. However, for
a long time nobody could figure out how to extract and measure the fossil air reliably.
| In the early years, the most useful work
was done from the ice itself. The method had been worked out back
in 1954 by an ingenious Danish scientist, Willi Dansgaard. He showed
that the ratio of oxygen isotopes (O18/O16) in the ice measured the
temperature of the clouds at the time the snow had fallen the
warmer the air, the more of the heavy isotope got into the ice crystals.(33) It was an exhilarating day for the researchers at Camp
Century, making measurements along each cylinder of ice after it was
pulled up from the borehole, when they saw the isotope ratios change
and realized they had reached the last ice age. The preliminary study
of the ice cores, published in 1969, showed variations that indicated
an average temperature change of perhaps 10°C (that is, 18°F). Some cycles were
tentatively identified, including one with a 13,000 year
length.(34) Comparison of the Greenland and Antarctic
cores showed that the climate changes were truly global, coming at
essentially the same time in both hemispheres. That put a strict constraint
on theories about the cause of cycles.(35)
<=Uses of shells
There is a supplementary site on the History of Greenland Ice Drilling, with
some documentation of the U.S. "GISP" projects of the 1980s.
| Ice core
studies also confirmed a feature that researchers had already noticed
in deep-sea cores: the glacial cycle followed a sawtooth curve. In
each cycle, a spurt of rapid warming was followed by a more gradual,
irregular descent back into the cold over tens of thousands of years.
A closer look showed that temperatures tended to cluster at the two
ends of the curve. It seemed that the climate system had two fairly
stable modes, brief warmth and more enduring cold, with relatively
rapid shifts between them. Warm intervals like the past few thousand
years normally did not last long.(36)
Beyond such fascinating hints, however, the Greenland ice cores could
say little about long-term cycles. They were too short to reach past
a single glacial cycle. And the ice flowed like tar at great depths,
confusing the record. In the 1970s, despite the arduous efforts of
the ice drillers, the most reliable data were still coming from deep-sea
That work too was strenuous and hazardous, manhandling long wet pipes on a heaving deck.
Oceanographers (like ice drillers) lived close together for weeks or months at a time under
Spartan conditions, far from their families. The teams might function smoothly or not.
Either way, the scientists labored long hours, for the problems were stimulating, the results could
be exciting, and dedication to work seemed normal with everyone around them doing the same.
To make it worthwhile, scientists had to draw on all their knowledge and luck to find the right
places to drill on the ocean bed. In these few places, layers of silt had built up unusually swiftly
and steadily and without disturbance. Meanwhile, drilling techniques were finally worked out
that could extract the continuous hundred-meter cores of clay that Emiliani had been asking for
since the 1950s. Improved techniques for measuring the layers gave data good enough for
| The most prominent feature turned out to be a 100,000-year cycle
evidently a key to the entire climate puzzle. Several earlier
studies had tentatively identified this long-term cycle. Corroboration
was in hand from Kukla's loess layers in Czechoslovakia, at the opposite
end of the world from some of the deep-sea cores. Here too the 100,000-year
cycle stood out.(37)
Yet nobody could be entirely sure. Radiocarbon decayed too rapidly to give dates going back
more than a few tens of thousands of years. A deeper timescale could only be estimated by
measuring lengths down a core, and it was uncertain whether the sediments were laid down at a
uniform rate. For a decade controversy had smoldered between Emiliani, as usual sticking by his
original position, and other scientists who felt that his chronology was seriously in error.
According to their data, the prominent cycle he had seen and attributed to the 41,000-year orbital
shifts was actually the 100,000-year cycle.(38) Here again Emiliani had been bogged down by erroneous
assumptions, yet somehow had muddled through to the fundamental truth that Milankovitch
cycles were real.
| In 1973, Nicholas (Nick) Shackleton nailed
it all down for certain. What made it possible was the new magnetic-reversal
dates established by radioactive potassium, plus Shackleton's uncommon
combination of technical expertise in different fields. A splendid
deep-sea core had been pulled "one of the best and most complete
records of the entire Pleistocene that is known" the famous
core Vema 28-238 (named after the Lamont Observatory's oceanographic
research vessel, a converted luxury yacht). It reached back over a
million years, and included the most recent reversal of the Earth's
magnetic field, which geologists dated at a bit over 700,000 years
ago. This calibrated the chronology for the entire core. As a further
benefit, Shackleton managed to extract and analyze the rare shells of foraminifera
plankton that lived in the deep sea, and which reflected basic oceanic changes
independent of the fluctuating sea-surface temperatures. The deep-sea
foraminifera showed the same isotopic variations as surface ones, confirming
that the variations gave a record of the withdrawal of water to form
ice sheets. When Shackleton showed his graph of long-term change to
a roomful of climate scientists, a spontaneous cheer went up.(39*)
| The core Vema 28-238 and a few others contained
such a long run of consistent data that it was possible to analyze
the numbers with a mathematically sophisticated "frequency-domain"
calculation, a well-established technique for picking out the lengths
of cycles in a set of data.(40) Detailed measurements and numerical calculations found
a set of favored frequencies, a spectrum of regular cycles visible
amid the noise of random fluctuations. The first unimpeachable results
(well, almost unimpeachable) were achieved in 1976 by James Hays,
John Imbrie and Shackleton. The trio not only analyzed the oxygen-isotope
record in selected cores from the Indian Ocean, but checked their
curves against temperatures deduced from the assemblage of foraminifera
species found in each layer.
| The long cores proved beyond
doubt what Emiliani had stoutly maintained there had been not
four major ice ages, but dozens. The analysis showed cycles with lengths
roughly 20,000 and 40,000 years, and especially the very strong cycle
around 100,000 years, all in agreement with Milankovitch calculations.(41)
Extrapolating the curves ahead, the group predicted cooling for the
next 20,000 years. As Emiliani, Kukla, and other specialists had already
concluded several years earlier, the Earth was gradually — indeed,
perhaps quite soon as geologists reckoned time — heading into
a new ice age (see above).
|These results, like so many in paleoclimatology,
were promptly called into question.(42)
For one thing, there was no solid reason to suppose that our current
interglacial period would be of average length and was therefore nearing
its end. But
the main results withstood all criticism. Confirmation came from other
scientists who likewise found cycles near twenty and forty thousand
years, give or take a few thousand. The most impressive analysis remained
the pioneering work of Hays, Imbrie, and Shackleton. They could even
split the 20,000 year cycle into a pair of cycles with lengths
of 19,000 and 23,000 years exactly what the best new astronomical
calculations predicted. By the late 1970s, most scientists were convinced
that orbital variations acted as a "pacemaker" to set the timing of
ice ages.(43) Science magazine reported
in 1978 that the evidence for the Milankovitch theory was now "convincing,"
and the theory "has recently gained widespread acceptance as a factor"
in climate change.(44)
Yet the cause of the ice ages remained more a mystery than ever.
How could the "pacemaker" possibly work? The variation in the intensity of sunlight that was
computed for the 100,000-year astronomical cycle came from a minor change in orbital
eccentricity a slight stretching of the Earth's path around the sun out of a perfect circle. It
was a particularly tiny variation; the changes it caused should be trivial compared with the
shorter-term and larger orbital shifts, not to mention all the other influences on climate. Yet it
was the 100,000-year cycle that dominated the record. Scientists began to turn from hunting
down cycles to searching for the physical mechanisms that could make the climate system
respond so dramatically to subtle changes in sunlight. As a reviewer admitted, "failures to
support the Milankovitch theory may only reflect the inadequacies of the models."(45) A number of people took up the
challenge, devising elaborate numerical models that took into account the sluggish dynamics of
continental ice sheets. It seemed likely that eventually the modelers would produce a suite of
feedbacks that would entirely explain the schedule of the ice ages.
|Glimpsing a Greenhouse Future (1980s to Present)|
| During the 1980s, the work advanced steadily
with few surprises. Ocean drilling in particular, pursued on an international
scale, produced ever better cores. A costly project dedicated to "spectral
mapping" (SPECMAP) yielded a spectrum of cycles that matched the astronomical
calculations with gratifying precision going back hundreds of millennia.
Five separate cores confirmed that variations in the Earth's orbit
drove the coming and going of ice ages.(46)
But an unexpected finding brought in a new complication. The prominent
100,000-year cycle (due to changes in the orbit's eccentricity) had
dominated climate change only during the most recent million years.
During a long earlier phase of the Pleistocene epoch, the rise and
decay of ice sheets had followed the 41,000-year cycle (related to shifts
in the inclination of the Earth's axis).(47) Milankovitch and his
followers had originally expected that this cycle would have a much
stronger effect than the feeble 100,000-year shifts. They had recognized,
however, that the 41,000-year variations in sunlight might still have
been too small to cause ice ages without some kind of amplification.
The experts understood that "the response characteristics of the Earth's
climate system have themselves evolved," so that the details of cycling
could well change.(48) The shift in the dominant cycle surely gave a clue, if
an enigmatic one, to the variety of feedback mechanisms at work.
Meanwhile ice drillers, reaching ever farther into the past at locations where the flow of ice in the
depths did not introduce too much confusion, joined the deep-sea drillers as a main source of
information. The ice and seabed climate curves were found to go up and down in fine agreement,
and researchers began to combine data from both sources in a single discussion. The most
striking news from the ice was evidence that the level of carbon dioxide gas (CO2) in the atmosphere had risen and fallen more or less in time with
The outstanding record was extracted by
a French-Soviet team at the Soviets' Vostok Station in Antarctica.
It was a truly heroic feat of technology, wrestling with drills
stuck a kilometer down, at temperatures so low that a puff of breath
fell to the ground in glittering crystals. Vostok had been established
during the 1957-58 International Geophysical Year at the most remote
spot on the planet. Supplies were brought once a year by a train
of vehicles that clawed across 1400 kilometers of ice. Underfunded
and threadbare, the station was fueled by the typically Russian
combination of cigarettes, vodka, and stubborn persistence. ("What
do you do for recreation?" "Wash... you have a bath once every ten
days.") At one point the generator failed; the crew survived by huddling in an ice cave heated by candles.(49)
| The effort paid off. While
the Greenland record reached into the most recent ice age, by 1985
the Antarctic team had pulled up cores of ice stretching clear through
the cold period and into the preceding warm period—a complete
glacial cycle.(50) During
the cold period, the CO2 level had been much
lower than during the warm periods before and after. Indeed the curves
of gas level and temperature tracked one another remarkably closely.
Measurements in ice cores of an even more potent greenhouse gas, methane,
showed a similar rise and fall that matched the rise and fall of temperature.(51)
This work fulfilled the old dream that studying the different climates
of the past could be almost like putting the Earth on a laboratory
bench, switching conditions back and forth and observing the consequences.
=>Venus & Mars
The Vostok team pointed out
that the swings in greenhouse gas levels might be amplifying the
effect of the orbital shifts. A small rise or fall in temperature
seemed likely to cause a rise or fall in the gas levels (for example,
when seawater got warmer it would evaporate some CO2
into the atmosphere, whereas it would absorb the gas during a cooling
period). More or less greenhouse gases in the atmosphere would make
for further changes in temperature, which would in turn raise or
lower the gas levels some more... and so on. It was the first truly
plausible theory for how minor shifts of sunlight could make the
entire planet's temperature lurch back and forth.
|The changes in the atmosphere also answered
the old persuasive objection to Milankovitch's theory if the
timing of ice ages was set by variations in the sunlight falling on
a given hemisphere, why didn't the Southern Hemisphere get warmer
as the Northern Hemisphere cooled, and vice-versa? The answer was
that changes in atmospheric CO2 and methane physically
linked the two hemispheres, powerfully warming or cooling the planet
as a whole.(52*)
Variations over 420,000 years of CO2, methane(CH4), and temperature, from the Vostok ice core after it reached bedrock (1999): four complete glacial cycles.
Age in years Before Present, older to right.
Adapted from Petit et al., Nature 399:429-36 (1999).
|Looking at the rhythmic curves of past cycles, one could hardly
resist the temptation to extrapolate into the future. By the late
1980s, most calculations had converged on the familiar prediction
that the natural Milankovitch cycle should bring a mild but steady
cooling over the next few thousand years. As climate models and
studies of past ice ages improved, however, worries about a swift
descent into the next great glaciation — what many in the
1970s had tentatively expected — died away. New calculations
said that the next ice age would not come naturally within the next
ten thousand years or so. The calculations were backed up in 2004
by data from a heroic new drilling effort in Antarctica that brought
up ice spanning the past eight glacial cycles. Among these was an unusually long previous cycle where the orbital elements had been similar to those in our own cycle. On the other hand, in 2012 a team using a different ancient cycle as an analogy to the present claimed that the world should indeed be descending into an ice age over the next thousand years or so.(53*)
The scientists who published these calculations always added
a caveat. In the Antarctic record, atmospheric CO2
levels over the past 750,000 years had cycled between about 180
and 280 parts per million. The level by 2012 had climbed almost to 400 and kept climbing. (The other main greenhouse
gas, methane, was soaring even farther above any level seen in the
long ice record.) Greenhouse warming and other human influences
seemed strong enough to overwhelm any natural trend. One environmental
scientist, William Ruddiman, even argued that the rise of human
agriculture had already produced enough greenhouse gases to counteract the gradual cooling that should
have come during the past several thousand years in the normal descent
from a post-glacial temperature peak.(53a) As emissions climbed exponentially,
we might not only cancel the next ice age, but launch our planet
into an altogether new climate regime.
|The ice cores themselves
gave convincing evidence of the threat, according to analyses published beginning
in the early 1990s. The "climate sensitivity" — the
response of temperature to changes in carbon dioxide — could
be measured for the last glacial maximum. The answer was in the same
range that computer models were predicting for our future, namely, about 3°C of warming, give or take a degree or so, if the CO2 level doubled. It was just the same range that the models got. By 2012 this finding had been confirmed not only for recent ice ages but for many other geological epochs. That raised
confidence that the models could not
be far wrong.(53b)
|In climate science, where everything
is subtle and complex, it is rare for an issue to be entirely settled. By the
late 1980s, it did seem to be an established fact that ice ages were
timed by orbital variations. But what kind of feedbacks amplified the
effect? Some people challenged whether any of this was really
understood. The cycles, most scientists now agreed, involved not only
orbital variations in solar irradiation, but also a variety of geological
effects. First came the massive creation, settling and flow of continental ice
sheets, which naturally would work on a timescale of tens of thousands of years. But large-scale physical and chemical changes in the oceans
might be important too. New evidence gave a particularly crucial role
to changes in CO2 and other greenhouse gases, changes apparently driven not just by geochemistry and
ocean circulation, but still more by changes in biological activity.
And of course the biosphere depended in turn on climate and
not just temperature, but also trickier matters like fertilization
of the seas by minerals eroded from glacial era deserts. Further unexpected
influences were added to the list of possibilities almost every year.(54) It would take much more study
to determine just what combination of effects determined the shape
of glacial cycles.
| In 1992, a more fundamental challenge was raised by the ingenious
exploitation of a novel source of data: layers of calcite laid down
in the desert oasis of Devils Hole, Nevada. The layers showed glacial
and interglacial periods much like those seen in the ice cores. But the
dating (using uranium isotopes) failed to agree with Milankovitch
calculations. The authors suggested that the timing of ice ages followed
no regular cycle at all, but was driven wholly by "internal nonlinear
feedbacks within the atmosphere-ice sheet-ocean system."A vigorous controversy followed, but in the end most climate
scientists stuck by the Milankovitch theory. The Devils Hole measurements
looked solid, but might they not represent only a strictly local effect? By 2001 there was evidence that the Nevada temperatures were indeed local, perhaps related to changing currents in the Pacific Ocean. Yet debate over this "great paleoclimate enigma" continued until 2016, when a team showed that complex local mechanisms had biased the Devils Hole data, and reconciled it with the Milankovitch chronology.(55)
The controversy had highlighted the complexities of the climate system. As two experts reviewing the problem put
it, "climate is too complicated to be predicted by a single parameter."(56)
The faint variations of summer sunlight in Northern latitudes were
effective only because the astronomical schedule somehow resonated
with other factors — the dynamics of continental ice
sheets and tropical ocean currents, the bio-geochemical CO2
system, and who knew what else. The more precise the data got, the
less precise seemed the match between sunlight in the Northern Hemisphere and ice age cycles; probably Southern Hemisphere sunlight and other Milankovitch features played a role.
Evidently when orbital effects served as a pacemaker, it was by
adjusting the timing of greater forces working through their own
complex cycles. As one reviewer said in 2002, "The sheer number of explanations
for the 100,000-year cycle... seems to have dulled the scientific
community into a semipermanent state of wariness about accepting
any particular explanation."(57)
The long collective
trudge through the intricacies of field data and models gradually increased understanding of all the interacting forces that drive climate cycles. By around 2013 computer models had finally advanced to the point where modelers could get convincing glacial cycles. They ran a climate model to take account of variations in sunlight and the rise and fall of CO2, then took snapshots from this model and fed them into a model for ice-sheet behavior and fed the result back into their climate model. The 100,000-year cycle was explained by the slow settling of rock under the colossal weight of the North American ice sheet. After several 23,000-year cycles the Earth's crust sagged so far that the ice's surface was at a low enough altitude to melt in summer — but only when orbital conditions brought increased sunlight in northern latitudes.(57a) This was a good start, but more work would be needed before the entire pattern could be well understood.
The invaluable fruit of a century of ice ages research
was the recognition of how complex and powerful the feedbacks
could be. Another important clue came from
some especially good Antarctic ice core records that timed precisely the changes
in the levels of CO2 and methane. The levels
apparently rose or fell a few centuries after a rise or
fall in temperature. At first this lag puzzled scientists, but they
quickly realized that this was just what they should have expected.
For it strongly confirmed that the Milankovitch-cycle orbital changes
initiated a powerful feedback loop. The close of an ice age came
when a shift in sunlight caused a slight rise of temperature, and
that evidently raised the gas levels over the next few centuries. The greenhouse effect then
slowly drove the planet's temperature a bit higher, which drove a
further rise in the gas levels... and so forth. On the other hand,
when the sunlight in key latitudes weakened, that would not only
bring more ice and snow, but also a shift from emission to absorption
of gases, eventually causing a further fall in temperature... and so forth. Confirmation came in 2012 from evidence that the lag was confined to the Antarctic ice cores: after an initial temperature rise followed by evaporation of CO2 from the Southern Ocean, globally the rise of CO2 had preceded the rise of temperature.(58)
|Our current situation was altogether
different. The warming was not started by a small shift of sunlight,
as in previous epochs. Our addition of gases to the atmosphere was
initiating the process, with the temperature rise lagging behind the
rise of gas levels. Emissions were climbing at a far
swifter rate than anything
in the Pleistocene record, so the lag was measured not in centuries, but mere decades. And already by the 1980s the levels of greenhouse gases had climbed far higher than anything seen for many millions of years. Even if we stopped our emissions, would feedbacks drive things higher on their own? There were
disturbing signs that feedbacks were indeed kicking in. Drying forests
and warmer seawater were getting less efficient at taking CO2
out of the air, and methane was seen bubbling up from Arctic wetlands.
By the start of the 21st century,
it was clear that the connection between global temperature and
greenhouse gas levels was a major geological force. All through the Pleistocene, the
greenhouse gas feedback had turned the planet's orbital cycles from
minor climate variations to grand transformations that affected
all life on the planet. The geological record gave a
striking verification, with wholly independent methods and data,
of the processes that computer models were predicting would bring a rapid and severe global warming — a disruption of climate exceeding anything seen since the emergence of the human species.
Simple Models of Climate
The Carbon Dioxide Greenhouse Effect
Changing Sun, Changing Climate
Temperatures from Fossil Shells
1. Callendar (1961),
p. 1. BACK
1a. The history is reviewed by Imbrie
and Imbrie (1986); the scheme of four ice ages was propounded by Albrecht
Penck, Penck and Brückner (1901-1909). BACK
2. Nuts: G. Andersson in 1902 as cited in Lamb (1977), p. 397; for the history overall, see Lamb pp. 193,
378ff. and Webb (1980).
3. Davis (1933).
4. Croll (1864); Croll (1875); Croll predicted glaciation when the Earth was
at aphelion in winter. But summer aphelion (with the distant Sun less
likely to melt the snow away) was more likely to do it, as pointed out
by Murphy (1876); Croll (1886) defends his views; Imbrie
and Imbrie (1979), pp. 77-88. BACK
5. For example, Arrhenius (1896),
p. 274; Brooks (1922), p. 18-19.
6. Milankovitch (1920) ; Milankovitch (1930), see pp. 118-21 for additional history; Milankovitch (1941) ; for this history I have used Imbrie and Imbrie (1986).
7. Simpson (1939-40), p. 203.
8. E.g., Landsberg (1941, rev. ed.
1947, 1960), pp. 191-92.
9. Bradley (1929) ; Zeuner (1946 [4th ed., 1958]).
10. "dogma... illusory," Öpik
(1957); "This theory has been answered devastatingly by... Sir George Simpson," Wexler (1952), p. 74.
11. van Woerkom (1953); see
also Science Newsletter (1952); similarly, "the changes of solar
radiation due to changes in the Earth's orbit are always too small to be of practical importance,"
Simpson (1939-40), p. 209.
12. Kuiper to H. Sverdrup, 28 May 1952, and reply, 11 June
1952, Box 11, G.P. Kuiper files, Special Collections, U. Ariz., kindly reported to me by Ron
Doel.; similarly, the theory "has failed utterly," Humphreys
(1920), pp. 564-66, quote p. 568, on the Croll theory, but repeated without change in the 3rd
(1940) edition, p. 586, without reference to Milankovitch.
13. Faegri et al. (1964); Manten (1966).
14. Urey (1947).
15. Emiliani (1955); see Emiliani (1958).
16. The effect was never expected to correlate with sunlight in
the Southern Hemisphere, which is mostly ocean where snow would never accumulate. Emiliani (1955), p. 509; see also Emiliani
(1958); on evolution, Emiliani (1958) p. 63.
17. Suess (1956).
18. Emiliani and Geiss (1959).
19. Hsü (1992), pp.
20. Quote: Broecker (1968),
p. 139; for early work, see Broecker (1966).
For a detailed account of this and following events see Broecker
and Kunzig (2008), pp. 46-56. BACK
21. Emiliani (1966).
22. Emiliani (1966).
23. Dansgaard and Tauber
(1969). Later work found many further refinements; any large change in precipitation or in ecosystems that processed oxygen would alter the oxygen-isotope ratio throughout the global atmosphere. For example, certain abrupt global climate changes thousands of years ago (Dansgaard-Oeschger events, see the essay on Ocean Currents) included large changes in monsoon rainfall over Asia, which produced noticeable isotope shifts, Severinghaus et al. (2009).
24. "sweepstakes": Broecker
et al. (1968) p. 300; as 125, 105, and 82,000 in Mesolella
et al. (1969); see also summary in Broecker
and van Donk (1970) .On this and the history of ice-age cycle observations in general see Broecker (2010), ch. 1. An important confirmation, using boreholes drilled
in Barbados reefs now drowned, was Fairbanks and Matthews (1978); the objection that the sea
level changes might be due to local uplift in Barbados, and not a world-wide
phenomenon, was refuted by an expedition to another fine set of coral
terraces on a rarely visited coast of New Guinea, Bloom et al. (1974); for discussion Berger (1988). BACK
25. Emiliani (1972).
26. Glen (1982).
27. Kukla and Kocí (1972),
p. 383. BACK
28. Chambers and Brain
(2002), p. 239.
29. Kukla et al. (1972), p.
191; Kukla and Matthews (1972); "large majority" according to
Flohn (1974), p. 385.
30. The first long core (411m), using a drill developed by B.
Lyle Hansen, was extracted at another site in Greenland in 1956: Dansgaard et al. (1973); for brief history and references, see also
Langway et al. (1985); Levenson
(1989) pp. 40-41; for a firsthand account, Alley (2000).
31. Epstein et al. (1970).
32. Hamilton and Seliga
33. Dansgaard (1954); Dansgaard (1964); for further bibliography on gases in ice, see Broecker (1995), pp. 279-84.
34. Dansgaard et al. (1969).
Exciting day: oral history interview of Klaus Hammer by Finn Aaserud, 1993, GISP interviews,
records of Study of Multi-Institutional Collaborations, AIP. Extensive autobiographical histories: Langway (2008), Dansgaard (2005).
35. Epstein et al. (1970).
36. Newell (1974); using
results of Johnsen et al. (1972).
37. Kukla and Kocí
(1972); see Schneider and Londer (1984), p. 53.
38. Broecker and van Donk (1970);
cf. Ericson and Wollin (1968), using foram temperatures.
39. Later revised to 780,000. Shackleton and Opdyke (1973), quote p. 40. They determined
temperatures by oxygen isotopes. Opdyke did the magnetic work. Cheer: John Imbrie, oral
history interview by Ron Doel, 1997, AIP; see Imbrie and Imbrie
(1979), p. 164.
40. For history and comments, see Imbrie (1982); Imbrie and Imbrie
41. Hays et al. (1976); for other
work, see Imbrie et al. (1975).
42. Evans and Freeland (1977).
43. Hays et al. (1976); Berger (1977); other data: Berger
(1978); see review, Berger (1988).
44. Kerr (1978).
45. Shift of emphasis: paraphrase of Imbrie (1982), p. 408; for example, see North and Coakley (1979); review: North et al. (1981), p. 107.
46. Imbrie et al. (1984). The
definitive "SPECMAP" chronology was published by Martinson
et. al. (1987) BACK
47. Pisias and Moore (1981);
Ruddiman et al. (1986).
48. Imbrie (1982), p. 411.
49. Quote: J.-R. Petit in Walker
(2000). For additional details see Broecker (2010), ch. 1.
50. Lorius et al.(1985);
Barnola et al.(1987); Genthon et al. (1987).
51. Stauffer et al. (1988).
52. E.g., Pisias and Shackleton
(1984); "The existence of the 100-kyr [kiloyear] cycle and the synchronism
between Northern and Southern Hemisphere climates may have their origin
in the large glacial-interglacial CO2 changes." Genthon
et al. (1987), p. 414. BACK
53. Calculations: e.g., Berger (1988),
p. 649; see Falkowski et al. (2000); Berger and Loutre (2002) discusses a long interglacial.
The new Antarctic "Dome C" record of climate went back 750,000
years. EPICA community members
(2004). On the drilling see Flannery (2005),
p. 58. See also reports in Science (November 25, 2005): 1285-87,
1313-21. Tzedakis et al. (2012). BACK
53a. Ruddiman (2005, 2010); Ruddiman
(2006). Evidence against the hypothesis was advanced by Elsig et al. (2009). BACK
53b. Lorius et
al. (1990); Hoffert and Covey (1992); Annan and Hargreaves (2006) ; Skinner (2012); PALAEOSENS (2012). BACK
54. A review of ice sheets (which added yet another factor,
permafrost thawing beneath a sheet) is Clark et al. (1999).
55. Winograd et al. (1992), p.
255; Ludwig et al. (1992). Pacific currents: Herbert et al. (2001). See also Burns et al. (2011). Moseley et al. (2016), "enigma" p. 168. BACK
56."Too complicated:" Karner and Muller (2000).
57. Reviewer: Crowley (2002),
p. 1474; see also Wunsch (2004); Drysdale et al. (2009). An attempt at a comprehensive theory: Chang et al. (2009).
57a. Abe-Ouchi et al. (2013), with historical references to earlier versions of this model. BACK
(2000); changes of CO2 preceding changes in
ice sheet volume were reported in Shackleton and Pisias (1985). The feedback is mentioned
or assumed in many of the references cited above, e.g., see quote
by Genthon; Lorius et al. (1990) remarked that Greenland drilling then underway "should allow a better determination of the relative timing (phase lag) of climate and greenhouse forcing" (p. 145), but the wider community had thought little about a lag. A summary
noting some of the complexities is Severinghaus
(2009). Confirmation: Shakun et al. (2012). BACK
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