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Rising Seas
A big enough rise of global temperatures would eventually melt the
world's glaciers, and indeed a retreat of mountain glaciers since the
19th century was apparent in some regions. That would release enough water
to raise the sea level a bit. Worse, beginning in the 1960s, several glacier
experts warned that part of the Antarctic ice sheet seemed unstable. If
the huge mass slid into the ocean, the rise of sea level would wreak great
harm, perhaps within the next century or two. While that seemed unlikely
(although not impossible), by the 1980s scientists realized that global
warming would probably raise sea level enough to damage populous coastal
regions.
Glaciologists, the scientists who study how ice behaves in
seriously large quantities, have a special interest in floods. They
even have their own word, jökulhlaup (from Icelandic), to describe
the spectacular outbursts when water builds up behind a glacier and
then breaks loose. An example was the 1922 jökulhlaup in Iceland.
Some seven cubic kilometers of water, melted by a volcano under a
glacier, had rushed out in a few days. Still grander, almost unimaginably
grand, were floods that had swept across Oregon toward the end of
the last ice age when a vast lake dammed behind a glacier broke loose.
In the 1940s, after decades of arguing, geologists admitted that high
ridges in the Oregon "scablands" were the equivalent of the little
ripples one sees in mud on a streambed, magnified ten thousand times.
By the 1950s, glaciologists were accustomed to thinking about catastrophic
regional floods. |
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Also within their purview was flooding on
a far grander, but much slower, scale. Since the heroic polar explorations
of the late 19th century the world had known that great volumes of
water are locked up in ice sheets. If there were substantial melting
of the Greenland ice cap, and especially of the titanic volume of
ice that buries Antarctica, the water released would raise the oceans
in a tide that crept higher and higher for centuries. It had happened
before geologists identified beaches far above the present
sea level, cut by waves in warmer periods when the Earth was entirely
free of ice. In the last warm interglacial period, some 130,000 years
ago, even though most of Antarctica had remained ice-covered, the
sea level had been about six meters (20 feet) higher than at present.
The next time that happened, sea water would swamp coastal regions
where a good fraction of the world's population now lived. All this
became familiar to anyone who followed scientific discussions of global
warming. |
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Up to the 1960s, if
there was any global warming, scientists expected it to steal in over
thousands of years, so the threat of flooding lay in a comfortably
vague and remote future. To be sure, a few scientists had begun to
imagine more abrupt change if the melting of the ice itself brought
on conditions that accelerated the warming. Transitions between glacial
and warm climates and back again might come in a matter
of mere centuries. As one example, in 1947 the New York Times
quoted a prominent Swedish geophysicist, Hans Ahlmann, who suggested
that a global warming might be underway that could eventually bring
a "catastrophic" rise of sea level as glaciers melted. "Peoples living
in lowlands along the shores would be inundated," he explained, calling
on international agencies to undertake studies as an urgent task.
Most scientists, however, expected that within the foreseeable future,
the main effect of any global warming on ice would be to shrink the
icepack on the Arctic Ocean. Since that ice was floating, it could
melt entirely away without changing sea level at all.(1)
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Glaciers on land could affect sea level, and they were notoriously
sensitive to climate. Advances and retreats of glaciers in the Alps
in particular had been conspicuous for generations, reacting to small
changes not just in temperature but also in the amount of snowfall.(2*) In 1962, John Hollin opened
up speculation about how relatively small climate changes might also
affect ice in Antarctica. He argued that great volumes of ice there,
piled up kilometers high and pushing slowly toward the ocean, were
held in place by their fringes. These edge sheets were pinned at the
marginal "grounding line" where they rested on the ocean floor. A
rise of sea level could float an ice sheet up off the floor, releasing
the entire stupendous mass behind it to flow more rapidly into
the sea.(3*) |
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The idea was picked
up by Alex Wilson, who pointed to the spectacle of a "surge." Glaciologists
had long been fascinated by the way a mountain glacier might suddenly
give up its usual slow creeping, to race forward at a rate of hundreds
of meters a day. They figured this happened when the pressure at the
bottom melted ice so that water lubricated the flow. As the ice began
to move, friction melted more water and the flow accelerated. Could
the ice in Antarctica become unstable in this fashion? If so, the
consequences sketched by Wilson would be appalling. As the ice surged
into the sea, the world's sea-coasts would flood. And that would not
be the worst of humanity's problems. Immense sheets of ice would float
across the southern oceans, cooling the world by reflecting sunlight.
It could bring a new ice age.(4*)
Hollin joined in with observations of deposits in England that recorded
past sea levels, showing rapid rises of as much as ten meters. It
could happen any time, he thought, perhaps in mere decades
or even faster if the sea-level change set off tsunamis. He pointed
to unusual features that suggested an abrupt disaster, such as "the
curiously intact remains of large mammals" buried whole.(5)
Few scientists gave much credence to any of these speculations. The
ice that covered most of Antarctica, in places more than four kilometers
thick, seemed firmly grounded on the continent's bedrock. |
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Serious concern began
to spread when John Mercer, a glaciologist at Ohio State University,
drew attention to the West Antarctic Ice Sheet. This is a smaller
(but still enormous) mass of ice, separated by a mountain range from
the bulk of the continent. Mercer argued that this mass was held back,
in an especially delicate balance, by the ice shelves floating at
its rim. The shelves might disintegrate under a slight warming. Just
so, he suggested, a collapse of Arctic Ocean ice sheets might have
caused the more local, but remarkably sudden, cooling of the North
Atlantic around 11,000 years ago that other scientists had identified.
A West Antarctic Ice Sheet collapse could be very rapid, Mercer said.
The sea level would not rise as far as if all of Antarctica surged,
but it would be bad enough up to five meters (16 feet). Much
of the world's population lives near the shore. Such a rise would
displace perhaps two billion people and force the abandonment of many
great cities. Mercer thought it could happen within the next 40 years.(6*) |
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The West Antarctic
Ice Sheet (WAIS)
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The problem, one expert complained, "could be argued indefinitely if it is not
quantized."(7) In fact glaciologists had
been working for decades on ways to calculate numbers for the flow of ice masses. In the 1970s
they made rapid progress in formulating abstract mathematical models and putting the powerful
new computers to work. The calculations, with many approximations, suggested that the West
Antarctic Ice Sheet was indeed unstable. Apparently the ice shelf that held it back could break up
with surprising ease, and the whole mass might begin sliding forward. The idea was backed up
by data from adventurous survey expeditions that traversed parts of Antarctica during the
1957-58 International Geophysical Year and on many later occasions. One scientist who made a
landmark calculation, Johannes Weertman, concluded that it was "entirely possible" that the
West Antarctic Ice Sheet was already now starting its surge.(8) Most climate specialists and geologists felt that the ice sheet models
were highly speculative. It seemed scarcely possible that the West Antarctic Ice Sheet could
disintegrate in less than a few centuries. But a surge that dumped a fifth of a continent of ice into
the oceans over the next few centuries would be no small thing, and they could not rule it out.
The picture fitted with a new feeling that was emerging in the climate community, a feeling that
the climate system in general was unstable or even radically chaotic.
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Concern sharpened in
1975 when Cesare Emiliani at the University of Miami reported measuring
deep-sea cores that showed a shockingly rapid rise of sea level
a rate of meters per decade around 11,600 years ago. (He remarked
that this was exactly the time Plato had given for the fall of Atlantis!)
Emiliani thought the cause of the flooding might not have been an
Antarctic surge, but water rapidly released from enormous lakes that
had been penned up behind the North American ice sheet, a titanic
jökulhlaup. In places like Florida where the land sloped gently
into the ocean, he wrote, "the sea would have been seen to advance
inland 300 feet in... a single summer."(9) Other areas at risk included the Nile
Delta and the Netherlands. Science journalists made sure that the
more spectacular warnings reached a broad public.(10)
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Meanwhile radar surveys from airplanes showed that the ice of West
Antarctica moved toward the sea not as a single sheet but through
a set of enormous ice streams. Terence J. Hughes (who started out
studying metallurgy but moved on to a different sort of solid material)
and other glaciologists developed increasingly elaborate models of
ice sheet dynamics.(11) They
showed how a slight shift in conditions could prompt an ice shelf
to break up into flotillas of icebergs. Looking over the new data
and theories, Mercer worried that most climate experts still assumed
that ice sheet changes would take many centuries. In 1978, he published
an article contending that thanks to humanity's use of fossil fuels,
"a major disaster... may be imminent or in progress." Mercer admitted
that the computer models were loaded with uncertainties, but "there
is, at present, no way of knowing whether they err on the optimistic
or the pessimistic side."(12)
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Hollin, Mercer, and Hughes had a chance to argue their case to a group of experts at a meeting
convened in April 1979 in Annapolis, Maryland. One participant noted in his diary that their
arguments convinced him that the deglaciation of West Antarctica was "a plausible hypothesis."
The majority felt that this was "not a cause for immediate alarm however. We are talking about
centuries."(13) In a published review, a
trio of experts laid out arguments explaining why the collapse of an ice sheet would probably
take several centuries to run its course. Yet they admitted that "little is known about the glaciers,"
and a 5-meter rise in sea level could possibly happen within a century. "Mercer's warning," they
concluded, "cannot be dismissed lightly."(14)
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That continued to be the most common view through the 1980s. Most
studies found that an ice sheet collapse was likely to take centuries
rather than decades. But experts knew too little about the behavior
of Antarctica's mammoth ice rivers to agree on any firm conclusion.
Field glaciologists, a small but hardy group, measured one or another
ice sheet as best they could at a few scattered locations. They found
ice streams moving at speeds of hundreds of meters a year, far faster
than ordinary mountain glaciers. Meanwhile, their mathematically-minded
colleagues back home constructed simplified models for the flow.(15) Some studies foresaw the possibility of a sea-level rise
of two or three meters (6-10 feet) by 2100, but most found this unlikely
so soon. In particular, for a 1983 National Academy of Sciences report,
the dean of oceanographers, Roger Revelle, estimated that within the
next hundred years the sea level would probably rise some 70 cm (about
two feet). That would be harmful but not catastrophic. He did worry,
however, about an Antarctic collapse later on.(16*) |
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Some rise of sea level in the coming century seemed not just possible,
but nearly certain. The oceans had already risen 10 or 20 centimeters
in the 20th century, about ten times as fast as the average sea-level
rise in previous millennia. Just where all the water had come from
remained uncertain. As one example, it was not until the 1990s that
experts realized that significant volumes of water were engaged by
human activities like irrigation and building reservoirs, and they
could not say whether the net result of such activities was to take
water from the oceans or to put more in.(17)
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One contribution to the sea-level rise was entirely clear. Water expands when heated. The
consequences may seem obvious, but amid all the talk of melting glaciers, for decades nobody
seems to have given a thought to other simple effects. Finally in 1982 two groups separately
calculated that the global warming observed since the mid-19th century must have raised the sea
level significantly by plain thermal expansion of the upper ocean layers. But a thermal expansion
could not account for all of the observed rise. The scientists figured the rest came from melting
glaciers (most of the world's small mountain glaciers were in fact shrinking).(18)
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The rising waters might help the West Antarctic Ice Sheet float
off its moorings and slowly break up. Even if that never happened,
there would still be problems. Scientists warned that tides would
probably mount a half meter or even a meter and a half higher by the
end of the next century, bringing severe harm to coastal regions.
Beaches would erode back hundreds of feet. Salt water would advance
into fragile estuaries. Entire populations would flee from storm surges.(19*)
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While the calculations of thermal expansion were straightforward,
the actual sea level rise would depend on a much tougher problem
what would happen to the ice sheets of Greenland and Antarctica? So
long as they did not surge and disintegrate, global warming would
not necessarily make them dwindle. A warmer atmosphere would hold
and transport more water vapor, so it would drop more snow. Thus the
polar ice sheets might actually grow thicker, withdrawing water from
the oceans. The future sea level depended crucially on just what happened
to glaciers and ice sheets, one pair of experts concluded, and predicting
that would be "a daunting task."(20)
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To sketch out an answer to the great question of ice-sheet collapse,
since the early 1980s scientists had bundled up in parkas and gone
out onto the windswept wastes of Antarctica. Their difficult goal
was to measure the motions of the immense slow ice currents, using
radar pulses, seismic measurements, and boreholes to study how ice
moved over the rock beneath. One example was a scientist who had been
skeptical of surge models he recalled that he "felt the whole
thing was like a house of cards" but who changed his mind when
he discovered that a kilometer-thick Antarctic ice stream rested not
on bedrock but on a layer of slippery mud.Another unsettling discovery
was that in recent centuries some of the great ice streams had stopped
or started moving, for no clear reason.(21)
Far more such data would be needed to bring a definitive answer. The
dynamics of ice sheets and the streams that fed them turned out to
be, like most things geophysical, a complicated snarl of influences.
Experts could not even agree on whether the West Antarctic Ice Sheet
had disintegrated during previous warm epochs over the past few million
years. The past sea level rises might have come from Greenland ice,
or from something else entirely. But according to evidence developed
in the 1990s, during a dramatic episode at the end of the last ice
age, something had once raised the sea level 16 meters within
three centuries. The rate of rise might have reached two feet per
decade. Antarctica was the most likely source of all that water. The
rush of new data fed what one observer called "polite but emotional
debate" among experts about the future possibilities.(22*)
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Meanwhile satellite pictures revealed that some of the smaller
floating ice shelves poking out from Antarctica were rapidly disintegrating, perhaps because of
rising temperatures.(23) Back in 1978,
Mercer had called for keeping an eye on just these ice shelves, contending that their breakup
would be "one of the warning signs that a dangerous warming trend is under way in
Antarctica."(24) In fact it was not clear
whether the changes had anything to say about the possibility of a catastrophic ice-sheet collapse.
They might have been a type of normal, regional event, which just had not been noticed before
the age of intensive global monitoring. Yet the public's concern about global warming was
reinforced from time to time when satellite images showed tabular icebergs bigger than cities
floating off.
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Specialists in glacier flow worked up increasingly elaborate
ice-sheet models. Entirely aside from the question of Antarctic surging, these models might be
useful in explaining the ice ages. It seemed increasingly likely that the reason ice sheets came and
went in cycles of around 100,000 years had something to do with the length of time needed for a
continent of ice to form and flow and melt. Nothing else on Earth seemed to change on the right
timescale.
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The models failed to answer the question of how fast a major ice
sheet could surge into the ocean. The improved models did show, reassuringly,
that there was no plausible way for a large mass of Antarctic ice
to collapse altogether during the 21st century. Probably the West
Antarctic Ice Sheet, if it diminished at all, would discharge its
burden only slowly over the following centuries, not placing too heavy
a burden on human society. Yet scientists could not altogether rule
out the possibility of a shocking surprise in some future generation.
The West Antarctic Ice Sheet remained what one expert had called it
a quarter-century earlier "glaciology's grand unsolved problem."(25) |
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Scientists were still less able to answer the question of whether climate change was gradually
melting the rest of the world's glaciers and ice caps, or instead was adding snow to them. In
"those huge areas where little or no information is available," an expert explained, "almost
anything might be happening."(26)
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Certainly many things were happening. Scientists
had begun to turn up all sorts of changes, now that they were giving
close attention to ice movements, thanks to views from satellites
and airplane overflights along with increasingly precise measurements
on the ice itself. "Perhaps the most important finding of the past
20 years," a glaciologist reported in 2002, "has been the rapidity
with which substantial changes can occur on polar ice sheets." Antarctic
ice streams the size of a province suddenly accelerated or came to
a halt, entire ice sheets rapidly thinned or broke up completely,
grounding lines under ice sheets were altered by vigorous melting
of the sheet's underside. [Starting around 2000, a few studies raised
the additional possibility that the Greenland ice cap, contrary to
what most scientists had figured, might not be entirely stable over
the next few centuries. Under some scenarios, increased summer temperatures
would cause more water to drain to the bottom of the ice and lubricate
it. The ice cap would thin around the edges as the flow of its huge
glaciers accelerated.] The new observations made it seem only too
possible that in coming centuries, global warming would release huge
volumes of ice from Antarctica, Greenland, or both, raising the sea
level by many meters.(27*)
Glaciologists could only speculate about the likelihood and timing
of such a misfortune. |
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At least one thing was certain. If temperatures
climbed a few degrees, as most climate scientists now considered likely,
the sea level would rise simply because water expands when heated.
This is almost the only thing about global change that can be calculated
directly from basic physics. The additional effects of glacier melting
are highly uncertain (scientists were still arguing over how much
of the 20th century’s sea level rise was due to heat expansion
and how much to ice melting). The rough best guess for the total rise
in the 21st century was perhaps half a meter.(28*) While such a rise will not
be a world disaster, by the late 21st century it will bring significant
everyday problems, and occasional storm-surge catastrophes, to populous
coastal areas from New Orleans to Bangladesh. More likely than not,
low-lying areas where tens of millions of people live will be obliterated.
Entire island nations are at risk. Then it will get worse. Even if
humanity controls greenhouse emissions enough to halt global warming,
the heat already in the air will work its way gradually deeper into
the oceans, so the tides will continue to creep higher, century after
century. |
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RELATED:
Home
Rapid Climate Change
The Public and Climate
1. Gladwin Hill, "Warming Arctic climate melting
glaciers faster, raising ocean level, scientist says," New York Times,
May 30, 1947. Ewing and Donn (1956); Budyko (1962).
BACK
2. Glaciers as "sensitive indicators of climate" are stressed in the
pioneering theoretical treatment of surges, Nye (1960).
BACK
3. "The chief conclusion of this paper is that the greatest glacial
fluctuations in Antarctica were produced by changes in sea-level." The paper was motivated by
the idea that the timing of Antarctic glacial movements was set by sea-level changes that
reflected Northern Hemisphere glaciation. Hollin (1962), p. 174.
BACK
4. Wilson (1964); Wilson (1966); Wilson (1969);
Wilson's starting-point was the suggestion that the center of Antarctica was at the pressure
melting point, see Robin (1962), p. 141, who adds that "one
would not expect the ice to surge over a large part of Antarctica at one time"; the role of
frictional heat in ice-sheet instability was pointed out back in 1961 (in partial support of
Ewing-Donn theory), drawing on earlier work by G. Bodvarsson, by Weertman (1961).
BACK
5. Hollin (1965), quote p. 15.
BACK
6. Mercer's basic argument was that "fringing ice shelves... will
rapidly disintegrate by calving if the average temperature of the warmest month rises above
freezing point," Mercer and Emiliani (1970); see Mercer (1968) ; North Atlantic: Mercer (1969); meanwhile a suggestion about a more gradual
disappearance of the Greenland ice cap was advanced by Emiliani
(1969); earlier, Robin and Adie (1964), said that
catastrophic deglaciation of West Antarctica was "unlikely, but not necessarily impossible," p.
117.
BACK
7. W.J. Campbell in discussion of Wilson (1969), p. 915.
BACK
8. Data were analyzed by Hughes
(1973); Weertman (1974), "entirely possible," p. 3; the
classic theory was Thomas (1973); and Thomas (1973); Flohn (1974)
gave a more general model; on ice modeling, see also Hughes
(1977).
BACK
9. Emiliani et al. (1975);
see Science (9/24/76): 1268 for criticism. Quote: Emiliani
(1980), p. 87. BACK
10. E.g., Calder (1975); note
also the semi-popular article: Emiliani (1980).
BACK
11. Hughes (1977); Hughes et al. (1977); Thomas and
Bentley (1978).
BACK
12. Mercer (1978), quotes pp.
321, 325.
BACK
13. Elliott (1977-89), vol. 1,
4/8/79.
BACK
14. Thomas et al. (1979), p.
355.
BACK
15. E.g., Herterich (1987).
BACK
16. Revelle (1983); similarly
Thomas et al. (1979); Bentley
(1980) saw a possible ice sheet collapse in the next 500 years; but Bentley (1982) said melting could take thousands of years; this
was disputed by Hughes (1982); Hollin (1980) tried to demonstrate an East Antarctic ice sheet surge
about 95,000 years ago; for predictions of meter-scale rises, see Jones
and Henderson-Sellers (1990), pp. 10-11, 15; a skeptic: Van
der Veen (1985); Van der Veen (1988).
BACK
17. IPCC (2001), pp. 657-58.
BACK
18. Etkins and Epstein (1982);
Gornitz et al. (1982).
BACK
19. "Most workers" project 0.5-1.5m rise in next 50-100 years if
warming continues, according to Schneider (1989), p. 777; he
cites i.a. Meier et al. (1985); this range was taken as plausible
for 2100 in National Research Council (1987); but only a few
cm rise by 2025 according to the most cited of these papers, Wigley
and Raper (1987).
BACK
20. Wigley and Raper (1987),
p. 131.
BACK
21. Barclay Kamb quoted by Walker
(1999); the slippage was predicted by Blankenship
et al. (1986). For this and other history see Bindschadler
and Bentley (2002). BACK
22. Later work confirmed Antarctic ice as the source:
Clark et al. (2002); "debate": W. Sullivan, New York
Times, May 2, 1995, p. C4. 16m rise: Bard
et al. (1990); Hanebuth et al. (2000). BACK
23. Doake and Vaughan
(1991); Rott et al. (1996).
BACK
24. Mercer (1978), p. 325.
BACK
25. Oppenheimer (1998); IPCC (2001), pp. 678-79; "Unsolved Problem" was the title of Weertman (1976); repeated in Van
der Veen and Oerlemans (1987), p. 14.
BACK
26. Thomas (1993), p. 398.
BACK
27. Rignot and Thomas (2002),
p. 1505. Subsequent work pointing in the same direction included De
Angelis and Skvarca (2003), who found that Antarctic grounded ice
surged after an ice shelf breakup, and Bindschadler
et al.(2003), who reported that a major West Antarctic ice stream
started and stopped flowing as the tide went up and down. Concern about
Greenland glacier surging was spurred by Krabill
et al. (1999); see review by Schiermeier
(2004). BACK
28. Following measurements of total heat absorbed
by the oceans by Levitus et al. (2000) and
Levitus et al. (2001), "20th-century sea
level remains an enigma — we do not know whether warming or melting
was dominant, and the budget is far from closed," according to Munk
(2003). IPCC (2001), pp. 641-42, estimates
0.1 to 0.4 m rise from thermal expansion with a total anywhere between
0.1 and 0.88 m. The problem will be compounded in many river deltas (Nile,
Ganges, Mississippi, etc.) by a half meter or so of subsidence, thanks
to dams that impound sediment and the withdrawal of water from aquifers.
BACK
copyright
© 2003-2004 Spencer Weart & American Institute of Physics
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