|
Once scientists asked the question and it was not an obvious question the answer
was obvious: where are the main ingredients of climate? Not in the Earth's tenuous atmosphere,
but in the oceans. The top few meters alone store as much heat energy as the entire atmosphere,
and the oceans average 3.7 kilometers deep. Most of the world's water is there too, of course, and
even most of the gases, dissolved in the water.
|
- LINKS -
|
It was during the 19th
century that the significance of these simple facts became clear.
The first thing scientists recognized was how winds passing over the
oceans brought moisture and warmth to neighboring lands. Those who
sought explanations for climate change included sea changes in their
long list of possible causes, and some made this the linchpin. For
example, in 1897 a geologist pointed out that a deviation of the Gulf
Stream, due perhaps to a gradual raising of land, might set off a
glacial epoch.(1*) |
Full discussion in
<=Simple models
<=Climate
cycles
|
Currents like the Gulf Stream were only minor actors in the story.
A far grander feature of the Earth’s surface heat circulation
was recognized in the 19th century when scientists tracked down the
fact that water hauled up from the deeps is nearly freezing, everywhere
in the world (there’s a story that the inspiration for these
studies came from an old practice of ships in the tropics, chilling
bottles of wine by dunking them overboard). This must be water that
had sunk in Arctic regions and slowly flowed equatorwards along the
bottom. It was a reasonable idea, since water would be expected to
sink where the winds made it colder and thus denser. |
|
On the other hand, the warm tropical seas would evaporate moisture, which would eventually
come down as rain and snow farther north; this would leave the equatorial waters more salty and
therefore denser. So wouldn't ocean waters sink in the tropics instead? The question became part
of a long-running debate over what mainly drove ocean circulation: was it differences in density,
whether due to cold or salt, or was it the steady push of winds?
| |
Around the turn of the century, the versatile American scientist T.C. Chamberlin took up the
question as part of his general program of studying causes of climate variations. He estimated
that "the battle between temperature and salinity is a close one... no profound change is necessary
to turn the balance." Perhaps in earlier geological eras when the poles had been warmer, salty
ocean waters had plunged in the tropics and come up near the poles. This reversal of the present
circulation, he speculated, could have helped maintain the uniform global warmth seen in the
distant past.(2) (Link from below)
| |
Chamberlin and a student of his also drew
attention to the crucial role of the ocean in regulating the composition
of the atmosphere as one example, there were "eighteen potential
atmospheres of carbon dioxide in the ocean." They noted that a warmer
ocean would tend to evaporate more of its carbon dioxide gas ( CO2)
and also water vapor into the air, whereas a colder ocean would tend
to absorb both gases. These were gases that helped keep the Earth
warm through the greenhouse effect. So it appeared that if the planet
began to warm up or cool down, the oceans might accelerate the tendency
by releasing or taking up the gases. Chamberlin and his student recognized,
however, that it was no simple matter to calculate just how the oceans
might absorb and emit CO2. It depended not
only on temperature and concentration but on complex chains of chemical
reactions.(3) |
<=>Simple models
|
Through the first half of the 20th century, scientists ignored the intractable
chemical complexities, which hardly seemed worth trying to unravel.
Most people assumed as a general principle that over the time-spans
relevant to human civilization, natural systems automatically regulated
the amount of water vapor and other gases in the atmosphere. In particular,
if burning fossil fuels added more CO2, then
as some of the gas dissolved in sea water it would modify the concentration
of carbonic acid there, in just such a way that the oceans could absorb
all the extra gas.(4) The view was fixed in a widely read statement by Alfred J.
Lotka. Since the oceans hold many times as much CO2
as the atmosphere, he explained, it seemed obvious that they must
eventually swallow up 95% of any new gas, regardless of the details
of the chemistry. The argument was roughly correct in principle (there
are about 50 carbon atoms dissolved in the oceans for every one in
the atmosphere). But Lotka, in tune with the common assumption that
a "balance of nature" kept everything stable, had failed to wonder
whether the oceans' absorption could keep up with a really rapid production
of CO2. Scientists of the time assumed without
much thought that any change in the atmospheric concentration of any
gas could happen only over a geological timescale, hundreds of thousands
if not millions of years.(5) |
=>Simple models
=>CO2 greenhouse
=>Revelle's result
|
The circulation of the oceans was likewise
pictured as a placid equilibrium. One pioneer later called this the
"underlying theology" of a perpetual steady state of circulation.
That was what scientists observed, if only because measurements at
sea were few and difficult. Oceanographers traced currents simply
by throwing bottles into the ocean. It took them a century to work
out the general pattern. "The first law of ocean research," a leader
of the field recalled, "was to never waste your assets by occupying
the same station twice! And when this law was violated and the results
differed, the differences could be attributed to equipment malfunctioning."
The inevitable consequence, he remarked, was "a climatology steady
in time."(6) |
=>Climatologists
|
The classic picture of steady-state circulation was laid out in
Harald Sverdrup's definitive textbook of 1942, drawing on the pathbreaking
expeditions of the German oceanographic vessel Meteor in
the 1920s. Sverdrup described, as one item in a list of many ocean
features, how cold, dense water sinks near Iceland and Greenland and
flows southward in the deeps. To complete the North Atlantic cycle,
warm water from the tropics drifts slowly northward near the surface.
Winds presumably added a push to this heat-driven cycle, although
the effect of trade winds and the like was entirely uncertain. Sverdrup
did not remark that the immense volume of warm water drifting northward
might be significant for climate. Like all oceanographers of his time,
he gave most of his attention to rapid surface currents like the Gulf
Stream.(7*) |
|
Through the 1950s, few scientists found much reason or opportunity to study the slow
circulations in the depths. Oceanography was a poorly organized field of research. There were
only a few oceanographic institutes, jealously isolated from one another, each dominated by one
or several forceful personalities. The funds for research at sea were wholly inadequate to the vast
subject. The economics of shipping and fishing supported only studies of practical interest such
as surface currents, and little data had been gathered about anything else. The field as a whole
scarcely looked like solid science. Theories about ocean circulation had what one expert called "a
peculiarly dream-like quality."(8)
| |
Nobody could see a way to do much better. Samples pulled up from
thousands of feet down had allowed oceanographers to label the main
water masses by temperature and salt content. Thus they could see,
among much else, that the water that sank near Iceland had made its
way along the bottom as far as the South Pacific. Little more could
be said. It seemed impossible to actually measure the motion of these
enormous, sluggish slabs of water.(9) |
|
Oceanographers concentrated on surface winds and currents, far
easier to measure and far more important for seafarers than the slow
overall circulation. They had not settled the old debate over how
much of the general circulation was driven by the winds, and how much
by density changes related to temperature and salinity. Oceanographers
who attempted to build theoretical models of the circulation gave
their greatest attention to the winds, so meaningful to all who went
to sea. Besides, as one of them confessed, "the wind-driven models
were easier to formulate."(10)
Although the calculations were primitive, they gave a starting-point.
Bit by bit, important features of the ocean circulation were explained.
In particular, in the mid 1950s Henry Stommel threw light on some
puzzling old observations by calculating the way cold, salty water
could sink in only a few small northern regions and creep along the
bottom of the oceans.(11) |
|
Change in the Oceans (1950s-1960s)
TOP
OF PAGE |
|
The 1950s gave oceanography,
like many other fields of geophysics, a breakthrough in organization
and funding because of two institutions. First came the U.S. Navy's
Office of Naval Research, which naturally took great interest in every
aspect of the subject. The ONR liberally dispensed money for all sorts
of research projects, imposing some coordination on the isolated research
institutions. Second was the International Geophysical Year (IGY)
of 1957-58, which further expanded and strengthened ocean research
under the leadership of an international committee. Oceanography was
a central player in the IGY, for here as in no other field it was
undeniable that progress depended on genuine cooperation among nations,
setting aside their political rivalries. Aside from the advantages
of having many ships on the seas doing coordinated studies at the
same time, even a single survey vessel would be hamstrung without
access to foreign ports.(12) |
<=Government
<=>International
|
From the outset, the scientists who planned the IGY believed that
the role of the oceans in climate change was something they should gather data on, if only for the
benefit of future researchers. The point was explained magisterially by the influential
meteorologist Carl-Gustav Rossby. Considering how temperature was balanced against salt
density, he thought it "not unlikely" that the oceanic circulation "must undergo strong and
probably rather irregular, slow fluctuations." Thus over the course of a few centuries vast
amounts of heat might be buried in the oceans or emerge, perhaps greatly affecting the planet's
climate. In sum, putting climate change and oceanography together would generate important
questions and fine opportunities for research. Combining these disparate fields would not be
easy, however, and not only because it posed a severe intellectual challenge. Oceanographers and
meteorologists worked in separate communities; it would take them decades to establish regular
communication and cooperation.(13)
|
<=Climatologists
|
The first impressive result of the combined approach was published
by a meteorologist, Jerome Namias, in 1963. The previous winter had
been phenomenally cold and snowy in North America and Europe. Namias
argued plausibly that this was caused, paradoxically, by some unusually
warm surface water lingering in a region of the North Pacific. By
now oceanographers were taking enough measurements at sea to detect
such anomalies, and meteorologists were getting a feel for how a patch
of warm sea water might change wind patterns across the entire hemisphere.
The patch itself had apparently been maintained by an unusual wind
pattern that pushed tropical surface waters northward. It was a persuasive
example of what Namias called "complexly coupled mechanisms" leading
to a "self-perpetuating system." A change in prevailing winds changed
the ocean surface temperature, which in turn influenced the prevailing
winds, shifting the planet’s weather system at least for a while.(14*) |
|
Namias's work attracted
no special notice at the time. It was just one of a number of studies
that led to the recognition, in the 1970s, that there were ocean-atmosphere
feedback oscillations on a timescale of a few years to a few decades.
Most important was the "El Niño-Southern Oscillation"
(ENSO) in the mid-Pacific. The breakthough came in 1969 when Jacob
Bjerknes presented a persuasive hypothesis for interactions between
what had long been known as separate phenomena: the "El Niño"
surface temperature changes in the South Pacific Ocean, and the
"Southern Oscillation" of pressure changes in the atmosphere
above it.(14a) Bjerknes's study attracted intense interest once scientists
recognized that the El Niño events were connected with powerful
if temporary climate anomalies around the world, from torrential
rains in Peru to droughts in Kansas. (In the 1980s and 1990s, decade-scale
irregular cycles were also detected in the North Pacific, North
Atlantic, and Arctic Oceans. These oscillations provided some useful
insights for the study of longer-term climate change, and a good
workout for computer modelers, but they are not covered in this
essay.)
|
=>Solar variation
=>Models
(GCMs)
|
Another set of observations
meanwhile cast into doubt the old assumption that the world-ocean
maintained an unvarying circulation over many thousands of years.
In the mid 1950s, oceanographers managed to drill into the floor of
the deep sea, extracting long cores of ooze and clay sediment. Analysis
of fossil shells in the cores told much about the condition of the
sea water when the sediments had been laid down. Although interpretation
of the data was tricky, it seemed to say that the temperature could
make a giant jump in as little as a thousand years. Wallace (Wally)
Broecker, a young geochemist who had been studying climate changes
recorded in ancient lake levels and comparing them with ocean data,
began to ask whether "the present configuration is a transient one."
Could it change abruptly with serious consequences for climate? Broecker
saw no way to tell whether that could really happen, or ever had.
The available data on ocean waters could be interpreted well enough
using the traditional model of a torpid, steady-state
circulation.(15) |
<=Uses of shells
<=Rapid
change
|
A supplementary essay describes how scientists got Temperatures from Fossil Shells, a good example of the ingenious
oceanographic techniques and the controversies they could engender. | |
Broecker was well aware of a bold theory
about how ocean changes could turn ice ages on and off rapidly. In
1956 two of his senior colleagues, Maurice Ewing and William Donn,
had suggested that raising or lowering sea level could do the trick
by letting warm water from the Atlantic spill into the Arctic Ocean
or shutting it off. They were drawing on the old tradition of hand-waving
ideas about how climate might change in response to the opening or
closing of straits, which acted like "valves" controlling the ocean
currents that warmed or cooled a region.(16)
This tradition had imagined a gradual geological process, with currents
responding passively. But now a few people were ready to speculate,
if not in scientific articles than in comments to colleagues, about
a more sensitive ocean system. |
<=Simple
models |
In 1957 Columbus Iselin, director of the Woods Hole Oceanographic
Institution, shared with a journalist some of the talk in the air
at Woods Hole. It seemed possible, he said, that during warmer past
epochs the North Atlantic water had not been cold enough to sink,
so the oceans had stopped overturning. That might happen in future
if the greenhouse effect warmed the planet past some critical point.
At that point the ocean waters would not so readily absorb CO2
and carry it into the depths. Thus the level of the gas would climb
and greenhouse effect warming would accelerate. It was hard to predict
the outcome. If the Arctic Ocean became warm enough to lose its cover
of ice, so much moisture might evaporate and come down as snow that
it would trigger the formation of continental ice sheets. "Are we
making a tropical epoch...," Iselin wondered, or "starting another
ice age?"(17) |
|
Henry Stommel explored the idea of a radical shift more analytically. He
sketched a simple model of the oceans as tanks connected by pipes,
with circulation driven by differences of density due to both temperature
and salinity. Working through the equations turned up critical points.
At these points small change in conditions, even a temporary perturbation,
could provoke a "jump" between states. The system, Stommel noted demurely,
"is inherently fraught with possibilities for speculation about climatic
change."(18) Broecker took up the challenge, speculating that "the Earth
has two stable modes of operation of the ocean-atmosphere system,
glacial and interglacial." (Link from below) That would explain
a puzzle that came up in the mid 1960s from studies of deep-sea cores.
It seemed that slight variations in the planet’s orbit had somehow
set the timing for major glacial periods. The orbital variations made
only minor changes in the sunlight falling at a given point, and something
had to be amplifying the effect. Ocean circulation was a leading suspect.(19) At a conference held in Boulder, Colorado
in 1965, where climate specialists put together for the first time
a variety of evidence and ideas about how the climate system could
lurch into a new state, speculations about modes of ocean circulation
came to the fore. (Link from below) |
<=>Simple models
<=>Climate cycles
=>Rapid change
|
The circulation of the
world-ocean was better charted now, thanks to nuclear physics. Since
the 1950s important practical concerns had augmented the purely scientific
curiosity of oceanographers. Government officials hoped to bury radioactive
waste from nuclear reactors in the abyss. Meanwhile fallout from bomb
tests was already mixing into the Pacific Ocean, sparking an international
outcry and demands to know exactly where the poisons were going. All
these demands to study ocean circulation were fulfilled with a new
technique which likewise came from nuclear physics. The radioactive
isotope carbon-14 could now be measured in the CO2
dissolved in a volume of sea water. Pull up a canister filled with
water from the depths, and the isotope would tell you how many years
had passed since that water had been on the surface, absorbing the
gas from the atmosphere. A number of groups took up the challenge.
They were financed by agencies that oceanographers of the 1930s never
dreamed of, ranging from the International Geophysical Year to the
U.S. Atomic Energy Commission. |
<=Carbon dates
<=Revelle's
result
|
In the lead was a group
at Columbia University's Lamont Geological Observatory, established
by Maurice ("Doc") Ewing in 1947. Isolated amid woods overlooking
the Hudson River outside New York City, Lamont scientists were combining
geological interests with oceanography and the new radioactive and
geochemical techniques in a burst of creative research. The intense
interaction between oceanography and radiochemistry might seem surprising,
except that a good fraction of the institution's funding came from
government agencies concerned about the fate of fallout from nuclear
weapons tests. Some nine-tenths of Lamont's funding in its first quarter
century derived from military contracts. Not all the scientists were
well aware of this strong military connection, which was veiled in
secrecy, and they could pursue their research with little attention
to anything beyond the purely scientific implications.(20)
Yet the Lamont group was never far from Cold War concerns as they
painstakingly measured carbon isotopes in more than a hundred samples
drawn from waters around the world. |
<=>Government
<=>Revelle's result
|
When the group began this work in
1955, nobody could say whether the oceans took a hundred years to turn over or several
thousand, nor just what paths the circulation followed. The pattern of flow turned out to be
different from what Sverdrup had supposed. Tracking down the ages of various water masses
showed that water was moving northward across the surface of the Atlantic all the way up from
Antarctica. The return flow of cold water underneath went all the way into the middle of the
Pacific. Equally significant was the time scale, which turned out to be half a millennium or so (in
particular, the deep water of the North Atlantic had been down there an average of 650 years).(21) Other groups using carbon-14 data
agreed that on average the ocean waters took at least several hundred years to turn over. Would
that suffice to bury greenhouse gases as fast as humankind produced them? The question
prompted Roger Revelle at the Scripps Institution of Oceanography in California to take a close
look at the chemistry of
CO2 dissolved in sea water. He showed that the uptake was slow: it
would take many hundreds of years for the oceans to dispose of the extra gas we added to the
atmosphere.
|
=>CO2 greenhouse
<=Revelle's
result
|
The full story of the crucial discovery that the oceans cannot rapidly absorb
CO2 is given in a supplementary essay on Revelle's Discovery
| |
Mapping and Modeling the Circulation
(1970s)
TOP
OF PAGE |
|
As scientists came to see that the ocean circulation might be
crucial for climate change, not just over geological time but more
immediately, more of them paid attention to the problem. A particularly
stimulating idea came from Peter Weyl of Oregon State University
in the mid 1960s. He noticed that the moist trade winds that cross
the Isthmus of Panama and drop rain into the Pacific Ocean carry
fresh water out of the Atlantic, leaving behind saltier water. Weyl
built on this to develop a theory of the ice ages, involving the
way changes of saltiness might affect the formation of sea ice.
He did not publish his model until 1968, but he presented the rudiments
at the 1965 Boulder conference. The theory would scarcely have been
noticed among the many other speculative and idiosyncratic models
for climate change, except for a novel insight.
Weyl pointed out that if the North Atlantic around Iceland should
become less salty as might happen if melting ice sheets diluted
the upper ocean layer with fresh water the entire circulation
could lurch to a halt. Without the vast drift of tropical waters
northward, a new glacial period could begin. Others since Chamberlin
had speculated that the circulation might stop if global warming
somehow made northern surface waters less dense. Now explicit calculations
confirmed the idea. The circulation, which was coming to be called
the "thermohaline circulation," really was precariously balanced.(22*) (See above)
|
|
As the 1970s began, the picture of
large shifts in the ocean-atmosphere system, only hinted at in cores of deep-sea clay, began to get
support from studies of ice cores drilled from the Greenland ice cap. That helped stimulate yet
more models for the causes of the ice ages. For example, in 1974 Reginald Newell suggested
how oceanic ice sheets could help create "the two preferred modes" for the global movement of
heat. When sea ice spread widely (say, around Antarctica) it insulated the sea from the frigid air.
The water would no longer get cold enough to sink, and the ocean circulation would decrease. As
Newell admitted, all this was guesswork and needed much more study, including numerical
modeling.(23)
|
=>Simple models
<=Climate
cycles |
By this point scientists recognized that they would never understand
climate change at all until they knew how the oceans worked. "We may
find that the ocean plays a more important role than the atmosphere
in climatic change," a panel of experts remarked in 1975. They said
that should be "a major motivation for the accelerated development
of numerical models for the oceanic general circulation."(24)
Computer ocean models, however, were primitive compared with atmospheric
models. The climate models of the 1960s calculated the general circulation
of the atmosphere with, in place of oceans, a "swamp" a mere
motionless wet surface. Yet ocean currents were surely a main component
of the climate system. In 1969, the leading modeler Syukuro Manabe
used crude measurements by oceanographers to estimate that the currents
carried roughly as much heat from the tropics to the Arctic as the
general circulation of the atmosphere carried. It seemed that something
like half the motor of climate was simply absent from the models.(25) (In later decades, better data would show this to be an
exaggeration, but it was true that energy transfers through the oceans
were a crucial part of the climate system.) |
|
Two obstacles kept modelers from handling the oceans in the same
way as the atmosphere. First, while meteorologists measured the atmosphere
daily in thousands of places, oceanographers had only a scattering
of occasional data for the oceans. And second, while atmospheric models
could bypass many difficulties by using a simple equation or a single
number to stand in for a complex process like a storm, ocean models
could not use that trick. For in the seas, analogous processes lasted
months or decades, and had to be computed in full detail. Even the
fastest computers of the 1970s lacked the capacity to calculate central
features of the movements of energy in the ocean system. They could
not even handle something as fundamental and apparently simple as
the vertical transport of heat from one layer to the next. |
|
Direct observation showed that heat from the atmosphere was absorbed rather quickly
by the upper few dozen meters of sea water (the "mixed layer"), but
below that the heat penetrated much more slowly into the cold bulk
of the oceans. That bare information was enough for some simple models
of climate developed in the early 1970s. Modelers pointed out that
if anything added heat to the atmosphere, such as the increase of
greenhouse gases, much of it would be absorbed into the upper layer
of the oceans. While that was warming up, the world’s perception
of climate change would be delayed for a few decades. The first generation
of atmospheric general circulation models had entirely ignored that,
treating the oceans as simply a wet surface in equilibrium. "We may
not be given a warning until the CO2 loading
is such that an appreciable climate change is inevitable," a panel
of experts explained in 1979. "The equilibrium warming will eventually
occur; it will merely have been postponed."(26) Beyond such elementary effects, all was obscure. As one
senior oceanographer remarked, scientists had no understanding of
the physical processes that brought heat into the depths, but only
"a set of recipes." And even the recipes "may be completely wrong."(27)
|
=>Modern temp's
=>Government
=>Models (GCMs)
|
Another great unknown was the interaction between currents like the Gulf Stream and the giant
eddies that the currents spun off as they meandered. Evidence of these broad, sluggishly rotating
columns of water had turned up during a survey voyage across the North Atlantic in 1960. This
was confirmed by an international campaign carried out by six ships and two aircraft in the early
1970s another example of how studying ocean phenomena needed international
cooperation. The survey discovered eddies bigger than Belgium that plowed through the seas for
months. What oceanographers had supposed were static differences in the oceans between their
sparse measuring-points, had often actually been changes over time, not over space.
| |
The oceanographers had been vaguely aware that when meteorologists
built atmospheric models, they included the energy carried by wind
eddies as an important factor (physical oceanography, as one practitioner
remarked, was "to some extent a mirror of meteorology"). Yet it was
astounding to see what prodigious quantities of heat, salt, and kinetic
energy the ocean eddies carried. Indeed nearly all the energy in the
ocean system was in these middle-sized movements, not in the ocean
currents at all.(28*) |
|
Water is not like air, and computers that could handle
meteorological computations were far too slow to work through comparable models for this
swirling ocean "weather." As one ocean modeler complained in 1974, "Extensive research efforts
have not yet yielded much more than a greater appreciation of the difficulty of these
questions."(29) To get a handle on the
problem, oceanographers had to understand the oceans from top to bottom. But they had little
data on the depths the occasional expeditions, retrieving bottles of water here and there
from kilometers down, were like a few blind men trying to map a vast prairie. Oceanographers
liked to remark that we had better maps of the face of the Moon than of the deep sea. After all,
there was little economic incentive, nor much military interest either, in studying the colossal
slow movements of water, salts, and heat through the abyss.
|
<=>Models (GCMs)
|
The decades-long variations as currents and giant eddies sloshed about in ocean basins were on a
scale too great not only for computers, but for human lives. A significant ocean change took
longer than making a typical doctoral thesis, sometimes longer than an entire career. "This is bad
for morale," as one oceanographer remarked wryly, and an intellectual obstacle besides. How
long would it have taken meteorologists to develop ways to predict weather, if they saw only a
handful of storms and cold fronts pass through in a lifetime?(30)
| |
Stommel worried that oceanographers did not even know how they could start attacking the
problem with their current supply of ideas, techniques, and funds. He felt that researchers were
spending their time on "tractable side problems... skirting problems of the ocean circulation" as
too tough to handle. The only way forward, he said, would be a concerted group effort.(31) But it would not be easy to persuade
people that this would be worth their time and money. "Thinking about the climate is a relatively
new business for oceanographers," a science journalist reported in 1974, "and despite pressure
from their meteorological colleagues many believe that global monitoring and modeling of the
oceans... is simply beyond the present capacity of the field."(32)
| |
The scientists who made such complaints meant to spur
action, and action did get underway. The U.S. government and a few
other governments began to give oceanography more money and attention.
Already in 1968 the Glomar Challenger had put to sea to begin
a Deep Sea Drilling Project. Technologies for working on the ocean
floor at great depths had been developed extensively for commercial
purposes such as oil prospecting, and for Cold War missions including
the recovery of sunken submarines and lost nuclear weapons. Now these
technologies were put to use for scientific oceanography, including
some work related to climate. In particular, increasingly accurate
methods had been devised to analyze fossil shells as a "thermometer"
for past temperatures. The technique was put to use in the "CLIMAP"
project, which in 1976 produced maps of sea temperatures at the peak
of the last ice age, roughly 20,000 years ago. As expected, the oceans
had looked quite different then cooler overall, and probably
with "a more energetic circulation system."(33) But other studies, using different markers, found hints
that North Atlantic waters had sunk less readily during the last ice
age.(34) A number of features were hard to explain, challenging
computer modelers to reproduce them. |
<=Government
<=Uses
of shells
=>Models (GCMs)
=>Simple models
|
Another major project
that the U.S. government funded in the 1970s was GEOSECS (Geochemical
Ocean Sections Study), which studied the present ocean circulation.
Teams of researchers sampled sea water at many points, and not only
for natural carbon-14. Nuclear bomb tests in the late 1950s had spewed
radioactive carbon, tritium, and other debris into the atmosphere.
The fallout had landed on the surface of the oceans around the world
and was gradually being carried into the depths. Thanks to its radioactivity,
even the most minute traces could be detected. The bomb fallout "tracers"
gave enough information to map accurately, for the first time, all
the main features of ocean circulation. "Now that we have the GEOSECS
data," Broecker boasted, "what more can be said on this subject?"(35)
|
<=External input
<=Government
|
The improved grasp of ocean circulation came
just in time for a problem that was especially troubling oceanographers:
exactly how much CO2 were the oceans currently
absorbing? The GEOSECS data helped them win a long debate with other
scientists over the global balance of carbon, taking into account
how burning fossil fuels and the destruction of forests emitted gases.(36) Yet questions remained about just how the masses of water
moved about. Only full-scale computer modeling could give answers,
using the GEOSECS data as a reality check. Broecker admitted that
"At least a decade will pass before a realistic ocean model can be
developed."(37) |
=>Biosphere
|
Attempts to represent ocean circulation on computers had begun
in the late 1960s. In the lead was Kirk Bryan, a Woods Hole oceanographer
who had picked up computer modeling from the enthusiasts at the Massachusetts
Institute of Technology while working there for a Ph.D. in meteorology.
Joseph Smagorinsky’s atmosphere modeling group in Princeton
recruited Bryan to add an oceanographic dimension. Here Bryan and
a collaborator, Michael Cox, managed to build a numerical model for
a highly simplified ocean basin with five levels. Their computer produced
a map of numbers that looked roughly like the Atlantic Ocean's Gulf
Stream and equatorial flow. Bryan later recalled that it was not easy
to get such work published. For modeling "was looked at with deep
suspicion by many of the oceanographic colleagues as... premature."
Most oceanographers were still struggling to map out what was actually
in the oceans, and to understand basic processes (like the giant eddies)
which computers were nowhere near able to calculate.(38) |
|
Nevertheless Bryan pushed ahead, motivated,
as he put it, by "the pressing need for a more quantitative understanding
of climate." One example of the tricks he devised was to revise the
equations so they did not include vertical movements of the water
surface the mathematical equivalent of clamping a rigid lid
on the oceans. Bryan was pretending that the most obvious feature
of oceans, their surging waves, did not exist. That scarcely mattered
for the slow circulation, and it speeded up computations tremendously.
One major influence remained to account for. Winds helped drive the
ocean currents that moved heat from the tropics poleward, and the
movement of heat in turn was a main influence on climate. So in 1969
Bryan coupled his ocean basin model to Syukuro Manabe's model of atmospheric
circulation. The pair got a recognizable simulation of a slice of
climate (if not exactly our own planet's climate), and signs of a
thermohaline ocean circulation.(39) |
<=>Models (GCMs) |
Others now gravitated toward ocean modeling. A research program
that had once seemed "a lonely frontier like a camp of the Lewis and
Clark Expedition," as an ocean modeler recalled in 1975, took on "more
of the character of a Colorado gold camp." One reason was breathtaking
advances in computer power. Equally important was the extraordinary
improvement in oceanographic data, thanks to GEOSECS and other large-scale
ocean surveys. These gave for the first time a three-dimensional picture
of the actual oceans in motion a target the modelers could
aim at. They constructed plausible models for individual basins like
the North Atlantic and Indian Ocean. It was Bryan's work that "established
the paradigm," as another expert later remarked. In contrast to the
wide variety of approaches in atmospheric models, most ocean modelers
created "varietal forms" based on similar physical assumptions and
numerical methods.(40) |
|
Now that highly simplified systems had established
that the thing could work, modelers forged ahead with more realistic
geography. First to plausibly model the entire global ocean was Bryan's
collaborator Cox, using nine levels of ocean and a grid with boxes
measuring two degrees of latitude by two of longitude. The calculations
used up so much computer time that he could only follow the ocean
circulation a few years in its centuries-long progress, but overall
the simulated ocean moved somewhat like the real one. Coupling this
to a model global atmosphere gave results that had "some of the basic
features of the actual climate," as Manabe and Bryan quietly boasted.
Many unrealistic features remained. Modelers had a long way to go
before they could calculate ocean circulation well enough to furnish
accurate models of climate.(41*)
|
<=>Models (GCMs)
|
Unpleasant Surprises? (1980-1988)
TOP
OF PAGE |
|
Another handle on the problem was provided
by the Deep Sea Drilling Program (which was followed in 1985 by an
international Ocean Drilling Program using the American JOIDES
Resolution, converted from an oil-drilling ship). In expeditions
across the seven seas, year after year workers pulled up thick cylinders
of clay and ooze, totaling many kilometers. These were stored in "libraries"
which any scientist could exploit for climate studies alongside many
other topics. |
=>Climate cycles
|
Studies of fossil shells in the cores gave clues about ocean waters
in the past, with a striking conclusion. It now seemed beyond doubt
that there had been shifts in the North Atlantic particularly around
the end of the last ice age some 11,000 years ago a time geologists
on land had long known as the "Younger Dryas" climate shift. The entire
pattern of ocean circulation had evidently changed within a couple
of thousand years, or perhaps only a few hundred.(42*)
|
|
That resonated with studies by Willi Dansgaard,
Hans Oeschger, and others using cores drilled out of the Greenland
ice sheet in the early 1980s. Certain periods such as the Younger
Dryas had seen very abrupt cooling around the North Atlantic, episodes
so striking that they got a name of their own, the "Dansgaard-Oeschger
events." Meanwhile a study of changes in microscopic deep-sea fossil
species showed that the cooling had extended clear to the ocean floor.
Such studies using microbiology were not given much credence at the
time, however. A bit more convincing was a 1983 report, using the
geochemistry of isotopes in fossils, with complex evidence pointing
to "a dramatic change in ocean circulation" in the last glacial period.
The deep waters of the North Atlantic had apparently grown cold and
still. Scientists were being gradually pushed to think about dramatic
transitions in the circulation of the North Atlantic, or even the
entire world-ocean.(43*) |
<=Rapid change
|
Oeschger was particularly struck by a rapid
rise in the atmospheric concentration of CO2
at the end of the last ice age, which others had recently discovered
in ice and deep-sea cores. The vexing problem of how the gas got in
and out of the atmosphere had intrigued him ever since 1958, when
he had worked with Revelle's group at Scripps just as they were discovering
that greenhouse warming was plausible. Oeschger also understood that
a feedback that released more and more of the gas might accelerate
the end of an ice age. |
<=CO2
greenhouse |
The main reservoir of
CO2 was the oceans, so that was the first place to think about. In
1982 Broecker visited Oeschger's group in Bern, Switzerland, and explained current ideas about
the North Atlantic circulation. Broecker also shared an intriguing new idea: the ocean's uptake of
CO2 during an ice age depended on biochemical changes involving
the growth and death of plankton. Oeschger reflected that Broecker's biochemical mechanism
would take thousands of years to operate, too slow for the rapid changes found in ice cores.
Perhaps, he thought, there had been a transition in the ocean from "a relatively stagnant state" to
a state where more rapid mixing brought nutrients to the surface which changed the
biochemistry.
|
<=Biosphere
|
The stagnant state might have been caused (as some had earlier speculated) by fresh water
flowing in as continental ice sheets melted. That would have diluted the surface salt water until it
would not sink, halting the circulation. Many questions remained, Oeschger conceded. But major
circulation changes might well have been involved perhaps triggered by some little
perturbation.(44)
| |
Oeschger was one of the first to worry that a switch between ocean
circulation modes might be set off by the greenhouse gases that humanity
was adding to the atmosphere. But as a colleague recalled, "his early
warnings were often greeted with disbelief." Oeschger tried to find
collaborators to write a paper on circulation modes for submission
to a top scientific journal like Nature or Science,
but he met only skepticism and gave up the effort. Two colleagues
at Bern did publish a paper in Nature suggesting that "ocean
circulation changes were the essential cause" of the rapid CO2
variations seen in ice cores, giving Oeschger credit for the idea,
but like Broecker they concentrated on biochemical changes rather
than the circulation as such. Oeschger continued to bring the idea
up at scientific meetings. Broecker heard him, and his interest was
stimulated.(45) |
|
As we have seen, Broecker had been thinking for decades about possible ocean instabilities. (See above) The reports of big,
rapid
CO2 variations in Greenland ice cores stimulated him to put this
interest into conjunction with his oceanographic interests, since nothing but a major change in
the oceans could cause such a swift and global shift in the atmosphere. In fact, scientists later
realized that the rapid variations seen in the ice cores had been misinterpreted. They did not
reflect changes in atmospheric
CO2, but only changes in the ice's acidity due to dust layers
(something had indeed changed swiftly, but not necessarily the
CO2 level). No matter: the error had served a good purpose, pushing
Broecker to a novel and momentous calculation. Broecker recalled that one day as he sat in Bern,
listening to a lecture by Oeschger describing the abrupt variations in his data, "an idea hit my
brain.... As quick as that, my studies in oceanography and paleoclimatology merged."(46)
| |
In 1985, Broecker and two colleagues published a paper in Nature titled, "Does the
Ocean-atmosphere System Have More than One Stable Mode of Operation?" Crediting Oeschger
as the first to suggest that the apparent
CO2 changes in Greenland ice cores represented a jump between
"two modes of ocean-atmosphere-biosphere-cryosphere operation," the paper continued that "it is
tempting to speculate" that Oeschger's two modes corresponded to different states of the North
Atlantic circulation.
| |
Broecker and his collaborators now identified
the key. It was what he later described as a "great conveyor
belt" of sea water carrying heat northward. Although the GEOSECS
survey of radioactive tracers had laid out the gross properties of
the circulation a decade earlier, it was only now, as Broecker and
others worked through the numbers in enough detail to make crude computational
models, that they fully grasped what was happening. They saw that
the vast mass of water that gradually creeps northward near the surface
of the Atlantic is as important in carrying heat as the familiar and
visible Gulf Stream. "It was an easy calculation," recalled Broecker,
"and I was astounded by the amount of heat that it had." The energy
carried to the neighborhood of Iceland was "staggering," Broecker
explained nearly a third as much as the Sun sheds upon the
entire North Atlantic. If something shut down the conveyor belt, climate
would change across much of the Northern Hemisphere.(47) |
= Milestone
|
In one sense this was no discovery, but
only an extension of an idea that could be traced back to Chamberlin
at the start of the century. (See above) Few scientific "discoveries" are wholly new
ideas. An idea becomes a discovery when it begins to look real.
Broecker made that happen by providing solid numbers and plausible
mechanisms. Chamberlin had speculated that the circulation could shut
down if the North Atlantic surface water became less salty. Now the
effect had been calculated. And Broecker pointed out geological evidence
that might actually have happened, at the start of Younger Dryas times.
Just then a vast lake dammed up behind the melting North American
ice sheet had suddenly drained, releasing a colossal surge of fresh
water into the ocean. |
Wally
Broecker
|
Broecker's impressive idea was typical of many
ideas in geophysics for the way it drew upon several different areas
of data and theory. His own career (as may be seen elsewhere in these
essays) had rambled through a variety of fields. Ever since the days
when he had trudged around fossil lake basins in Nevada for his doctoral
thesis, Broecker had been interested in sudden climate shifts. The
idea had remained in his mind while he studied the Atlantic Ocean's
circulation as revealed by radioactive tracers, the geo-biochemistry
of surface sea water as reflected in deep-sea cores, the timing of
sea level changes as measured in coral reefs in New Guinea, and numerous
other seemingly unrelated topics. "It's like doing a picture puzzle,"
he remarked. "You get stuck on one, and then it just sits there. And
then along comes an idea, and you say, 'Oh my God, that's a piece
that fits right there.' " The trick was to keep many pieces on the
table, which meant keeping several different lines of research going
at the same time.(48) When one piece fitted into another
an unexpected picture could appear, like the possibility of a sudden
shutdown of the North Atlantic conveyor belt. |
|
The paper by Broecker and his collaborators
made a stir among scientists, less for its new ideas than for putting
forth in a plausible and dramatic way hypotheses that until then had
been hazy and unappreciated. "Until now," the authors wrote, "our
thinking about past and future climate changes has been dominated
by the assumption that the response to any gradual forcing will be
smooth." Even the most elaborate computer models of climate had shown
only gradual transitions but by their very structure that was
all they could be expected to show. In the real world, when you push
on something steadily it may remain in place for a while, then move
with a jerk. |
=>Rapid change
|
The numerical ocean models of the 1980s were inadequate to explore
such a jerk. Even the fastest computers could still scarcely handle
the immense number of calculations that even a quite simple model
required. Modelers normally began with a static ocean and ran it through
a few simulated decades (or if they could get enough computer time,
a century or so) of "spin-up" to watch the currents establish themselves.
The models did not get through even a single complete cycle of the
globe-spanning circulation that interacted with climate change. As
a real-world check, scientists also needed to get a much closer look
at the details of the fossil climate record. "Unless we intensify
research in these areas," Broecker warned, "the major impacts of CO2
will occur before we are prepared fully to deal with them."(49) |
|
In 1987, Broecker followed up with an
even more provocative Nature paper titled, "Unpleasant surprises in the
greenhouse?" Here he emphasized the risk that the current buildup of greenhouse gases might set
off a catastrophe. "We play Russian roulette with climate," he exclaimed.(50) Meanwhile he issued the same warning
in testimony to Congressional committees, in discussions with journalists and in a magazine
article.(51)
|
=>Rapid change
=>Government
|
A few scientists and the science writers
who listened to them began to warn that the ocean circulation might
shut down without much warning, making temperatures plunge drastically
all around the North Atlantic. London and Berlin are in the same
latitude as Labrador, they pointed out, and would be as barren if
not for the heat they get from the ocean. The more attentive members
of the public heard the warning, and in the following decades it
got a lot of journalistic play, although many ignored it as just
another science fiction speculation.
|
=>Public opinion
|
There was in fact a large measure of myth
in the old tale that "the Gulf Stream warms Europe," although
at first even scientists failed to catch the fallacy. Not until around
2002 did some experts point out forcefully that a halt in circulation
could never make Europe's climate anything like Labrador's. England
gets its extra warmth because the prevailing Westerly winds pick up
the ocean's heat, much of which is simply retained heat from the summer.
Labrador, on the other hand, is downwind from tundra that freezes
in winter. Computer modeling suggested that a shutdown of the ocean
circulation conveyor would bring a fairly modest cooling, a few degrees
at most, in North America as well as Europe. That would still be a
significant climate change (although by the time it came, it might
bring only a temporary pause in the warming from greenhouse
gas emissions).(52*)
Among the scientists and others who did pay attention to climate
change, prospects of a North Atlantic shutdown became one of the
most persistent concerns of the 1990s. A few worried that the North
Atlantic region was precisely where most of the data on abrupt climate
change came from, and where most of the people who thought about
it lived. They wondered whether other trigger mechanisms elsewhere
remained to be discovered. |
=>Public opinion
|
"Does the ocean-atmosphere
system have more than one stable mode of operation?" Broecker's question
was already on the mind of computer modelers concerned with future
climate change. Even before Broecker published his ideas, Kirk Bryan
and others had been working up numerical simulations that included
changes in ocean salinity as well as wind patterns. What they found
was troubling. A 1985 study suggested that if the level of atmospheric
CO2 jumped fourfold, the ocean's thermohaline
conveyor belt circulation could cease altogether.(53)
Another study found that even small perturbations could give rise
to radically different modes of ocean circulation. In particular,
a spurt of fresh water suddenly released from a melting continental
ice sheet the kind of event that some thought might have triggered
the Younger Dryas could switch the circulation pattern in as
little as a century.(54*) |
<=Models (GCMs)
<=>Rapid
change
|
These studies were no more than suggestive, for the models of the
mid 1980s were still extremely limited. The planet might be represented
in the computer as, to take one example, three equal continents and
three equal oceans, extending from pole to pole like the segments
of a grapefruit, with the oceans all of uniform depth and the continents
without mountains. To keep computation time within reason, Bryan had
to hold the cloudiness constant, although he knew clouds would interact
with climate change in crucial feedbacks. Along with all that, as
Bryan remarked, "uncertainties abound concerning the interaction of
the ocean circulation and the carbon cycle."(55)
|
|
Syukuro Manabe and Ron Stouffer developed a coupled atmosphere-ocean model with more
realistic geography. As they were varying the
CO2 to see how that might change climate, they made an inadvertent
discovery. If they started two computer runs with the same
CO2 level and other overall physical parameters (the "boundary
conditions"), but with different random "initial conditions" for the first day's weather, they could
wind up with two radically different but stable states. In one state, the thermohaline conveyor belt
was operating. In the other, it wasn't. The model was still packed with unrealistic simplifications,
of course. Yet it seemed at least an "intriguing possibility," as they put it, that global warming
might shut down the North Atlantic circulation within the next century or so, with grave
implications for regional climates.(56)
| |
Most groups still had too little computer power and too little
understanding to manage full-scale models of both ocean and atmospheric circulation and to link
them together. They continued to treat the oceans as a passive "swamp," which exchanged
moisture with the air but did little else. That forced the model atmosphere to handle all the heat
transport from the tropics to the poles, whereas in the real world ocean currents do a good share
of the work. And it entirely missed how heat might sink into the ocean deeps.(57)
|
<=>Models (GCMs)
|
After 1988
|
=>after88 |
Coupled ocean-atmosphere computer models
improved rapidly through the 1990s, and gradually took a central role
in thinking about climate change. A variety of studies showed it was
quite possible that global warming would indeed shut down the North
Atlantic circulation eventually, although the process would probably
take centuries.(58) Confidence in the validity of models
increased as some reproduced the striking El Niño oscillations
quite well. Still more encouraging, computer specialists managed to
reproduce not only the current state of the atmosphere and oceans
but also, using the same models without artificial adjustments, the
radically different climate that had prevailed at the height of the
last ice age. |
<=Models (GCMs)
|
Despite these triumphs,
much remained to be done before anyone could form a clear picture
of how the oceans connected to long-term climate change. Perhaps
the most vexing of the many difficulties was figuring in the large
amount of CO2 that the ocean's plankton absorbed
from the atmosphere. The plankton population depended on the sea
surface temperature, and still more on nutrients brought in by rivers,
by wind-borne dust, and by the upwelling of ocean currents
all of which could change as climate changed. The plankton's biochemical
behavior meanwhile would affect the chemical balance of sea water,
which was also crucial for CO2 uptake or release.
Alongside these intricately indirect effects, scientists gradually
learned to worry about a problem that stemmed directly from the
rise of atmospheric CO2. As ever more of
the gas dissolved in the oceans, the acidity of the surface water
was measurably increasing. This would make it harder for the water
to continue to absorb gas (by the mechanism Revelle had reported
back in 1957). The acid might eventually dissolve the calcium-carbonate
shells of plankton and other creatures important in marine food
chains, with uncertain effects on seawater chemistry. Scientists
would have to untangle these complexities before they could truly
understand how the oceans' uptake of CO2 would
influence the future climate. |
<=>Biosphere
=>CO2 greenhouse
|
Plenty of surprises were still coming from
new data. Especially striking were studies in the 1980s that turned
up layers of tiny pebbles in North Atlantic deep-sea cores. The debris
could have traveled across thousands of kilometers of ocean in only
one way: rafted within far-traveling icebergs. Apparently the North
American ice sheet had disintegrated at the edges perhaps in
a gigantic surge? so that great numbers of icebergs had broken
off and sailed the North Atlantic as far as Spain. This fitted with
speculations about the breakup of Arctic Ocean ice that had been circulating
for decades. In 1988, a German graduate student, Hartmut Heinrich,
published evidence that the "iceberg armadas" had swarmed across the
North Atlantic regularly at particular phases of the glacial cycle.(59)
Further studies showed that these "Heinrich events" connected with
the more frequent "Dansgaard-Oeschger" periods of cooling. The exact
sequence of cause and effect was obscure, but there was evidence of
a link to massive surges of the North American ice sheet and changes
in the thermohaline circulation.(60*)
In any case, it was now certain that catastrophic climate shifts,
connected with shifts in ocean circulation, could affect the entire
North Atlantic region, and probably other parts of the globe as well.
|
=>Rapid change
|
Whatever had set off the abrupt shifts, they seemed to have been
a feature of glacial epochs, not of warmer times like the present.
However, many oceanographers suspected that the present climate was
not immune.(61) Experts looking into the complexities of the North Atlantic
system began to think that it might have a variety of possible modes,
not just "glacial" and the present stable "interglacial."
Meanwhile,
increasingly
in the 1990s and after, the oceanographers' traditional preoccupation
with the North Atlantic gave way to a broader perspective. They
began to suspect that the tropical oceans could easily be as important
as the North Atlantic in rapid climate change. For one thing, it
was becoming plain that even ordinary El Niño events in the
tropical Pacific seriously affected weather right around the world.
For another, new studies showed that equatorial waters had undergone
major changes during past ice ages. Climatologists had believed
for generations that ice ages had scarcely affected the equatorial
jungles. This was now replaced by a view of the globe as a system
where every region reacted to changes everywhere else. Suppose,
to take just one possibility, a variation of tropical climate altered
how winds carried moisture from the Atlantic to the Pacific, altering
the salinity? Broecker and co-workers argued that such variations
could drive a feedback cycle that might bring "massive and abrupt
reorganizations of the ocean-atmosphere system."(62) And there
were probably other mechanisms, so far barely visible, adding their
own complexities. |
=>Rapid change
<=Models (GCMs)
|
Only computer models
could say which of these ideas might really work, if any. Modelers
successfully simulated abrupt shifts of the North Atlantic circulation,
confirming that during a glacial period it could shut off and on by
itself. A change of circulation also looked only too likely within
the next few centuries as global warming took hold. Paradoxically,
that might bring severe cooling to regions from Chicago to Moscow.
The modelers' best guess was that the ocean circulation would gradually
slow down during the 21st century. Indeed observations early in the
century showed that salinity had already decreased and the water was
overturning more slowly. That might be part of a normal cycle (see
below), and most experts did not expect any dramatic shift in the
next hundred years. But they could not entirely rule out a possibility
that the expected greenhouse warming would push the circulation system
across some threshold, bringing an abrupt and
complete shutdown.(63*) The most widely used
and elaborate models could hardly be expected to show abrupt changes,
for they had been built to be stable. Indeed these models
failed to show the sort of climate system jumps that were abundantly
clear in the actual geological record of glacial times. |
<=>Rapid change
<=>Models
(GCMs) |
Modelers had not yet fully grasped even the current global ocean
circulation. Among other shortcomings, their grid boxes were still
too large to realistically represent giant eddies or narrow currents
like the Gulf Stream. The models' failings were underlined by new
data, which hinted that much of the heat energy that was carried vertically
from layer to layer in the oceans was not transported by some kind
of average convection (as the models had assumed), but was moved by
tides(64) The strength of ocean tides varies
in predictable long-term cycles, ranging from a few years to well
over a thousand. Strong tides would mix the waters, perhaps introducing
lunar rhythms into the climate system. Could that explain the hints
that had been cropping up of an oceanic cycle a thousand or so years
long, which in turn would explain the main temperature trends of the
past millennium?(65) No matter how that particular idea
turned out, it remained a necessary but daunting task to find how
water mixed up and down through the layers.(66*) |
|
There was also new evidence that the North Atlantic Ocean, all on its
own, goes through quasi-regular oscillations. This tended to confirm
the cycles around 60-80 years long that Dansgaard and others had
found for the region. Since the 1920s, meteorologists had been talking
about a decades-long variation of weather patterns in the region.
Perhaps it was a slow sloshing of water masses that had made temperatures
around the North Atlantic rise so noticeably until the 1940s, then
dip until the 1970s (helping to bring drought to the African Sahel),
and then rise again.
Whateverthe cause, would
a new long-term fluctuation bring another temporary halt in the
warming of this crucial region once again confusing the public
about the greenhouse future?(67*) Until scientists understood such major effects, and constructed
better models, and stopped interrupting one another with surprising
new evidence and ideas, the ocean circulation would remain one of
the biggest uncertainties in the equation of climate change. |
=>Modern temp's
=>Simple
models
=>Models (GCMs)
|
Progress would
depend on data, and oceanographers still had sampled only a minute
fraction of the world-ocean. Beginning in the 1970s, collaborative
projects mobilized thousands of people from scores of nations. The
march of acronyms started under the international Global Atmospheric
Research Program (GARP) with regional studies like the groundbreaking
GARP Atlantic Tropical Experiment (GATE), carried out in 1974. Next
came a Tropical Ocean-Global Atmosphere study (TOGA) that surveyed
the equatorial Pacific, inspired by the devastating El Niño
of 1982-83. The advent of realistic supercomputer models in the 1980s
fostered a more global view of ocean dynamics, by calculating how
waters from the North Atlantic circulated all the way to the mid-Pacific
and back. To feed the models, there were now satellites (starting
with the short-lived SEASAT of 1978) that could measure winds, waves,
temperatures, and currents in the remotest reaches of the seas. But
the satellites could not measure everything, and what their instruments
did measure required "ground truth" observations for checking and
calibration. The global approach was embodied in a World Ocean Circulation
Experiment (WOCE), planned in the 1980s and carried out in the 1990s
by some thirty nations. It was supplemented by a Joint Global Ocean
Flux Study (JGOFS) that looked at CO2 uptake
and other ocean chemistry.(68) |
<=>International
=>Biosphere
|
Mining old
data could also tell many things. One project burrowed through historical
records to transcribe literally millions of thermometer readings,
assembling a database for the most basic of all climate numbers —
the temperatures within the seas. Since the world-ocean absorbs dozens
of times more heat than any other component of the climate system,
it was here if anywhere that the reality of global warming should
be visible. The team found that the heat content of the upper oceans
had risen markedly in the second half of the 20th century —
in a pattern that matched the "signature" that computer
modelers predicted from the greenhouse effect.(69) |
=>Models (GCMs)
=>Modern temp's
|
It would be harder to get reliable measures for the more complex
numbers that represented currents, movement of heat into the deeps,
and other dynamic features. Improvements would have to wait on data
from yet more grand international data-gathering projects, and on
a clearer grasp of fundamental processes, worked into computer models.
The coupled ocean-atmosphere models were now good enough to give a
general idea of the warming that was likely to come in the 21st century
as greenhouse gases built up in the atmosphere. But nobody could confidently
rule out the possibility of some future climate shock, caused by processes
perhaps not yet imagined in the convoluted systems that linked air,
ice, and seas.(70*) |
|
|
RELATED:
Home
Rapid Climate Change
General Circulation Models of the Atmosphere
1. Hull (1897), noting that "the
increased snowfall which would thus be caused... would tend to intensify the cold," p. 107;
deflection of currents was likewise seen as central in the scheme of Croll (1875).
BACK
2. Chamberlin (1906), quote p.
371; Fleming (1998), p. 89.
BACK
3. Tolman (1899), quote p. 587.
BACK
4. E.g., Gregory (1908), p. 348.
BACK
5. Lotka (1924), pp. 222-24; for
the state of thinking in the 1950s, see Hutchinson (1954), pp.
383-84.
BACK
6. "theology": Munk (2000), p.
45; similarly Wunsch (1981), p. 342; "first law": Munk (2000), p. 1.
BACK
7. Sverdrup et al. (1942), pp.
628-29, 635-37, 647, 685; Sverdrup (1957) likewise sees much
more heat transport in the Gulf Stream than across the equator, and describes the North Atlantic
deep circulation as driven by winds and heat but not salinity.
BACK
8. Wenk (1972), pp. 38-41. H. Stommel, privately circulated
document, 1954, titled, "Why do our ideas about the ocean circulation have such a peculiarly
dream-like quality?" referenced in Warren and Wunsch (1981),
p. 601.
BACK
9. Deacon (1957), p. 81.
BACK
10. Stommel (1987), p. 58.
BACK
11. Mills (1998), p. 634; Warren and Wunsch (1981), pp. xvi-xvii, xxi, 12-13.
BACK
12. Wenk (1972), pp. 49-50;
Miles (1981); Weir (2001).
BACK
13. Rossby (1956); translated as
Rossby (1959), p. 13.
BACK
14. Namias's phrases here referred to how snow cover on
land would add to the cold, but elsewhere he made a point of wind-sea
feedbacks. Namias (1963), quotes pp. 6717, 6185; such a wind-ocean
interaction was also reported by Bjerknes (1966).
BACK
14a. Bjerknes
(1969), see also Bjerknes (1966).
BACK
15. Broecker (1957), p. III-12;
Broecker et al. (1960); Broecker et al.
(1960).
BACK
16. Ewing and Donn (1956);
"valves" e.g., Humphreys (1940), pp. 623-24.
BACK
17. Robert C. Cowen, "Are men changing the Earth's
weather?" Christian Science Monitor, Dec. 4, 1957. Iselin did
not mention, but was plainly referring to, the Ewing and Donn model.
BACK
18. Stommel (1961), p. 228.
BACK
19. Broecker (1966), p. 301.
BACK
20. Doel (2001).
BACK
21. Broecker et al. (1960); the
circulation pattern was mentioned already by Rossby (1956);
translated as Rossby (1959), p. 13.
BACK
22. Weyl (1968), speculating
that the "temporary stagnation" of the bottom water would end because of warming by the
interior heat of the Earth; the role of glacial meltwater suppressing North Atlantic Deep Water
production was also pioneered by Worthington (1968); a neat
explanation of the entire circulation in terms of water evaporating from the North Atlantic more
than from the cooler North Pacific was indicated by Warren
(1983).
BACK
23. Newell (1974).
BACK
24. GARP (1975), pp. 4, 219.
BACK
25. Manabe and Bryan (1969);
for empirical evidence they cite Sverdrup (1957).
BACK
26. National Academy of Sciences
(1979), p. 2.
BACK
27. Munk (1966), "recipes"
quote p. 728. This example of the struggle with vertical mixing includes a hypothesis about tidal
effects; recipes for vertical diffusion may be wrong because mixing may actually happen only "in
special places": Munk (1975) (at a 1972 conference).
BACK
28. Survey: J. Swallow and J. Crease on the British
ship Aries. Campaign: the Mid-Ocean Dynamics Experiment (MODE)
1971-1973, promoted especially by H. Stommel. For history, see Wunsch
(1981), 358-59, "mirror" p. 343. BACK
29. "extensive research": Schneider
and Dickinson (1974), p. 465.
BACK
30. Young (2000), p. 166.
BACK
31. Stommel (1970), p. 1531.
BACK
32. Hammond (1974), p. 1147.
BACK
33. "Climate: Long Range Investigation, Mapping
and Prediction." CLIMAP project members (A. McIntyre
et al.) (1976), quote p. 1136; Cline and Hays
(1976); the final product was maps (which I have not seen), CLIMAP
(1981); note also CLIMAP (1984). The maps were for 18,000 carbon-14 years
ago, now estimated at about 21,000 calendar years. BACK
34. Curry and Lohmann
(1982); Boyle and Keigwin (1982).
BACK
35. Broecker (1981), p. 449.
BACK
36. A key model, including diffusion by eddies into the deeps,
was Oeschger et al. (1975); see also Broecker et al. (1979), q.v. for references to GEOSECS reports by
H.G. Ostlund et al., University of Miami; Broecker et al. (1980).
BACK
37. Broecker et al. (1980),
quote p. 582.
BACK
38. Bryan and Cox (1968).
Bryan, interview by Weart, Dec. 1989, AIP.
BACK
39. Manabe and Bryan (1969);
Manabe (1969); Bryan
(1969), quote p. 806; "rigid lid": Bryan (1969).
BACK
40. Frontier: Reid et al.
(1975), Introduction, p. 3; "paradigm... varietal" McWilliams
(1996).
BACK
41. Cox (1975) ("the most
ambitious ocean simulation so far," according to W.L. Gates, p. 116); published at the same time
was a somewhat cruder whole-ocean model, aimed at integration with the Mintz-Arakawa model,
Takano (1975); Manabe et al.
(1975), quote p. 3; together with Bryan et al. (1975);
further landmarks included Manabe et al. (1979); Washington et al. (1980).
BACK
42. Ruddiman and McIntyre
(1981); Boyle and Keigwin (1982) (using Cd as tracer for
nutrients); for further refs., see Broecker et al. (1985); later
Boyle and Keigwin, using a core from a spot where deposits had built up exceptionally fast,
found that "the deep ocean can undergo dramatic changes in its circulation regime" within 500
years, Boyle and Keigwin (1987), p. 36.
BACK
43. "A basinwide change of deep water occurred," Schnitker (1979), quote p. 265; Schnitker (1982) speculated about unstable ocean feedback loops;
"dramatic change": Shackleton et al. (1983), p. 242; Rooth (1982) wrote that "catastrophic transitions in the structure of
the thermohaline circulation are not only possible, but have probably occurred on many
occasions...," p. 131.
BACK
44. "large-scale circulation changes," Oeschger et al. (1984), p. 303; he cited Broecker (1982); for meltwater effect he cited Worthington (1968); and in more detail Ruddiman and McIntyre (1981); in 1990 Broecker cited Oeschger's
paper as the first suggestion "that the Greenland events constitute jumps between two modes of
operation of the climate system," Broecker et al. (1990).
BACK
45. Oeschger to Broecker, 11/23/95 and reply 12/4/95, Broecker
office files, Lamont-Doherty Geophysical Observatory, Palisades, NY. "Disbelief:" Stocker (1999); Siegenthaler and Wenk
(1984).
BACK
46. "Idea hit:" Broecker
(2000), p. 13; Broecker also recalls seeing Oeschger at a 1984 Florida meeting. On this and
faulty data: Broecker, interview by Weart, Nov. 1997, AIP.
BACK
47. "Astounded": Broecker, interview by Weart, Nov. 1997,
see also Dec. 1997, AIP. Broecker et al. (1985),
"jumps... speculate," p. 25; "conveyor belt" and "staggering" heat flow
were publicized in Broecker (1987), p. 87, and laid out fully in Broecker
(1991). BACK
48. Broecker, interview by Weart, Dec. 21, 1997, AIP.
BACK
49. Broecker et al. (1985), p.
25.
BACK
50. Broecker (1987), p. 123.
BACK
51. Broecker (1987); Broecker (1987); U.S. Senate, Subcommittee on Environmental
Protection, Hearings, Jan. 26-28 1987, pp. 21-23.
BACK
52. Seager et al. (2002).
A typical modern model shows a roughly 2°C drop in Europe: Vellinga
and Wood (2002). BACK
53. Bryan and Spelman (1985);
the question is the title of Broecker et al. (1985).
BACK
54. Bryan (1986). N.b.
this is Frank Bryan, not Kirk. See Broecker et al. (1990). The cause of the event is still
uncertain and under study. For full references see note
45 in "Rapid climate change". BACK
55. Bryan and Spelman (1985),
p. 11,687.
BACK
56. Manabe and Stouffer
(1988), p. 841.
BACK
57. Schlesinger and Mitchell
(1987), p. 796; McGuffie and Henderson-Sellers (1997),
pp. 55-56; 1980s work is reviewed in Haidvogel and Bryan
(1992); Meehl (1992).
BACK
58. E.g., Wood et al. (1999);
see summary: Rahmstorf (1999).
BACK
59. Heinrich (1988); earlier
speculations: Mercer (1969); Ruddiman and McIntyre (1981).
BACK
60. Bond et al. (1992); Bond et al. (1993); Broecker suggested that when fresh water was
brought into the North Atlantic in a million melting icebergs, it might have halted the North
Atlantic thermohaline circulation. Broecker et al. (1992).
BACK
61. Bauch et al. (2000); Alley (2000), ch. 15.
BACK
62. Broecker and Denton
(1989), quote p. 2489; Broecker et al. (1990); for the
evaporation cycle Warren (1983); a detailed review is Broecker and Denton (1990); for a more recent review, Broecker (2000).
BACK
63. Manabe and Stouffer
(1993) pioneered the demonstration of a transition under future warming; an improved
model showed a shutdown was especially likely with rapid increase of greenhouse gas emissions,
Stocker and Schnitter (1997); see also Broecker (1997); Ganopolski and
Rahmstorf (2001) for instability during a glacial period; IPCC
(2001), pp. 439-40.
BACK
64. Egbert and Ray (2000).
BACK
65. See discussion in Keeling
(1998), pp. 70-73.
BACK
66. Mapping global patterns and incorporating the
results in models was described as "...a daunting task... requires a large
effort, but ... feasible" for one important type of mixing, the breaking
of the internal waves on the surfaces between layers of different densities,
Gregg et al. (2003). Merryfield
(2005)reviews recent mixing studies. BACK
67. New studies suggested that this "Atlantic
Meridional Overturning" (AMO, also "North Atlantic Oscillation")
was driven not by ocean interactions, but by changes in upper atmosphere
wind patterns around the entire hemisphere, Wallace
and Thompson (2002), see also Science 289
(28 July 2000): 547-48. BACK
68. Thompson et al. (2001).
BACK
69. Levitus et al. (2001)
(the data compilation was by the NOAA group Levitus headed); Barnett
et al. (2001), updated and improved by Levitus
et al. (2005). BACK
70. E.g., "The threshold separating stable and unstable climate
regimes represents a relatively small departure from the modern ice sheet configurations,"
according to McManus et al. (1999), p. 1.
BACK
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© 2003-2006 Spencer Weart & American Institute of Physics
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