| Like many Victorian natural philosophers,
John Tyndall was fascinated by a great many questions. While he was
preparing an important treatise on "Heat as a Mode of Motion" he took
time to consider geology. Tyndall had hands-on knowledge of the subject,
for he was an ardent Alpinist (in 1861 he made the first ascent of
the Weisshorn). Familiar with glaciers, he had been convinced by the
evidence hotly debated among scientists of his day that
tens of thousands of years ago, colossal layers of ice had covered
all of northern Europe. How could climate possibly change so radically?
Full discussion in
|One possible answer was
a change in the composition of the Earth's atmosphere. Beginning with
work by Joseph Fourier in the 1820s, scientists had understood that
gases in the atmosphere might trap the heat received from the Sun.
This was the effect that would later be called, by an inaccurate analogy,
the "greenhouse effect." The equations and data available to 19th-century
scientists were far too poor to allow an accurate calculation. Yet
the physics was straightforward enough to show that a bare rock at
the Earth's distance from the Sun should be far colder than the Earth
actually is. Tyndall set out to find whether there was in fact any
gas that could trap heat rays. In 1859, his careful laboratory work
identified several gases that did just that. The most important was
simple water vapor (H2O). Also effective was
carbon dioxide (CO2), although in the atmosphere the gas is only a few parts in ten
thousand. Just as a sheet of paper will block more light than an entire
pool of clear water, so the trace of CO2 altered the balance of heat radiation through the entire atmosphere.
(For full explanation of the science, follow the link at right
to the essay on Simple Models of Climate.)(1)
| Greenhouse Speculations: Arrhenius
| The next major scientist to consider the
question was another man with broad interests, Svante Arrhenius in
Stockholm. He too was attracted by the great riddle of the prehistoric
ice ages. In 1896 Arrhenius completed a laborious numerical computation
which suggested that cutting the amount of CO2
in the atmosphere by half could lower the temperature in Europe some
4-5°C (roughly 7-9°F) that is, to an ice age level.
But this idea could only answer the riddle of the ice ages if such
large changes in atmospheric composition really were possible. For
that question Arrhenius turned to a colleague, Arvid Högbom.
It happened that Högbom had compiled estimates for how carbon
dioxide cycles through natural geochemical processes, including emission
from volcanoes, uptake by the oceans, and so forth. Along the way
he had come up with a strange, almost incredible new idea.
| It had occurred to Högbom to calculate
the amounts of CO2 emitted by factories and other
industrial sources. Surprisingly, he found that human activities were
adding CO2 to the atmosphere at a rate roughly
comparable to the natural geochemical processes that emitted or absorbed
the gas. The added gas was not much compared with the volume of CO2
already in the atmosphere the CO2 released
from the burning of coal in the year 1896 would raise the level by
scarcely a thousandth part. But the additions might matter if they
continued long enough.(2) (By
recent calculations, the total amount of carbon laid up in coal and
other fossil deposits that humanity can readily get at and burn is
some ten times greater than the total amount in the atmosphere.) So
the next CO2 change might not be a cooling decrease, but an increase. Arrhenius
made a calculation for doubling the CO2 in the
atmosphere, and estimated it would raise the Earth's temperature some
| Arrhenius did not see that as a problem. He figured that if industry
continued to burn fuel at the current (1896) rate, it would take perhaps
three thousand years for the CO2 level to rise
so high. Högbom doubted it would ever rise that much. One thing
holding back the rise was the oceans. According to a simple calculation,
sea water would absorb 5/6ths of any additional gas. (That is roughly
true over a long run of many thousand years, but Högbom and Arrhenius
did not realize that if the gas were emitted more rapidly than they
expected, the ocean absorption could lag behind.) Anyway temperatures
a few degrees higher hardly sounded like a bad idea in chilly Sweden.
Another highly respected scientist, Walter Nernst, even fantasized
about setting fire to useless coal seams in order to release enough
CO2 to deliberately warm the Earth's climate.(4*)
| Arrhenius brought up about the possibility of future warming in
an impressive scientific article and a widely read book. By the time
the book was published, 1908, the rate of coal burning was already
much higher than in 1896, and Arrhenius suggested warming might appear
wihin a few centuries rather than millenia. Yet here as in his first
article, the possibility of warming in some distant future was far
from his main point. He mentioned it only in passing, during a detailed
discussion of what really interested scientists of his time —
the cause of the ice ages. Arrhenius had not quite discovered global
warming, but only a curious theoretical concept.(5)
| An American geologist, T. C. Chamberlin,
and a few others took an interest in CO2. How,
they wondered, is the gas stored and released as it cycles through
the Earth's reservoirs of sea water and minerals, and also through
living matter like forests? Chamberlin was emphatic that the level
of CO2 in the atmosphere did not necessarily
stay the same over the long term. But these scientists too were pursuing
the ice ages and other, yet more ancient climate changes gradual
shifts over millions of years. Very different climates, like the balmy
age of dinosaurs a hundred million years ago, puzzled geologists but
seemed to have nothing to do with changes on a human time scale. Nobody
took much interest in the hypothetical future warming caused by human
| Experts could dismiss the hypothesis because they found Arrhenius's
calculation implausible on many grounds. In the first place, he had
grossly oversimplified the climate system. Among other things, he
had failed to consider how cloudiness might change if the Earth got
a little warmer and more humid.(6)
A still weightier objection came from a simple laboratory measurement.
A few years after Arrhenius published his hypothesis, another scientist
in Sweden, Knut Ångström, asked an assistant to measure
the passage of infrared radiation through a tube filled with carbon
dioxide. The assistant ("Herr J. Koch," otherwise unrecorded
in history) put in rather less of the gas in total than would be found
in a column of air reaching to the top of the atmosphere. The assistant
reported that the amount of radiation that got through the tube scarcely
changed when he cut the quantity of gas back by a third. Apparently
it took only a trace of the gas to "saturate" the absorption
— that is, in the bands of the spectrum where CO2
blocked radiation, it did it so thoroughly that more gas could make
Still more persuasive
was the fact that water vapor, which is far more abundant in the
air than carbon dioxide, also intercepts infrared radiation. In
the crude spectrographs of the time, the smeared-out bands of the
two gases entirely overlapped one another. More CO2
could not affect radiation in bands of the spectrum that water vapor,
as well as CO2 itself, were already blocking
These measurements and arguments had fatal flaws. Herr Koch had
reported to Ångström that the absorption had not been
reduced by more than 0.4% when he lowered the pressure, but a modern
calculation shows that the absorption would have decreased about
1% — like many a researcher, the assistant was over confident
about his degree of precision.(8a) But even if he had seen
the1% shift, Ångström would have thought this an insignificant
perturbation. He failed to understand that the logic of the experiment
was altogether false.
The greenhouse effect will in fact operate even if the absorption
of radiation were totally saturated in the lower atmosphere. The
planet's temperature is regulated by the thin upper layers where
radiation does escape easily into space. Adding more greenhouse
gas there will change the balance. Moreover, even a 1% change in
that delicate balance would make a serious difference in the planet’s
surface temperature. The logic is rather simple once it is grasped,
but it takes a new way of looking at the atmosphere — not
as a single slab, like the gas in Koch's tube (or the glass over
a greenhouse), but as a set of interacting layers. (The full
explanation is in the essay on Simple Models, use link at right.)
|The subtle difference did not occur to anyone for many decades,
if only because hardly anyone thought the greenhouse effect was
worth their attention. For after Ångström published his
conclusions in 1900, the few scientists who had taken an interest
in the matter concluded that Arrhenius's hypothesis had been proven
wrong. Theoretical work on the question stagnated for decades, and
so did measurement of the level of CO2 in the
| A few scientists dissented from the view
that changes of CO2 could have no effect. An American physicist, E.O. Hulburt, pointed
out in 1931 that investigators had been mainly interested in pinning
down the intricate structure of the absorption bands (which offered
fascinating insights into the new theory of quantum mechanics) "and
not in getting accurate absorption coefficients." Hulburt's own calculations
supported Arrhenius's estimate that doubling or halving CO2
would bring something like a 4°C rise or fall of surface temperature,
and thus "the carbon dioxide theory of the ice ages... is a possible
theory."(10*) Hardly anyone noticed this paper.
Hulburt was an obscure worker at the U.S. Naval Research Laboratory,
and he published in a journal, the Physical Review, that
few meteorologists read. Their general consensus was the one stated
in such authoritative works as the American Meteorological Society's
1951 Compendium of Meteorology: the idea that adding CO2 would change the climate "was never widely accepted and was abandoned
when it was found that all the long-wave radiation [that would be]
absorbed by CO2 is [already] absorbed by water
|Even if people had recognized
this was untrue, there were other well-known reasons to deny any greenhouse
effect in the foreseeable future. These reasons reflected a nearly
universal conviction that the Earth automatically regulated itself
in a "balance of nature." Getting to specifics, scientists repeated
the plausible argument that the oceans would absorb any excess gases
that came into the atmosphere. Fifty times more carbon is dissolved
in sea water than in the wispy atmosphere. Thus the oceans would determine
the equilibrium concentration of CO2, and it
would not easily stray from the present numbers.
|If somehow the oceans
failed to stabilize the system, organic matter was another good candidate
for providing what one scientist called "homeostatic regulation."(12)
The amount of carbon in the atmosphere is only a small fraction of
what is bound up not only in the oceans but also in trees, peat bogs,
and so forth. Just as sea water would absorb more gas if the concentration
increased, so would plants grow more lushly in air that was "fertilized"
with extra carbon dioxide. Rough calculations seemed to confirm the
comfortable belief that biological systems would stabilize the atmosphere
by absorbing any surplus. One way or another, then, whatever gases
humanity added to the atmosphere would be absorbed if not at
once, then within a century or so and the equilibrium would
automatically restore itself. As one respected expert put it baldly
in 1948, "The self-regulating mechanisms of the carbon cycle can cope
with the present influx of carbon of fossil origin."(13)
| Yet the theory that atmospheric CO2 variations
could change the climate was never altogether forgotten. An idea so
simple on the face of it, an idea advanced (however briefly) by outstanding
figures like Arrhenius and Chamberlin, had to be mentioned in textbooks
and review articles if only to refute it. Arrhenius's outmoded hypothesis
persisted in a ghostly afterlife.
| It found a lone advocate. Around 1938 an English engineer, Guy Stewart
Callendar, took up the old idea. An expert on steam technology, Callendar
apparently took up meteorology as a hobby to fill his spare time.(14) Many people, looking at weather stories from the past,
had been saying that a warming trend was underway. When Callendar
compiled measurements of temperatures from the 19th century on, he
found they were right. He went on to dig up and evaluate old measurements
of atmospheric CO2 concentrations. He concluded
that over the past hundred years the concentration of the gas had
increased by about 10%. This rise, Callendar asserted
, could explain the observed warming. For he understood (perhaps from
Hulburt's calculation) that even if the CO2 in
the atmosphere did already absorb all the heat radiation passing through,
adding more gas would change the height in the atmosphere where the
absorption took place. That, he calculated, would make for warming.
| As for the future, Callendar estimated, on flimsy
grounds, that a doubling of CO2 could gradually
bring a 2°C rise in future centuries. He hinted that it might
even trigger a shift to a self-sustaining warmer climate (which did
not strike him as a bad prospect).(15)
But future warming was a side issue for Callendar. Like all his predecessors,
he was mainly interested in solving the mystery of the ice ages.
|| <=Simple models
| Callendar's publications attracted some
attention, and climatology textbooks of the 1940s and 1950s routinely
included a brief reference to his studies. But most meteorologists
gave Callendar's idea scant credence. In the first place, they doubted
that CO2 had increased at all in the atmosphere.
The old data were untrustworthy, for measurements could vary with
every change of wind that brought emissions from some factory
or forest.(16) If in fact CO2
was rising, that could only be detected by a meticulous program stretching
decades into the future.(17*)
The objections that had been raised against Arrhenius also had to
be faced. Wouldn't the immense volume of the oceans absorb all the
extra CO2? Callendar countered that the thin
layer of ocean surface waters would quickly saturate, and it would
take thousands of years for the rest of the oceans to turn over and
be fully exposed to the air.(18)
But nobody knew the actual turnover rate, and it seemed that the oceans
would have time to handle any extra gases. According to a well-known
estimate published in 1924, even without ocean absorption it would
take 500 years for fuel combustion to double the amount of CO2
in the atmosphere.(19)
| There was also the old objection, which most scientists continued
to find decisive, that the overlapping absorption bands of CO2
and water vapor already blocked all the radiation that those molecules
were capable of blocking. Callendar tried to explain that the laboratory
spectral measurements were woefully incomplete.(20) Some other scientists too kept an open mind on the question.
But it remained the standard view that, as an official U.S. Weather
Bureau publication put it, the masking of CO2
absorption by water vapor was a "fatal blow" to the CO2
theory. Therefore, said this authority, "no probable increase in atmospheric
CO2 could materially affect" the balance of radiation.(21)
| Most damaging of all, Callendar's calculations of the greenhouse
effect temperature rise ignored much of the real world's physics.
In particular, any rise in temperature would allow the air to hold
more moisture, which could mean more clouds. Callendar admitted that
the actual climate change would depend on interactions involving changes
of cloud cover and other processes that no scientist of the time could
reliably calculate. Few thought it worthwhile to speculate about such
dubious questions, where data were rudimentary and theory was no more
than hand-waving. Better to rest with the widespread conviction that
the atmosphere was a stable, automatically self-regulated system.
The notion that humanity could permanently change global climate was
implausible on the face of it, hardly worth a scientist's attention.(22)
| The scientists who brushed aside Callendar's claims were reasoning
well enough. (Subsequent work has shown that the temperature rise
up to 1940 was, as his critics thought, mainly caused by some kind
of natural cyclical effect, not by the still relatively low CO2 emissions. And the physics of radiation and climate was indeed
too poorly known at that time to show whether adding more gas could
make much difference.) Yet if Callendar was mistaken when he insisted
he could prove global warming had arrived, it was a fortunate mistake.
| Research by definition is done at the frontier
of ignorance. Like nearly everyone described in these essays, Callendar
had to use intuition as well as logic to draw any conclusions at all
from the murky data and theories at his disposal. Like nearly everyone,
he argued for conclusions that mingled the true with the false, leaving
it to later workers to peel away the bad parts. While he could not
prove that global warming was underway, he had given reasons to reconsider
the question. We owe much to Callendar's courage. His claims rescued
the idea of global warming from obscurity and thrust it into the marketplace
of scientific ideas. Not everyone dismissed his claims. Their very
uncertainty attracted scientific curiosity.
| The Speculations Vindicated (1950-1960)
| The complacent view that CO2
from human activity could never become a problem was overturned during
the 1950s by a series of costly observations. This was a consequence
of the Second World War and the Cold War, which brought a new urgency
to many fields of research. American scientists enjoyed massively
increased government funding, notably from military agencies. The
officials were not aiming to answer academic questions about future
climates, but to provide for pressing military needs. Almost anything
that happened in the atmosphere and oceans could be important for
national security. Among the first products were new data for the
absorption of infrared radiation, a topic of more interest to weapons
engineers than meteorologists.(23)
|The early experiments that sent radiation through
gases in a tube, measuring bands of the spectrum at sea-level pressure
and temperature, had been misleading. The bands seen at sea level
were actually made up of overlapping spectral lines, which in the
primitive early instruments had been smeared out into broad bands.
Improved physics theory and precise laboratory measurements in the
1940s and after encouraged a new way of looking at the absorption.
Scientists were especially struck to find that at low pressure and
temperature, each band resolved into a cluster of sharply defined
lines, like a picket fence, with gaps between the lines where radiation
would get through.(24) The most important CO2 absorption
lines did not lie exactly on top of water vapor lines. Instead of
two overlapping bands, there were two sets of narrow lines with spaces
for radiation to slip through. So even if water vapor in the lower
layers of the atmosphere did entirely block any radiation that could
have been absorbed by CO2, that would not keep
the gas from making a difference in the rarified and frigid upper
layers. Those layers held very little water vapor anyway. And scientists
were coming to see that you couldn't just calculate absorption for
radiation passing through the atmosphere as a whole, you had to understand
what happened in each layer — which was far harder to calculate.
|Digital computers were now at hand for such
calculations. The theoretical physicist Lewis D. Kaplan decided
it was worth taking some time away from what seemed like more important
matters to grind through extensive numerical computations. In 1952,
he showed that in the upper atmosphere, adding more CO2
must change the balance of radiation significantly.(25)
| But would adding carbon dioxide in the upper layers of the air significantly
change the surface temperature? Only detailed computations, point
by point across the infrared spectrum and layer by layer up through
the atmosphere, could answer that question. By 1956, such computations
could be carried out thanks to the increasing power of digital computers.
The physicist Gilbert N. Plass took up the challenge of calculating
the transmission of radiation through the atmosphere, nailing down
the likelihood that adding more CO2 would increase
the interference with infrared radiation.(26) Going beyond this qualitative result,
Plass announced that human activity would raise the average global
temperature "at the rate of 1.1 degree C per century."
like Callendar's, paid no attention to possible changes in water vapor
and clouds, and overall was too crude to convince scientists. "It
is almost certain," one authority scolded, "that these figures will
be subject to many strong revisions."(27)
Yet Plass had proved one central point: it was a mistake to dismiss
the greenhouse effect with spectroscopic arguments. He warned that
climate change could be "a serious problem to future generations"
although not for several centuries. Following the usual pattern,
Plass was mainly interested in the way variations in CO2
might solve the mystery of the ice ages. "If at the end of this century
the average temperature has continued to rise," he wrote, then it
would be "firmly established" that CO2 could
cause climate change.(28)
| None of this work met
the argument that the oceans would promptly absorb nearly all the
CO2 humanity might emit. Plass had estimated
that gas added to the atmosphere would stay there for a thousand years.
Equally plausible estimates suggested that the surface waters of the
oceans would absorb it in a matter of days.(29)
Fortunately, scientists could now track the movements of carbon with
a new tool the radioactive isotope carbon-14. This isotope
is created by cosmic rays in the upper atmosphere and then decays
over millennia. The carbon in ancient coal and oil is so old that
it entirely lacks the radioactive isotope. In 1955, the chemist Hans
Suess reported that he had detected this fossil carbon in the atmosphere.
| The amount that Suess measured in the atmosphere
was barely one percent, a fraction so low that he concluded that the
oceans were indeed taking up most of the carbon that came from burning
fossil fuels. A decade would pass before he reported more accurate
studies, which showed a far higher fraction of fossil carbon. Yet
already in 1955 it was evident that Suess's data were preliminary
and insecure. The important thing he had demonstrated was that fossil
carbon really was showing up in the atmosphere. More work on carbon-14
should tell just what was happening to the fossil carbon.(30)
| Suess took up the problem
in collaboration with Roger Revelle at the Scripps Institution of
Oceanography. (Some other carbon-14 experts attacked the topic independently,
all reaching much the same conclusions.) From measurements of how
much of the isotope was found in the air and how much in sea water,
they calculated the movements of CO2 (link from below). It turned out that
the ocean surface waters took up a typical molecule of CO2
from the atmosphere within a decade or so. Radiocarbon data also showed
that the oceans turned over completely in several hundred years, an
estimate soon confirmed by evidence from other studies.(31) At first sight that seemed fast enough to sweep any extra
CO2 into the depths.
| But Revelle had been studying the chemistry of the oceans through his entire
career, and he knew that the seas are not just salt water but a complex
stew of chemicals. These chemicals create a peculiar buffering mechanism
that stabilizes the acidity of sea water. The mechanism had been known
for decades, but Revelle now realized that it would prevent the water
from retaining all the extra CO2 it took up.
A careful look showed that the surface layer could not really absorb
much gas barely one-tenth the amount a naïve calculation
would have predicted.
| A supplementary essay on Revelle's Discovery
tells this crucial story in full, as a detailed example of the complex
interactions often found in geophysical research.
| Revelle did not at first
recognize the full significance of his work. He made a calculation
in which he assumed that industry would emit CO2
at a constant rate (like most people at the time, he scarcely grasped
how explosively population and industry were rising). This gave a
prediction that the concentration in the air would level off after
a few centuries, with an increase of no more than 40%. Revelle did
note that greenhouse effect warming "may become significant during
future decades if industrial fuel combustion continues to rise exponentially."
He also wrote that "Human beings are now carrying out a large scale
geophysical experiment of a kind that could not have happened in the
past nor be reproduced in the future."(32)
| As sometimes happens with landmark scientific papers, written in
haste while understanding just begins to dawn, Revelle's explanation
was hard to grasp. Other scientists failed to see the point that was
obscurely buried in the calculations, and continued to deny there
was a greenhouse effect problem. In 1958, when Callendar published
a paper to insist once again that CO2 observations showed a steady rise from the 19th century, he noted
Revelle's paper but still confessed that he did not understand why
"the oceans have not been accepting additional CO2
on anything like the accepted scale."(33) Finally in 1959 two meteorologists
in Sweden, Bert Bolin and Erik Eriksson, caught on. They explained
the sea water buffering clearly so clearly that during the
next few years, some scientists cited Bolin and Eriksson's paper for
this decisive insight rather than Revelle and Suess's (only in later
years was Revelle always cited for the discovery).(34)
The central insight was that although sea water did rapidly absorb
CO2, most of the added gas would promptly evaporate
back into the air before the slow oceanic circulation swept it into
the abyss. To be sure, the chemistry of air and sea water would eventually
reach an equilibrium but that could take thousands of years.
Arrhenius had not concerned himself with timescales shorter than
that, but geoscientists in the 1950s did.
| In the
late 1950s a few American scientists, starting with Plass, tentatively
began to inform the public that greenhouse gases might become a problem
within the next few centuries. Revelle in particular warned journalists
and government officials that greenhouse warming might come within
the foreseeable future, and deserved serious attention. The stakes
were revealed when Bolin and Eriksson pursued the consequences of
their calculation to the end. They assumed industrial production would
climb exponentially, and figured that atmospheric CO2
would rise some 25% by the end of the century. That was a far swifter
rise than anyone before had suggested. In 1962, a still stronger (although
not widely heeded) warning was sounded by the Russian climate expert
Mikhail Budyko. His calculations of the exponential growth of industrial
civilization suggested a drastic global warming within the next century
| Once meteorologists understood that ocean uptake was slow, they
found it possible that CO2 levels had been rising,
just as Callendar insisted.(35) Yet it was only a possibility,
for the measurements were all dubious. By the mid 1950s, researchers
were saying that it was important to measure, much more accurately,
the concentration of CO2 in the atmosphere.(36) A Scandinavian group accordingly set up a network of 15
measuring stations in their countries. Their only finding, however,
was a high noise level. Their measurements apparently fluctuated from
day to day as different air masses passed through, with differences
between stations as high as a factor of two. Only much later was it
recognized that their methods of analyzing the air had been inadequate,
and responsible for much of the noise.(37) A leading authority summarized the
scientific opinion of the late 1950s: "it seems almost hopeless to
arrive at reliable estimates [of CO2]... by such
measurements in limited areas." To find if the gas level was changing,
measurements would have to "be made concurrently and during a great
number of years" at many locations.(38)
| Charles David (Dave)
Keeling held a different view. As he pursued local measurements of
the gas in California, he saw that it might be possible to hunt down
and remove the sources of noise. Technical advances in infrared instrumentation
allowed an order of magnitude improvement over previous techniques
for measuring gases like CO2. Taking advantage
of that, however, would require many costly and exceedingly meticulous
measurements, carried out someplace far from disturbances. Most scientists,
looking at the large and apparently unavoidable fluctuations in the
raw data, thought such precision irrelevant and the instrumentation
too expensive. But Revelle and Suess had enough funds, provided by
the International Geophysical Year, to hire Keeling to measure CO2
with precision around the world.
| A supplementary essay tells the precarious story of Keeling's
funding and monitoring of CO2 levels as a
detailed example of how essential research and measurements might
be fed or starved.
| Revelle's simple aim
was to establish a baseline "snapshot" of CO2
values around the world, averaging over the large variations he expected
to see from place to place and from time to time. After a couple of
decades, somebody could come back, take another snapshot, and see
if the average CO2 concentration had risen. Keeling
did much better than that with his new instruments. With painstaking
series of measurements in the pristine air of Antarctica and high
atop the Mauna Loa volcano in Hawaii, he nailed down precisely a stable
baseline level of CO2 in the atmosphere. In 1960,
with only two full years of Antarctic data in hand, Keeling reported
that this baseline level had risen. The rate of the rise was approximately
what would be expected if the oceans were not swallowing up most industrial
funds soon closed down the Antarctic station, but Keeling managed
to keep the Mauna Loa measurements going with only a short hiatus.
As the CO2 record extended it became increasingly
impressive, each year noticeably higher. Soon Keeling's curve, jagged
but inexorably rising, was widely cited by scientific review panels
and science journalists.(40)
For both scientists and the public it became the primary icon of
the greenhouse effect.
Carbon Dioxide as the Key to Climate
|New carbon-14 measurements were giving scientists
solid data to chew on. They began to work out just how carbon moves
through its many forms in the air, ocean, minerals, soils, and living
creatures. They plugged their data into simple models, with boxes
representing each reservoir of carbon (ocean surface waters, plants,
etc.), and arrows showing the exchanges of CO2
among the reservoirs. The final goal of most researchers was to figure
out how much of the CO2 produced from fossil
fuels was sinking into the oceans, or perhaps was being absorbed by
vegetation (see above).
But along the way there were many curious puzzles, which forced researchers
to make inquiries among experts in far distant fields.
| During the 1960s, these
tentative contacts among almost entirely separate research communities
developed into ongoing interchanges. Scientists who studied biological
cycles of elements such as nitrogen and carbon (typically supported
by forestry and agriculture interests) got in touch with, among others,
geochemists (typically in academic retreats like the Scripps Institution
of Oceanography in La Jolla, California). This emerging carbon-cycle
community began to talk with atmospheric scientists who pursued interests
in weather and climate prediction (typically at government-funded
laboratories like the National Center for Atmospheric Research in
Boulder, Colorado, or the Geophysical Fluid Dynamics Laboratory in
Princeton, New Jersey). One valuable example of this crossover of
interests was a calculation published by Princeton computer specialists
in 1967: the first reasonably solid estimate of the global temperature
change that was likely if the amount of CO2 in
the atmosphere doubled.(41)
|Even before that, in 1965,
a prestigious group of scientists had suggested with remarkable foresight
that "By the year 2000 the increase in atmospheric CO2
... may be sufficient to produce measurable and perhaps marked changes
in climate." But most scientists at this time were scarcely concerned
about CO2 as an agent of future global warming.
They addressed the gas as simply one component in their study of biological,
oceanographic or meteorological systems.(42) Most stuck with the old assumption
that the Earth's geochemistry was dominated by stable mineral processes,
operating on a planetary scale over millions of years. People did
not easily grasp how sensitive the Earth's atmosphere was to biological
forces the totality of the planet's living activity
to say nothing of the small fraction of that activity affected by
| Leading scientists continued to doubt that
anyone needed to worry at all about the greenhouse effect. The veteran
climate expert Helmut Landsberg stressed in a 1970 review that little
was known about how humans might change the climate. At worst, he
thought, the rise of CO2 at the current rate
might bring a 2°C temperature rise over the next 400 years, which
"can hardly be called cataclysmic."(43) Meanwhile Hubert H. Lamb, the outstanding compiler of old
climate data, wrote that the effects of CO2 were
"doubtful... there are many uncertainties." The CO2
theory, he pointed out, failed to account for the numerous large shifts
that he had uncovered in records of climate from medieval times to
the present. Many agreed with Lamb that a "rather sharp decline" of
global temperature since the 1940s put the whole matter
Up to this point, about 1970, I have described a central core of CO2 atmospheric research, which only occasionally interacted with
other subjects. During the 1970s, the greenhouse effect became a major
topic in many overlapping fields. The description of these studies
is distributed among all the topical essays. The remainder of this
essay covers only the developments most directly related to the gas
| Research on changes in the atmosphere's CO2 had been, almost by definition, identical to research on the
greenhouse effect. But in the late 1970s and early 1980s, calculations
found that other gases emitted by human activities also have a strong
greenhouse effect sometimes molecule for molecule tens or hundreds
of times greater than CO2. Global climate change
could not be properly studied without taking into account methane,
emitted by both natural and artificial sources, and various other
industrial gases. Nevertheless most of the scientific interest continued
to revolve around CO2.
| Carbon cycle studies proliferated. A major
stimulus was a controversy that erupted in the early 1970s and stubbornly
resisted resolution. National economic statistics yielded reliable
figures for how much CO2 humanity put into the
air each year from burning fossil fuels. The measurements of the annual
increase by Keeling and others showed that less than half of the new
carbon could be found in the atmosphere. Where was the rest? Oceanographers
calculated how much of the gas the oceans took up, while other scientists
calculated how much the biosphere took up or emitted. The numbers
didn't add up some of the carbon was "missing." Plainly, scientists
did not understand important parts of the carbon cycle. Looking at
large-scale climate changes, such as between ice ages and warm periods,
they turned up a variety of interactions with climate involving plant
life and ocean chemistry. The papers addressing these topics became
| Some scientists took up the old argument that fertilization of plant life by
additional CO2, together with uptake by the oceans,
would keep the level of gas from rising too sharply. Keeling, however,
warned that by the middle of the next century, plants could well reach
their limit in taking up carbon (as every gardener knows, beyond some
point fertilization is useless or even harmful). Further, there would
eventually be so much CO2 in the ocean surface
waters that the oceans would not be able to absorb additional gas
as rapidly as at present.(45) He kept refining and improving his measurements of the
CO2 level in the atmosphere to extract more information.
The curve did not climb smoothly, but stuttered through a large seasonal
cycle, plus mysterious spells of faster and slower growth. It was
only over a long term, say a decade, that the rise was clearly as
inexorable as a tide.(46)
Meanwhile, computer models were coming into better agreement on the
future warming to be expected from increased CO2.
And global temperatures began to rise again. It was getting increasingly
difficult for scientists to believe that the greenhouse effect was
no cause for worry.
| Reader survey: Sorry to interrupt,
but it is really, really important to understand how people might
use this new form of historical text. Would you take just three minutes
to answer a few questions? Please click here.
| An especially convincing finding came from
holes arduously drilled into the Greenland and Antarctic ice caps.
The long cylinders of ice extracted by the drills contained tiny bubbles
with samples of ancient air by good fortune there was this
one thing on the planet that preserved CO2 intact.
Group after group cut samples from cores of ice in hopes of measuring
the level. For two decades, every attempt failed to give consistent
and plausible results. Finally reliable methods were developed. The
trick was to clean an ice sample scrupulously, crush it in a vacuum,
and quickly measure what came out. In 1980, a team published findings
that were definite, unexpected, and momentous.
| In the depths of the last ice age, the level
of CO2 in the atmosphere had been as much as
50% lower than in our own warmer times. (These Greenland measurements
were later called into question, but the dramatically lower ice-age
level was quickly confirmed by other studies.)(47) Pushing forward, by 1985 a French-Soviet
drilling team at Vostok Station in central Antarctica had produced
an ice core two kilometers long that carried a 150,000-year record,
a complete ice age cycle of warmth, cold and warmth. They found that
the level of atmospheric CO2 had gone up and
down in remarkably close step with temperature.(48)
| The Vostok core, an ice driller declared,
"turned the tide in the greenhouse gas controversy."(49)
At the least it nailed down what one expert called an "emerging consensus
that CO2 is an important component in the system of climatic feedbacks."
More generally, he added, it showed that further progress would "require
treating climate and the carbon cycle as parts of the same global
system rather than as separate entities."(50) The rise and fall of temperature was
tied up in a complex way with interlocking global cycles involving
not just the mineral geochemistry of CO2 in air
and sea water, but also methane emissions, the growth and decay of
forests and bogs, changes of the plankton population in the oceans,
and still more features of the planet's biosphere.
| All through these decades, a few geologists had continued to pursue
the original puzzle raised by Tyndall and Chamberlin had changes
of CO2 been responsible for the greatest of climate
changes? These were the vast slow swings, lasting tens of millions
of years, between eras like the age of dinosaurs with summer-like
climates almost from pole to pole, and eras like our own when continental
ice caps waxed and waned. There was no consensus about the causes
of these grand shifts, and it was nearly impossible to reliably measure
the atmosphere many millions of years back. Nevertheless, by the 1980s,
scientists turned up evidence that CO2 levels
had been elevated during the great warm eras of the past.
| Lines of thinking converged to emphasize the importance of the
greenhouse effect. For decades geologists had been puzzled by a calculation
that astrophysicists insisted was undeniable: the Sun had been dimmer
when the Earth was young. Billions of years ago the oceans would have
been permanently frozen, if not for high CO2
levels. Astrophysical theory showed that as the Sun had consumed its
nuclear fuel it had gradually grown brighter, yet somehow the Earth's
temperature had remained neither too cold nor too hot to sustain life.
The best guess was that CO2 acted as a thermostat for the planet. Volcanoes presumably put
the gas into the atmosphere at a fairly constant rate. But chemical
processes run faster at higher temperatures, so on a warmer Earth
the weathering of rocks would take up CO2 faster. As the rocks erode, rivers carry the soil into the seas,
where the carbon eventually winds up in compounds deposited on the
seabed. Thus a rough self-sustaining balance is maintained among the
forces of volcanic emissions, greenhouse warming, weathering, and
ocean uptake.(51) To be sure, the system might take thousands if not millions
of years to stabilize after some great disturbance.
| Such great disturbances even a totally
glaciated "snowball Earth" were not a fantasy of oversimplified
models. Geologists turned up evidence that more than half a billion
years ago the oceans had actually frozen over, if not entirely than
mostly. That seemed impossible, for how could the Earth have escaped
the trap and warmed up again? There was at least one obvious way (but
it was only obvious once someone thought of it, which took decades).
Over many thousands of years, volcanoes would have continued to inject
CO2 into the atmosphere. There the gas would
have accumulated, since it could not get into the frozen seas. Eventually
a colossal greenhouse effect might have melted
the ice.(52*) All this was speculative, and proved
little about recent climates. But it added to the gathering conviction
that CO2 was the very keystone of the planet's
climate system a system by no means as cozily stable as it
Another unusual disturbance had begun. The proof was in the
Vostok team’s 1987 report of their analysis of ice cores reaching
back some 160,000 years, through the entire previous glacial period
and into the warm time before. (And the drill was still only partway
down; by the time they stopped drilling a dozen years later, the
team had recovered ice going back 400,000 years, through four complete
glacial cycles.) The CO2 levels in their
record got as low as 180 parts per million in the cold periods and
reached 280 in the warm periods, never higher. But in the air above
the ice, the level of the gas had reached 350 — far above
anything seen in this geological era and
Level of CO2 in the atmosphere, 1958-2007
The curve has been climbing exponentially (except
in the mid 1990s when the economy of Eastern Europe and the Soviet
Union collapsed). The amount of gas added to the atmosphere is doubling
every 30-35 years. See
latest results from Scripps
Scripps Institution of Oceanography, reproduced by permission.
During the 1990s, further ice core measurements indicated that
during past glacial periods, temperature changes had preceded
CO2 changes by several centuries. Was it necessary to give up the
simple hypothesis that had attracted scientists ever since Tyndall
in the 19th century that changes in CO2
were a simple and direct cause of ice ages? Some scientists doubted
that the time lag could be measured so precisely, but most of the
evidence pointed to a lag.(54) It seemed that rises or falls in carbon dioxide levels
had not initiated the glacial cycles.
In fact most scientists had long since abandoned that hypothesis. In the
1960s, painstaking studies had shown that subtle shifts in our planet's
orbit around the Sun (called "Milankovitch cycles") set
the timing of ice ages. The amount of sunlight that fell in a given
latitude and season varied predictably over millenia, altering how
long snow lingered in the spring, which crucially affected how much
sunlight was absorbed. The fact that carbon dioxide levels lagged
behind the orbital effect should have been no surprise. But now
this could be seen as the first step in a powerful feedback cycle.
For even a small change in the gas level would bring further changes
in the global heat balance, which would in turn alter the gas level,
which... and so forth. This suggested how tiny shifts in the Earth's
orbit could be amplified into the enormous swings of glacial cycles.
Or, more ominously, how a change in the gas level initiated by humanity
might be amplified through a temperature feedback loop.
See the essay on
|There were many ways temperature or other climate
features could influence the carbon dioxide level one way or another.
Perhaps variations of temperature and of weather patterns caused land
vegetation to release extra CO2, or take it
up... perhaps the oceans were involved through massive changes in
their circulation or ice cover... or through changes in their CO2-absorbing
plankton, which would bloom or decline insofar as they were fertilized
by minerals, which reached them from dusty winds, rivers, and ocean
upwelling, all of which could change with the climate... or perhaps
there were still more complicated and obscure effects.
|A key point
stood out. The cycling of carbon through living systems was not something
to trifle with. In the network of feedbacks that made up the climate
system, CO2 was a main driving force. This did not prove by itself that the
greenhouse effect was responsible for the warming seen in the 20th
century. And it did not say how much warming the rise of CO2
might bring in the future. What was now beyond doubt was that the
greenhouse effect had to be taken very seriously indeed.(55)
|By now there were a dozen teams around the
world using computers to integrate every advance in observation or
theory. As the 21st century arrived, the growing agreement among the
rival teams, and the consistency of their models' results with many
different kinds of observations, became overwhelmingly convincing.
Scarcely any reputable expert now doubted that CO2
and other greenhouse gases were at least partly responsible for the
unprecedented warming all around the world since the 1980s. A final
nail in the skeptics' coffin came in 2005, when a team compared computer
calculations with long-term measurements of temperatures in the world's
ocean basins (it was not in the air but the massive oceans, after
all, that most of any heat added soon wound up). In each separate
ocean basin, they showed a close match between observations of rising
temperatures at particular depths, and calculations of where the greenhouse
effect should appear. This was telling evidence that the computer
models were on the right track. Nothing but greenhouse gases could
produce the observed ocean warming — and other changes that
were now showing up in many parts of the world, as predicted.
The computations pinned down an imbalance. The Earth was now taking
in from sunlight nearly a watt per square meter more than it was
radiating back into space, averaged over the planet’s entire
surface. That was enough energy to cause truly serious effects if
it continued. James Hansen, leader of one of the studies, called
it "smoking gun" proof of greenhouse effect
|Yet amid all the uncertainties about how carbon cycles operated,
how much could we trust the computer models? Scientists are more likely
to believe something if they can confirm it with an entirely independent
line of evidence, preferably from somewhere nobody had looked before.
Just such new evidence came up in the 1990s, thanks to an unexpected
alliance of paleontology and plant physiology. Studies of plant species
that had changed little since the rise of the dinosaurs (magnolia
for one) showed that if you exposed them to a higher level of CO2,
the structure of their leaves changed. Ancient fossil leaves showed
just such changes. Several kinds of chemical studies confirmed that
the level of the gas had swung widely over geological ages, and the
and their allies managed to get numbers for the “climate sensitivity”
in ancient eras, that is, the response of temperature to a rise
in the CO2 level. Over hundreds of millions
of years, a doubled level of the gas had always gone along with
a temperature rise of three degrees, give or take a couple of degrees.
That agreed almost exactly with the numbers coming from many computer
|It was good to see that the models had not missed something huge.
There seemed scant possibility of a runaway greenhouse catastrophe.
It was less reassuring to notice what the climate had looked like
in certain ancient times when CO2 had stood
at a high level... a level that humanity would eventually reach if
we went on burning all available oil and coal. The Earth had been
virtually a different planet, with tropical forests near the poles
and sea levels a hundred meters higher. Worse, as one group pointed
out, unchecked emissions seemed bound to bring not only "a warming
unprecedented in the past million years," but changes "much
faster than previously experienced
by natural ecosystems..."(57)
If the planet warmed up by several several degrees during the 21st century,
as paleontologists and computer modelers agreed was likely, what
would be the consequences? This became the new center for most of
the research. It was becoming clear that the consequences would
be severe in many parts of the world, perhaps in some places catastrophic.
See the summary of expected Impacts
of Global Warming.
Through all these discoveries, Keeling and
others had kept on monitoring and analyzing the ongoing changes
in atmospheric CO2 levels. Since the 1980s, a cooperative international program
had been measuring the gas at land stations around the world and
along shipping lanes. The baseline continued to rise ominously,
but not smoothly. There had been years when the world's atmosphere
had gained one billion metric tonnes of the gas, while in other
years it gained as much as six billion. How much did changes in
the world's industries and agricultural practices affect the rate
of the rise? Economic statistics allowed a good reckoning of how
much gas humanity emitted in burning fossil fuels — and also
of some significance, in the manufacture of cement — but the
effects of deforestation and other land use changes were not so
easy to figure.
|Beyond that, how much did changes in the level of CO2
reflect changes in the growth or decay of plants, perhaps related
to some fluctuation in the oceans or on the Sun? What could one learn
from the way the curve reacted to temporary climate changes brought
on by El Niño events, volcanic eruptions, and so forth?(58)
Further clues came from world-wide measurements of other biologically
active gases, especially oxygen (the exacting techniques for measuring
the tiny variations were pioneered by Keeling's son, Ralph Keeling).(59) Most of the "missing" carbon
was finally located, with gradually increasing precision, in rapidly
changing forests, soils, and other biological reservoirs.
| The basic physics and chemistry of the problems raised by Tyndall
were now well in hand. There were reliable calculations of the direct
effects of CO2 on radiation, of how the gas was
dissolved in sea water, and other physical phenomena. Further progress
would center on understanding the complex interactions of the entire
planetary system, and especially living creatures... most of all,
| What can people do about global warming, and
what should we do? See my Personal Note and
Past Cycles: Ice Age Speculations
The Modern Temperature Trend
Simple Models of Climate
Roger Revelle's Discovery
Other Greenhouse Gases
1. Tyndall (1861).
2. Högbom (1894) ;
the essentials are quoted by Arrhenius (1896), pp. 269-73; see also
Berner (1995); for further background, see Arrhenius (1997).
3. He also did computations for 1.5-, 2.5- and 3-fold increases.
Arrhenius (1896), 266; see Crawford
(1996), chap. 10; Crawford (1997); reprinted with further
articles in Rodhe and Charlson (1998).
4. Nernst also noted that the additional CO2 would fertilize crops. James Franck, interview by Thomas Kuhn,
p. 6, Archive for History of Quantum Physics, copies at AIP and other repositories.
5. Arrhenius (1896); revised
calculations, finding a somewhat lower effect, were given in Arrhenius (1901) ; popularization: Arrhenius (1908), chap. 2.
6. The following discussion to ca. 1960 is taken
with some changes from a published study that includes some additional
discussion and references, Weart (1997). BACK
7. Ångström (1900).
8. Ångström (1900),
pp. 731-32; Abbot and Fowle (1908), pp. 172-73;
for spectrographs, e.g., Weber and Randall (1932).
8a. Koch had only a thermocouple to measure
heat across the entire infrared spectrum. He accurately reported that
about 10% of the radiation from a 100°C black body was absorbed in
his tube, and that at lower pressure at most 9.6% was absorbed, whereas
in fact it must have been about 9%. For the modern calculation I thank
Raymond T. Pierrehumbert. BACK
9. Fleming (2000), p.
301; for early measurements and additional background and references,
see Mudge (1997). Ångström’s argument was
immediately accepted in for example, Monthly Weather
Review (1901). A leading expert dismissing CO2
because of saturation was Humphreys (1913),
pp. 134-35, although while denying that doubling the amount in the atmosphere
would "appreciably affect the total amount of radiation actually absorbed,"
he did note that it would "affect the vertical distribution or location
of the absorption," Humphreys (1920), p. 58; on CO2 saturation,
Schaefer (1905), p. 104
10. Hulburt (1931), quote p.
1876; note also Simpson (1928), who finds CO2 adds a correction but only a small one to water
11. Brooks (1951), p. 1016.
12. Redfield (1958), 221. The
atmospheric elements he addressed were oxygen and other elementary gases, not carbon.
13. Hutchinson (1948), quote
p. 228; see also Hutchinson (1954), 389-90; another example:
Eriksson and Welander (1956), 155.
14. (Obituary) (1965);
Fleming (2007). BACK
15. Callendar (1938); see also
Callendar (1940); Callendar
(1939); Callendar (1949).
16. For bibliography on CO2
measurements and ideas to 1951, see Stepanova (1952); criticism: Slocum
(1955); Fonselius et al. (1956); however,
some evidence for a gradual increase was summarized by Junge
(1958); measurements are reviewed by Bolin (1972);
From and Keeling (1986). BACK
17. E.g., it "may require a period of [data] collection of many
decades to detect the real trends" according to Eriksson and Welander
18. Callendar (1940).
19. Lotka (1924).
20. Callendar (1941).
21. Russell (1941), 94.
22. For additional discussion and references, see Weart (1997).
23. This section is condensed from a more detailed published
study, Weart (1997).
24. Martin and Baker (1932);
for review, see Smith et al. (1968), pp. 476-483.
25. Kaplan (1952); for other
workers see, e.g., Möller (1951), pp. 46-47.
26. Plass (1956); see also Plass (1956).
27. Rossby (1959), p. 14; the
chief critic was Kaplan (1960).
28. Plass (1956), quotes on
306, 311, 315, 316; see also Plass (1959); Plass (1956); Plass (1956).
29. Plass (1956); Dingle (1954).
30. Suess (1955); see also Suess (1953); a confirmation: Münnich (1957); Revision: Houtermans et al. (1967), see p. 68.
31. Revelle and Suess (1957);
Craig (1957); Arnold and Anderson
32. Revelle and Suess (1957),
pp. 18-20, 26.
33. Callendar (1958),
p. 246. Here Callendar was one of those who quickly picked up Revelle's
"geophysical experiment" phrase. A typical denial of any future greenhouse
effect problem was Bray (1959), see p. 228. BACK
34. Bolin and Eriksson (1959);
example of paper citing Bolin & Eriksson but not Revelle: Mitchell (1961), p. 243; review citing them: Skirrow (1965), pp. 282-84, 308.
35. e.g., Mitchell (1961).
36. Eriksson (1954).
37. Fonselius et al. (1955); Fonselius et al. (1956); for critique, see From and Keeling (1986), p. 88, and passim for history of
CO2 measurements generally; also Keeling
(1998), p. 43.
38. Rossby (1959), p. 15; this
is a translation of Rossby (1956).
39. The paper also described the seasonal cycle of CO2 emissions. Keeling (1960); in the
1970s, it was found that the 1959-1960 rise had been exaggerated by an unusual natural release
of the gas related to an El Niño event. Keeling (1998);
for the history, see also Keeling (1978).
40. For example in the landmark "SMIC" report,
Wilson and Matthews (1971), p. 234.
41. Manabe and Wetherald
42. President's Science Advisory
Committee (1965); Hart and Victor (1993),
43. Landsberg (1970).
44. Lamb (1969), p. 245.
45. The partial pressure of CO2 in
sea water would grow and the chemical buffering would
change. Keeling (1973), p. 291.
46. E.g., Keeling et al. (1976).
47. Berner et al. (1980); Delmas et al. (1980); Neftel et al.
(1982); Shackleton et al. (1983).
48. Lorius et al. (1985); see
also Barnola et al. (1987); Genthon
et al. (1987).
49. Mayewski and White
(2002), pp. 39, 77.
50. Sundquist (1987).
51. Walker et al. (1981).
52. Also, the discovery in the late 1970s that life
is sustained at hot springs in the deep ocean showed that the continuous
fossil record of sea life did not rule out the possibility that the oceans
had frozen. Studies include Berner et al. (1983);
Kasting and Ackerman (1986); for review, Crowley and North (1991); more recently, Hoffmann et al. (1998); the term "snowball Earth" was coined
by Joseph Kirschvink Kirschvink (1992); earlier
Manabe called it "White Earth" according to Gleick
(1987), p. 332; for references and popular-level discussion, see Ward
and Brownlee (2000). BACK
53. Genthon et al. (1987);
Petit et al. (1999). BACK
54. Shackleton (2000);
changes of CO2 preceding changes in ice sheet volume were
reported in Shackleton and Pisias (1985).
55. Petit et al. (1999);
IPCC (2001), p. 202; Lorius et
al. (1990); Pälike et al. (2006) (detecting
the carbon cycle feedback over millions of years). BACK
56. Barnett et al. (2005);
Hansen et al. (2005). BACK
57. A recent review: Royer
et al. (2007). Among many references: Berner
(1991) (geochemical); Cerling (1991) (carbon
in soils); McElwain and Chaloner (1995) (leaves);
Royer et al. (2001). Quote: Hoffert
and Covey (1992), p. 576. BACK
58. Keeling (1998).
59. Keeling et
al. (1993); Keeling et al. (1996). BACK
© 2003-2007 Spencer Weart & American Institute of Physics