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The Modern Temperature Trend
Tracking the world's average temperature from the late 19th century,
people in the 1930s realized there had been a pronounced warming trend.
During the 1960s, scientists found that over the past couple of decades
the trend had shifted to cooling. Many scientists predicted a continued
and prolonged cooling, perhaps a phase of a long natural cycle or perhaps
caused by human activities. Others insisted that humanity's emission of
gases would bring warming over the long run. In the late 1970s, this group's
views became predominant. By the late 1980s, it was plain that the cooling
spell, whose cause remained mysterious, had been a temporary distraction.
For whatever reason, unprecedented global warming was underway.
"The subject... is a vast one, and only
too easily submerged in an ocean of repelling statistics, unless firm
measures are taken to reduce the mass of data into a form which eliminates
distracting or irrelevant detail..." G.S. Callendar(1)
climate change, urban heat island, cities, Little Ice Age, Medieval Warm Period,
satellites, upper atmosphere, hockey stick
If you had a certain type of mind, temperature
statistics could be more absorbing than a book of crossword puzzles.
Ever since the invention of the thermometer, some amateur and professional
scientists had recorded the temperature wherever they happened to
be living or visiting. During the 19th century, government weather
services began to record measurements more systematically. By the
1930s, observers had accumulated millions of numbers for temperatures
at stations around the world. It was an endlessly challenging task
to weed out the unreliable data, average the rest in clever combinations,
and compare the results with other weather features such as droughts.
Many of the players in this game pursued a hope of discovering cycles
of weather that could lead to predictions. Perhaps, for example, one
could correlate rainfall trends with the eleven-year sunspot cycle.
|
- LINKS -
More discussion in
<=>Solar variation
|
Adding interest to
the game was a suspicion that temperatures had generally increased
since the late 19th century at least in eastern North America
and western Europe, the only parts of the world where reliable measurements
went back so far.(2) In the 1930s, the press began to call attention to numerous
anecdotes of above-normal temperatures. The head of the U.S. Weather
Bureau's Division of Climate and Crop Weather responded in 1934. "With
'Grand-Dad' insisting that the winters were colder and the snows deeper
when he was a lad," he said, "...it was decided to make a rather exhaustive
study of the question." Averaging results from many stations in the
eastern United States and some scattered locations elsewhere around
the world, the weather services found that 'Grand-Dad' was right:
average temperatures had risen several degrees Fahrenheit (°F)
since 1865 in most regions. Experts thought this was simply one phase
of a cycle of rising and falling temperatures that probably ambled
along for centuries. As one scientist explained, when he spoke of
the current "climate change" he did not mean any permanent
shift, but a long-term cyclical change "like all other climate fluctuations."(3)
|
<=>Public opinion
= Milestone
|
It may have been the press reports of warming that stimulated an English
engineer, Guy Stewart Callendar, to take up climate study as an amateur
enthusiast. He studied global temperature change systematically and
thoroughly. A 19th-century German had made an attempt at this in seeking
a connection with sunspot cycles. Otherwise, if anyone else had thought
about it, they had probably been discouraged by the scattered and
irregular character of the weather records, plus the common assumption
that the average climate scarcely changed over the span of a century.
After countless hours of sorting out data and penciling sums, Callendar
announced that the temperature had definitely risen between 1890 and
1935, all around the world, by close to half a degree Celsius (0.5°C,
equal to 0.9°F).(4) Callendar's statistics gave him confidence
to push ahead with another and more audacious claim. Reviving an old
theory that human emissions of carbon dioxide gas (CO2)
from burning fuel could cause a "greenhouse effect," Callendar said
this was the cause of the warming. (For the old theory, follow
the link in the righthand column from the essay on Simple
Models of Climate. For scientific views of Callendar's day
on the theory, follow the link to the essay on The CO2
Greenhouse Effect.) |
=>CO2 greenhouse
Callendar's
warming
<=Simple
models |
It all sounded dubious to most meteorologists.
Temperature data were such a mess of random fluctuations that with
enough manipulation you could derive all sorts of spurious trends.
Taking a broader look, experts believed that climate was comfortably
uniform. "There is no scientific reason to believe that our climate
will change radically in the next few decades," the highly respected
climatologist Helmut Landsberg explained in 1946. "Good and poor years
will occur with approximately the same frequency as heretofore."(5) If during some decades there was an unmistakable climate
change in some region, the change must be just part of some local
cycle, and in due time the climate of the region would revert to its
average. |
<=Simple models
|
(By the end of the 20th century, scientists
were able to check Callendar's figures. They had done far more extensive
and sophisticated analysis of the weather records, confirmed by "proxy"
data such as studies of tree rings and measurements of old temperatures
that lingered in deep boreholes. The data showed that the world had
in fact been warming from the mid 19th century up to about 1940, mostly
because of natural fluctuations. As it happened, most of the warming
had been in the relatively small patch of the planet that contained
the United States and Europe and thus contained the great majority
of scientists and of those who paid attention to scientists. But for
this accident, it is not likely that people would have paid attention
to the idea of global warming for another generation. That would have
severely delayed our understanding of what we face.) |
<=>CO2 greenhouse
|
During the 1940s only a few people looked into the question of
warming. A prominent example was the Swedish scientist Hans Ahlmann,
who voiced concern about the strong warming seen in some northern
regions since early in the century. But in 1952, he reported that
northern temperatures had begun to fall again since around 1940.(6) The argument for warming caused by CO2 emissions, another eminent climatologist wrote in 1949, "has
rather broken down in the last few years" when temperatures in some
regions fell.(7) In any case (as yet another authority
remarked), compared with the vast slow swings of ice ages, "the recent
oscillations of climate have been relatively small."(8)
|
|
If the North Atlantic region
was no longer warming, through the 1940s and 1950s it remained balmy
in comparison with earlier decades. People were beginning to doubt
the assumption of climate stability. Several scientists published
analyses of weather records that confirmed Callendar's finding of
an overall rise since the 1880s.(9) An example was a careful study of U.S. Weather Bureau data
by Landsberg, who was now the Bureau's chief climatologist. The results
persuaded him to abandon his belief that the climate was unchanging.
He found an undeniable and significant warming in the first half of
the century, especially in more northern latitudes. He thought it
might be due either to variations in the Sun's energy or to the rise
of CO2.(10) Others pitched in with reports of effects
plain enough to persuade attentive members of the public. Ahlmann
for one announced that glaciers were retreating, crops were growing
farther north, and the like.(11) Another striking example was a report
that in the Arctic "the ice is thinner at the present than ever before
in historic times," and before long we might even see an open polar
sea.(12) Such high-latitude effects were exactly what simple models
suggested would result from the greenhouse effect warming of increased
CO2. |
=>Public opinion
=>Simple
models
=>Aerosols
=>Chaos theory |
"Our attitude to climatic 'normals' must
clearly change," wrote the respected climate historian Hubert H. Lamb
in 1959. Recent decades could not be called normal by any standard
of the past, and he saw no reason to expect the next decades would
be "normal" either. Actually, since the 1930s the temperatures in
his own homeland, Britain, had been heading down, but Lamb would not
speculate whether that was the start of a cyclical downtrend. It could
be "merely another wobble" in one region. Lamb's main point, reinforced
by his scholarly studies of weather reports clear back to medieval
times, was that regional climate change could be serious and long-lasting.(13)
Most meteorologists nevertheless stuck to their belief that the only
changes to be expected were moderate swings in one part of the world
or another, with a fairly prompt return to the long-term average.
If there was almost a consensus that for the time being there was
a world-wide tendency to warming, the agreement was fragile. |
<=>Climatologists
|
In January 1961, on a snowy and unusually
cold day in New York City, J. Murray Mitchell, Jr. of the U.S. Weather
Bureau's Office of Climatology told a meeting of meteorologists that
the world's temperature was falling. Independently of Callendar, Mitchell
had trudged through all the exacting calculations, working out average
temperatures for most of the globe, and got plausible results. Global
temperatures had indeed risen until about 1940, Mitchell said. But
since then, temperatures had been falling. There was so much random
variation from place to place and from year to year that the reversal
to cooling had only now become visible.(14*)
|
=>Solar variation |
Acknowledging that the increasing amount of CO2 in the atmosphere
should give a tendency for warming, Mitchell tentatively suggested
that the reversal might be partly caused by smoke from volcanic eruptions
and perhaps cyclical changes in the Sun. But "such theories appear
to be insufficient to account for the recent cooling," and he could
only conclude that the downturn was "a curious enigma." He suspected
the cooling might be part of a natural "rhythm," a cycle lasting 80
years or so.(15) The veteran science correspondent Walter Sullivan was at
the meeting, and he reported in the New York Times (January
25 and 30, 1961) that after days of discussion the meteorologists
generally agreed on the existence of the cooling trend, but could
not agree on a cause for this or any other climate change. "Many schools
of thought were represented... and, while the debate remained good-humored,
there was energetic dueling with scientific facts." The confused state
of climate science was a public embarrassment. |
=>Public opinion
=>CO2 greenhouse
=>Simple models
= Milestone
|
Through the 1960s and into the 1970s, the
average global temperature remained relatively cool. Western Europe
in particular suffered some of the coldest winters on record. (Studies
in later decades found that a quasi-regular long-term weather cycle
in the North Atlantic Ocean had moved into a phase in the 1960s that
encouraged Arctic winds to move southward there.)(16) People will always give special attention
to the weather that they see when they walk out their doors, and what
they saw made them doubt that global warming was at hand. Experts
who had come to suspect greenhouse warming now began to have doubts.
Callendar found the turn worrisome, and contacted climate experts
to discuss it.(17) Landsberg returned to his earlier view that the climate
was probably showing only transient fluctuations, not a rising trend.
While pollution and CO2 might be altering the climate in limited regions, he wrote, "on
the global scale natural forces still prevail." He added, however,
that "this should not lead to complacency" about the risk of global
changes in the distant future.(18) |
temperature
hump
=>Aerosols
|
One source of confusion was increasingly debated. Weather watchers
had long recognized that the central parts of cities were distinctly
warmer than the surrounding countryside. In urban areas the absorption
of solar energy by smog, black roads and roofs, along with direct
outpouring of heat from furnaces and other energy sources, created
an "urban heat island" effect. This was the most striking of all human
modifications of local climates. It could be snowing in the suburbs
while raining downtown.(19)
Some people pushed ahead to suggest that as human civilization used
ever more energy, in a century or so the direct output of heat could
be great enough to disturb the entire global climate.(20) If so, that would not happen soon, and for the moment the
main consequences were statistical. |
|
Some experts began to ask whether the warming reported for the
decades before 1940 had been an illusion. Most temperature measurements
came from built-up areas. As the cities grew, so did their local heating,
which might have given a spurious impression of global warming.(21*) Callendar and others replied that
they were well aware of urban effects, and took them fully into account
in their calculations. Mitchell in particular agreed that population
growth could explain the "record high" temperatures often reported
in American cities but it could not explain the warming of
remote Arctic regions.(22*) Yet the statistical difficulties were
so complex that the global warming up to 1940 remained in doubt. Some
skeptics continued to argue that the warming was a mere illusion caused
by urbanization. |
|
While neither scientists
nor the public could be sure in the 1970s whether the world was warming
or cooling, people were increasingly inclined to believe that global
climate was on the move, and in no small way. The reassuring assumption
of a stable "normal" climate was rarely heard now. In the early 1970s,
a series of ruinous droughts and other exceptionally bad spells of
weather in various parts of the world provoked warnings that world
food stocks might run out. Fears increased that somehow humanity was
at fault for the bad weather if we were not causing global
warming with greenhouse gases, then perhaps we were cooling the globe
with our smoke and smog. Responding to public anxieties, in 1973 the
Japan Meteorological Agency sent a questionnaire to meteorological
services around the world. They found no consensus. Most agencies
reported that they saw no clear climate trend, but several (including
the Japanese themselves) noted a recent cooling in many regions. Many
experts thought it likely that the world had entered a long-term cool
spell.(23) |
<=Public opinion
<=Aerosols
|
Public pressure was urging scientists to declare where the climate was going.
But they could not do so without knowing what caused climate changes.
Haze in the air from volcanoes might explain some cooling, but not
as much as was observed. As for air pollution from human sources,
most experts doubted we were putting out enough to affect global climate.
A more acceptable explanation was a traditional one: the Earth was
responding to long-term fluctuations in the Sun's output of
energy.(24) |
<=Solar variation |
An alternative explanation was found in the "Milankovitch" cycles, tens of
thousands of years long, that astronomers calculated for minor variations
in the Earth's orbit. These variations brought cyclical changes in
the amount of sunlight reaching a given latitude on Earth. In 1966,
a leading climate expert analyzed the cycles and predicted that we
were starting on the descent into a new ice age.(25)
In the early 1970s, a variety of measurements pinned down the nature
and timing of the cycles as actually reflected in past climate shifts.
Projecting the cycles forward strengthened the prediction. A gradual
cooling was astronomically scheduled over the next few thousand years.(26)
Unless, that is, something intervened. |
<=Climate cycles
<=Climate
cycles |
It scarcely mattered
what the Milankovitch orbital changes might do, wrote Murray Mitchell
in 1972, since "man's intervention... would if anything tend to prolong
the present interglacial." Human industry would prevent an advance
of the ice by blanketing the Earth with CO2. A panel of top experts convened by the National Academy of Sciences
in 1975 tentatively agreed with Mitchell. True, in recent years the
temperature had been dropping (perhaps as part of some unknown "longer-period
climatic oscillation"). Nevertheless, they thought CO2
"could conceivably" bring half a degree of warming by the end of the
century.(27) The outspoken
geochemist and oceanographer Wallace Broecker went farther. He suspected
that there was indeed a natural cycle responsible for the cooling
in recent decades, perhaps originating in cyclical changes on the
Sun. If so, it was only temporarily canceling the greenhouse warming.
Within a few decades that would climb past any natural cycle. "Are
we on the brink of a pronounced global warming?" he asked.(28*) |
<=Solar variation
=>Public
opinion
|
Meanwhile in 1975, two New Zealand scientists
reported that while the Northern Hemisphere had been cooling over
the past thirty years, their own region, and probably other parts
of the Southern Hemisphere, had been warming.(29) There were too few weather stations in the vast unvisited
southern oceans to be certain, but other studies tended to confirm
it. The cooling since around 1940 had been observed mainly in northern
latitudes. Perhaps cooling from industrial haze counteracted the greenhouse
warming there? After all, the Northern Hemisphere was home to most
of the world's industry. It was also home to most of the world's population,
and as usual, people had been most impressed by the weather where
they lived.(30*) |
=>Aerosols |
If there had almost been a consensus in the early 1970s that the entire
world was cooling, the consensus now broke down. Science journalists
reported that climate scientists were openly divided, and those who
expected warming were increasingly numerous. In an attempt to force
scientists to agree on a useful answer, in 1977 the U.S. Department
of Defense persuaded two dozen of the world's top climate experts
to respond to a complicated survey. Their main conclusion was that
scientific knowledge was meager and all predictions were unreliable.
The panel was nearly equally divided among three opinions: some thought
further cooling was likely, others suspected that moderate greenhouse
warming would begin fairly soon, and most of the rest expected the
climate would stay about the same at least for the next couple of
decades. Only a few thought it probable that there would be considerable
global warming by the year 2000 (which was what would in fact happen).(31) |
=>Public opinion
=>Public
opinion
|
Government officials and scientists needed more definite statements
on what was happening to the weather. Thousands of stations around
the world were turning out daily numbers, but these represented many
different standards and degrees of reliability a disorderly,
almost indigestible mess. Around 1980 two groups undertook to work
through the numbers in all their grubby details, rejecting sets of
uncertain data and tidying up the rest. |
|
One group was in New York, funded by NASA and led by James Hansen.
They understood that the work by Mitchell and others mainly described
the Northern Hemisphere, since that was where the great majority of
reliable observations lay. Sorting through the more limited temperature
observations from the other half of the world, they got reasonable
averages by applying the same mathematical methods that they had used
to get average numbers in their computer models of climate. (After
all, Hansen remarked, when he studied other planets he might judge
the entire planet by the single station where a probe had landed.)
In 1981, the group reported that "the common misconception that the
world is cooling is based on Northern Hemisphere experience to 1970."
Just around the time that meteorologists had noticed the cooling trend,
such as it was, it had apparently reversed. From a low point in the
mid 1960s, by 1980 the world had warmed some 0.2°C.(32)
|
|
Hansen's group looked into the causes of
the fluctuations, and they got a rather good match for the temperature
record using volcanic eruptions plus solar variations. Greenhouse
warming by CO2 had not been a major factor (at
least, not yet). More sophisticated analyses in the 1990s would eventually
confirm these findings. From the 1940s to the early 1960s, the Northern
Hemisphere had indeed cooled while temperatures had held roughly steady
in the south. This was largely because of normal variations in natural
forces, perhaps including changes in the Sun’s output, although
industrial aerosol pollution had helped. Then the warming had resumed
in both hemispheres. |
<=Aerosols
|
The temporary northern cooling had been bad
luck for climate science. By feeding skepticism about the greenhouse
effect, while provoking some scientists and many journalists to speculate
publicly about the coming of a new ice age, the cool spell gave the
field a reputation for fecklessness that it would not soon live down.
|
=>Public opinion |
Any greenhouse warming had been masked by chance fluctuations
in solar activity, by pulses of volcanic aerosols, and by increased
haze from pollution. Furthermore, as a few scientists pointed out,
the upper layer of the oceans must have been absorbing heat. These
effects could only delay atmospheric warming by a few decades. Hansen's
group boldly predicted that considering how fast CO2
was accumulating, by the end of the 20th century "carbon dioxide warming
should emerge from the noise level of natural climatic variability."
Around the same time, a few other scientists using different calculations
came to the same conclusion the warming would show itself clearly
sometime around 2000.(33*) |
<=The oceans
=>Government
<=>Solar variation
= Milestone |
The second important group analyzing global temperatures
was the British government's Climatic Research Unit at the University
of East Anglia, led by Tom Wigley and Phil Jones. Help in assembling
data and funding came from American scientists and agencies. The British
results agreed overall with the NASA group's findings the world
was getting warmer. In 1982, East Anglia confirmed that the cooling
that began in the 1940s had turned around by the early 1970s. 1981
was the warmest year in a record that stretched back a century.(34*) Returning to old records, in 1986
the group produced the first truly solid and comprehensive global
analysis of average surface temperatures (including the vast ocean
regions, which most earlier studies had neglected). They found considerable
warming from the late 19th century up to 1940, followed by some regional
cooling in the Northern Hemisphere but roughly level conditions overall
to the mid-1970s. Then the warming had resumed with a vengeance. The
warmest three years in the entire 134-year record had all occurred
in the 1980s.(35) Convincing
confirmation came from Hansen and a collaborator, who analyzed old
records using quite different methods from the British, and came up
with substantially the same results. It was true: an unprecedented
warming was underway, at least 0.5°C in the past century.(36)
|
= Milestone
=>International
=>CO2 greenhouse
=>Public opinion
=>Simple models |
In such publications, the few pages of text
and numbers were the visible tip of a prodigious unseen volume of
work. Many thousands of people in many countries had spent most of
their working lives measuring the weather. Thousands more had devoted
themselves to organizing and administering the programs, improving
the instruments, standardizing the data, and maintaining the records
in archives. In geophysics not much came easily. One simple sentence
(like "last year was the warmest year on record") might be the distillation
of the labors of a multi-generational global community. And it still
had to be interpreted. |
<=International |
Most experts saw no solid proof that continued warming lay in the
future. After all, reliable records covered barely a century and showed
large fluctuations (especially the 1940-1970 dip). Couldn't the current
trend be just another temporary wobble? Stephen Schneider, one of
the scientists least shy about warning of climate dangers, acknowledged
that "a greenhouse signal cannot yet be said to be unambiguously detected
in the record." Like Hansen and some other scientists, he expected
that the signal would emerge clearly around the end of the century,
but not earlier.(37)
|
|
Global average temperatures
1860-2002 (difference from 1961-1990 normals, °C),
using air measurements at land stations and sea surface temperatures measured
by ships and buoys.
From the Hadley
Centre, U.K. © Crown copyright 2003, see the site for updated
data
After 1988 |
=>after88 |
A new major effort to track global temperature trends, joining
the work by groups in New York and East Anglia, was getting underway
at NOAA's National Climatic Data Center in Asheville, North Carolina.
The Center had been established in 1951 as the National Weather Records
Center, with the task of organizing the data that the Weather Bureau
and military services had accumulated since the 1940s. The staff had
assembled the world's largest collection of historical weather records.
A team led by Thomas Karl tediously reviewed the statistics for the
world and especially the United States. |
|
Each of the three groups began to issue annual updates, which the press
reported prominently. When all the figures were in for 1988, the year
proved to be a record-breaker (now the 1980s included the four warmest
years since global measurements began). But in the early 1990s, average
global temperatures dipped. Most experts figured the cause was the
huge 1991 Pinatubo volcanic eruption, whose emissions dimmed sunlight
around the world. After rains washed out the volcanic aerosols, the
temperature rise resumed. 1995 was the warmest year on record, but
that was topped by 1997, and 1998 beat that in turn by a surprisingly
large margin. Of course these were global averages of trends that
varied from one region to another. The citizens of the United States,
and in particular residents of the East Coast, had not felt the degree
of warming that came in some other parts of the world. But for the
world as a whole, for the first time most experts now agreed: a serious
warming trend was underway.(38*) |
=>Public opinion
<=>Aerosols
= Milestone
=>Public opinion
|
This consensus was sharply attacked by a few scientists. Some pulled
out the old argument that the advance of urbanization was biasing
temperature readings. In fact, around 1990 meticulous re-analysis
of old records had squeezed out the urban heat-island bias to the
satisfaction of all but the most stubborn critics. Moreover, long-term
warming trends showed up in various kinds of physical "proxy" data
measured far from cities. To be sure, in urban areas whatever global
warming the greenhouse effect might be causing got a strong addition
of heat, so that the combination significantly raised the mortality
from heat waves. But the larger global warming trend was no statistical
error.(39*) |
|
With the urbanization argument discredited,
the skeptics turned to measurements by satellites that monitored the
Earth. Since 1979, when the first of these satellites was launched,
they had provided the first truly comprehensive set of global temperature
data. The instruments did not measure temperatures on the surface,
but at middle heights in the atmosphere. At these levels, analysis
of the data indicated, there had been no rise of temperature, but
instead a slight cooling. The satellites were designed for observing
daily weather fluctuations, not the average that represented climate,
and it took an extraordinarily complex analysis to get numbers that
showed long-term changes. The analysis turned out to have pitfalls.
Some argued against the greenhouse skeptics that the satellite data
might even show a little warming.(40) |
=>Public opinion
|
In an attempt to settle the controversy, a panel of the National
Academy of Sciences conducted a full-scale review in 1999. The panel
concluded that the satellites seemed reliable (balloon measurements,
although far less comprehensive, also failed to find warming in mid-atmosphere).
The satellite instruments simply were not designed to see the warming
that was indeed taking place at the surface. The fact that the satellite
measurements indicated that middle layers of the atmosphere had not
noticeably warmed was embarrassing to the scientists who were constructing
computer models of climate, for their models predicted significant
warming there. They suspected the discrepancy could be explained by
temporary effects — volcanic eruptions such as Pinatubo, or
perhaps the chemical pollution that was depleting the ozone layer.
The skeptics persisted. But most scientists concluded that while the
computer models were surely imperfect, the satellite data analysis
was too ambiguous to pose a serious challenge to the global warming
consensus. (This hunch was confirmed in 2004 when a new analysis of
the data showed that the mid levels had in fact been warming much
as predicted. "This is the answer — I wish we had recognized
it ourselves," said the chair of the 1999 Academy survey. Richard
Kerr, the veteran climate reporter for Science magazine,
declared that "satellite temperatures can no longer be used to
portray a feeble greenhouse effect.")(41*)
|
= Milestone
=>Models (GCMs) |
By the late 1990s, many types of evidence showed a general warming
at ground level. For example, the Northern Hemisphere spring was
coming on average a week earlier than in the 1970s. This was confirmed
by such diverse measures as earlier dates for bud-break in European
botanical gardens, and a decline of Northern Hemisphere snow cover
in the spring as measured in satellite pictures. Turning to a more
fundamental indicator, the temperature of the upper layer of the
oceans where nearly all the heat entering the climate system
was stored again a serious rise was found in recent decades, and the greenhouse effect was the
only plausible cause.(42*) (Link from below) The 1990s were
unquestionably the warmest decade since thermometers came into common
use, and the trend was accelerating.
(We see this ourselves, who have lived
long enough. My home happens to be near where I grew up five decades
ago in New York State. While it’s easy to fool yourself on
such matters, my personal impressions agree with the statistics
on the Northeast that report a long-term trend of less snowfall,
an earlier spring and higher temperatures in general. And I have
stood on a Canadian glacier that was visibly in retreat.) |
=>Simple models
<=The oceans
|
Most people now took it for granted that the
cause was greenhouse warming, but critics pointed out that other things
might be responsible. After all, the greenhouse effect could not have
been responsible for much of the warming that had come between the
1890s and 1940, when industrial emissions had still been modest. So
announcements that a given year was the warmest on record, when the
record had started during the 19th-century cold spell, might not mean
as much as people supposed. The warming up to 1940 (and the dip that
followed until the 1970s) might have been caused by long-term cycles
in ocean currents, or by variations in the Sun’s radiation.
There were also decades-long fluctuations in the atmosphere-ocean
system and in the global pattern of winds, which drove gradual variations
in regional weather patterns. These had been suspected since the 1920s,
but only started to become clear in the late 1990s. Until these possibilities
were sorted out, the cause of the ground-level warming since 1970
would remain controversial. |
<=The oceans
|
However, "fingerprints" were found that pointed directly to greenhouse
warming. One measure was the difference of temperature between night
and day. Tyndall had pointed out more than a century back that basic
physics declared that the greenhouse effect would act most effectively
at night, as the gases impeded radiation from escaping into space.
Statistics did show that it was especially at night that the world
was warmer.
No less convincing, Arrhenius at the turn of the century, and everyone
since, had calculated that the Arctic would warm more than other
parts of the globe. That was largely because less snow and ice would
reflect less sunlight back into space. (This effect would not be
expected in Antarctica, with its colossal year-round ice cover,
and in fact warming was not seen there — except around the
coasts and on the long peninsula that projected beyond the ice sheet).
Arctic warming was glaringly obvious to scientists as they watched
trees take over mountain meadows in Sweden and the Arctic Ocean's
ice pack grow ever thinner. Alaskans and Siberians didn't need statistics
to tell them the weather was changing when they saw buildings sag
as the permafrost that supported them melted. |
<=Simple models
Ecosystem
changes
<=>Public opinion |
Pursuing this in a more
sophisticated way, computer models predicted that greenhouse gases
would cause a particular pattern of temperature change. It was different
from what might be caused by other external influences, such as solar
variations. The observed geographical pattern of change did in fact
bear a rough resemblance to the computers' greenhouse effect maps.
"It is likely that this trend is partially due to human activities,"
researchers concluded, "although many uncertainties remain."(43)
Even before it was published, the finding impressed the community
of climate scientists. In an important 1995 report, the world’s
leading experts offered the “fingerprint” as evidence
that greenhouse warming was truly underway. The leader of the team
at Lawrence Livermore Lab that found the “fingerprint,”
Benjamin Santer, helped write the summary of this report, and he was
deeply hurt when a few skeptics attacked not only the statement but
his personal scientific integrity. |
=>Models (GCMs)
<=>International
|
The skeptics, including a minority of climate
experts, continued to doubt that humans were causing global warming.
Santer’s model, like all models, admittedly relied on a lot
of guesswork. Or perhaps subtle changes involving the Sun (detectable
only with sophisticated instruments), or something else, had somehow
triggered changes in cloud cover or the like to mimic the strong night-time
and Arctic warming and other features of the greenhouse fingerprint?
Yet even if that were true, it just went to show how sensitive the
climate must be to delicate shifts in the forces at work in the atmosphere. |
<=Solar
variation |
A variety of new evidence suggested that the recent warming was
exceptional even if one looked back many centuries. Beginning in the
1960s, a few historians and meteorologists had labored to discover
variations of climate by digging through historical records of events
like freezes and storms. For example, had the disastrous harvest of
1788 helped spark the French Revolution?Scholars found it difficult
to derive an accurate picture, let alone quantitative data, from old
manuscripts. Increasingly laborious projects hacked away at the problem
(as one of many examples, by 2004 an international team had analyzed
hundreds of thousands of weather observations recorded in 18th and
19th century ships' logs). Another kind of data came from physical
analysis of tree rings, coral reefs, and other ingenious proxy measures,
which produced increasingly reliable numbers.(44) |
|
One important example was a uniquely straightforward method, the measurement
of old temperatures directly in boreholes. Data from various locations
in Alaska, published in 1986, showed that the top 100 meters of permafrost
was anomalously warm compared with deeper layers. The only possible
cause was a rise of average air temperature by a few degrees since
the last century, with the heat gradually seeping down into the earth.(45) In a burst of enthusiasm during the
1990s, scientists took the temperature of hundreds of deep boreholes
in rock layers around the planet. The averages gave a clear signal
of a recent rise in northern regions. A still more important example
of the far-flung efforts was a series of heroic expeditions that labored
high into the thin air of the Andes and even Tibet, hauling drill
rigs onto tropical ice caps. The hard-won data showed again that the
warming in the last few decades was greater than anything seen for
thousands of years before. The ice caps themselves, which had endured
since the last ice age, were melting away faster than the scientists
could measure them.(46) |
<=>Rapid change
|
By 2005 glaciologists had gathered
enough evidence to demonstrate that most of the world's glaciers were
in retreat.(47) Glaciers that had existed since
the last ice age were melting back, revealing mummies that had been
frozen for thousands of years. The changes in the Alps, in Glacier
National Park in the United States, and on Mount Kilimanjaro made
a particularly strong impression on the public. Scientists were more
impressed by surveys showing that tropical glaciers had not been so
warm for thousands of years.
Three scientists, combining a variety of measures, put estimated
Northern Hemisphere temperatures over the past ten centuries into
a graph that showed a sharp turn upward since the start of the industrial
revolution. The temperatures of the 1990s soared off the chart.
Apparently 1998 had been not just the warmest year of the century,
but of the millennium. The graph (shown below) was widely
reprinted and made a strong impression. It was dubbed the
"hockey stick."(48*) (For more on global
temperatures before the 19th century, see the essay on solar
influences.)
Attempts to reconstruct temperatures before the late nineteenth
century remained controversial, but the warming since then was now
as certain a fact as anything in science. A few skeptics continued
to play with weather and satellite statistics, but geophysicists
noted that the real buildup of heat energy was easily seen, less
in the thin and variable atmosphere than in the masses of solid
earth sampled by boreholes. Still more telling, layers in ocean
basins — which were gradually absorbing most of the heat energy
— showed a pattern of recent warming. (See above.)
The pattern precisely matched what computer modellers expected from
greenhouse gas accumulation and nothing else.
|
Glacier
in Alps 1875/2004
<=Models (GCMs
|
If you compared the upward curve of 20th-century
temperatures with curves showing the predictions of various computer
models which simulated the effects of the sharp rise in greenhouse
gases (with adjustments for volcanic eruptions and solar variations),
the match was close indeed, and now soaring in an unprecedented
way. Most scientists could not believe all this was mere coincidence.
An International Panel on Climate Change (IPCC) that gathered the
views of the world’s community of experts finally agreed,
with little dissent, that it was highly likely that the strong global
warming since the 1970s was in large part the work of humanity.(49)
As of this writing, the warmest year in a record going back to
1861 was still 1998. (In retrospect it was seen that in 1998 a "super
El Niño" event, the strongest of the century, had pumped
some heat from the Pacific Ocean into the atmosphere.) The year
2005 tied 1998. The next four warmest on record were 2002, 2003,
2001 and 2004. |
=>Models (GCMs)
= Milestone
=>International
Get latest figures from the Hadley
Centre for Climate Research. |
|
An influential
1999 reconstruction of temperatures for the past millenium,
averaged over the Northern Hemisphere, included
measured temperatures for the past century. The dark solid line shows temperatures
averaged over each half-century or so, and the shaded area gives the range
of possible averages. An apparent downward trend from a warmer Middle Ages
("Medieval Warm Period," roughly comparable to the 1950s) into
a cooler "Little Ice Age" is abruptly interrupted by a steep rise
in the 20th century. (In retrospect, the dark line, which in some graphics
was smoothed even more, caught too much attention. Steep changes might be
concealed in the gray area of incomplete data, and the smoothed average
curve could give a misleading impression of relative stability.)
This "hockey stick" graph, prominently featured in the IPCC's
2001 report, immediately became a powerful tool for people who were trying
to raise public awareness of global warming — to the regret of some
seasoned climate experts who recognized that, like all science at the
point of publication, it was preliminary and uncertain. The dedicated
contrarians who insisted that there was no global warming problem promptly
attacked the calculations. For example, in 2003 a few scientists argued
that the Medieval Warm Period had been as hot as the 20th century. But
other climatologists, looking at data for the entire world, found a scattering
of warm and cold periods in different places at different times, not comparable
to the recent general warming.
Iin 2004 other groups pointed out that the huge gaps and uncertainties
in the pre-19th century data, and the methods used to average the data,
could conceal changes of temperature in the past that might have been
as large and abrupt as anything seen in modern times — that is,
aside from the warming since the 1980s, which was truly unprecedented.
A National Academy of Sciences panel reviewed all the studies and in 2006
announced that the original conclusions held: (1) the world had recently
grown warmer than at any time in the last four centuries, and (2) while
earlier data were much less reliable, it was "plausible" that
the world was now hotter than at any time in the past millenium.
As so often in this story, no single scientific finding could bring conviction
by itself, but only in conjunction with many other lines of evidence.
The most worrisome result of paleotemperature studies was that global
climate had probably swung substantially over the centuries. As climate
scientists noted, variability in the past, presumably due to small changes
in the atmosphere caused by solar and volcanic-dust
variations, warned that climate must be highly sensitive to any perturbation
— "So greater past climate variations imply greater future
climate change." Worse, much evidence indicated that the current
warming was faster than anything in the historical record. And the data
neatly matched the warming that every variety of computer
model, consistently since the 1970s, had been predicting would result
from greenhouse gas buildup in the atmosphere. (For more on the controversy
see note 48*).
More recent (2004-2005) and more complete reconstructions of
global temperatures are shown in this figure.
RELATED: Home The
Public and Climate The Carbon Dioxide Greenhouse
Effect Changing Sun, Changing Climate
1. Callendar (1961), p. 1.
BACK
2. One early notice was Brooks
(1922). BACK
3. Kincer (1934), p. 62; "wie
bei allen anderen Klimaschwankugen": Scherhag (1937), p.
263; similarly, "no evidence" of a permanent shift: George E. McEwen of Scripps, Science Newsletter (1940). BACK
4. Callendar (1938). Early attempt:
Köppen (1873) BACK
5. Landsberg (1946), pp.
297-98. BACK
6. Abarbanel and McCluskey
(1950), p. 23, see New York Times, May 30, 1947 and August
7, 1952. BACK
7. Brooks (1949), p. 117. BACK
8. Willett (1949), p. 50. BACK
9. In particular, Lysgaard
(1950); this was cited by several authors in Shapley
(1953); see also Willett (1950); on the shift of views,
see Lamb (1966), 171-72, also ix, 1-2. BACK
10. Landsberg (1958); his
analysis found an average 0.8°F rise, more around the Great Lakes. Landsberg (1960). BACK
11. Ahlmann (1952). BACK
12. Crary et al. (1955). BACK
13. Lamb (1959), in
Changing Climate (1966) p. 19. BACK
14. Mitchell was spurred by some Scandinavian studies
showing a leveling off in the 1950s the Arctic was usually where
trends showed up first. Mitchell (1961); see
also Mitchell (1963), "rhythm" p. 180; for another
and similar temperature curve, computed by the Main Geophysical Observatory
in Leningrad (and attributed to volcanoes), see Budyko (1969), p. 612; according to Wigley, the work of
Mitchell and Budyko, together with a 1961 study by Callendar, were "the
first reasonably reliable estimates of large scale average temperatures,"
Wigley et al. (1986), p. 278; Callendar
(1961) found chiefly a temperature rise in the Arctic. One other attempt
was Willett (1950). BACK
15. Mitchell (1961), pp. 249,
247. BACK
16. For the North Atlantic Oscillation, see Fagan (2000), esp. pp. 207-08. BACK
17. Lamb (1997), p. 218.
BACK
18. Landsberg (1967); quote:
Landsberg (1970), p. 1273; on all this, see Mitchell (1991). BACK
19. Brief reviews of observations back to the 19th century
include Mitchell (1953); Landsberg
(1955); Landsberg (1970). BACK
20. Budyko (1962); others
such as Wilson and Matthews (1971) pp. 60, 166-68 agreed the
effect could be serious. BACK
21. e.g., Dronia (1967),
removing urban heat effects found no net warming since the 19th century. BACK
22. Mitchell (1953); already
in 1938 Callendar adjusted for the effect, while admitting that "this is a matter which is open to
controversy." Callendar (1938), p. 235. Additionally, the
common practice during the 1950s of moving weather stations from downtown locations to
airports, outside the "heat island," would give a spurious impression of cooling, but Mitchell and
others allowed for that too in their calculations. BACK
23. Lamb (1977), pp. 709-10.
BACK
24. Johnsen et al. (1970); Lamb (1977), pp. 529, 549. BACK
25. Emiliani (1966). BACK
26. Hays et al. (1976). BACK
27. Mitchell (1972), p. 445;
GARP (1975), pp. 37, 43; they cite a Manabe computer model
of 1971 and Mitchell (1973). BACK
28. He also suspected the natural cycle was scheduled to
reverse within decades, adding to the rise. Broecker (1975). BACK
29. Salinger and Gunn
(1975). BACK
30. Damon and Kunen
(1976); a brief argument on turbidity reducing high-latitude temperatures is in Bryson (1973), p. 9; see also Damon and
Kunen (1978). BACK
31. National Defense University
(1978); also published in Council on Environmental Quality
(1980), ch. 17. BACK
32. Hansen et al. (1981),
"misconception" p. 961, and Hansen, interview by Weart, Oct. and Nov. 2000, AIP. BACK
33. Specifically, they predicted the effect would
rise above the two-sigma level in the 1990s. Hansen
et al. (1981), "emerge" p. 957; another scientist who compared temperature
trends with a combination of CO2, emissions from volcanic eruptions, and supposed solar cycles,
likewise got a good match, and used the cycles to predict that greenhouse
warming would swamp other influences after about 2000. Gilliland
(1982); Madden and Ramanathan (1980) studied the climate "noise"
in comparison with warming predicted by various computer models and concluded
the effect "should be detectable anytime from the present to about the
year 2000," p. 767. Already in 1956, both Gilbert Plass and Roger Revelle
had expected an effect, if any existed, would be apparent by the end of
the century. But the editor of Nature, no critic of greenhouse
arguments, thought the effect would "become apparent only halfway through
the next century" if not later, Maddox (1983).
BACK
34. The news for 1981 was added in proof in mid-December.
Jones et al. (1982). For funding they thank the U.S. Dept. of
Energy and Office of Naval Research. On American help with data, see e-mail interview of
Raymond S. Bradley by Ted Feldman, 2000, copy at AIP. BACK
35. Jones et al. (1986); a
review is Wigley et al. (1986). BACK
36. Hansen and Lebedeff
(1987). BACK
37. Schneider (1992), p. 26;
Other examples: MacCracken and Luther (1985); Ramanathan (1988). BACK
38. There was strong U.S. warming 1976-2000, but only in
the winter, not the summer warming that would have been noticeable. See IPCC (2001), p. 117; Hansen et al.
(2001). BACK
39. Study of the U.S., the only place where sufficiently
good records were available, showed a large urban bias which, when removed,
left a mild warming from 1900 to the 1930s. Karl
and Jones (1989); Jones et al. (1990); irrigation and other changes in land
use also contribute, making for a large total effect, according to Kalnay
and Cai (2003); another debate was over whether a reported sea-surface
temperature rise in the 1980s was due to temporary distortions such as
an El Niño event rather than the greenhouse effect, Reynolds
et al. (1989); Robock and Strong (1989). BACK
40. Spencer and Christy
(1990); Spencer and Christy (1992); Christy et al. (1997) with reply by K.E. Trenberth and J.W. Hurrell
gives an idea of the technical problems of analysis; Christy et al.
(1998); on Christy see Royte (2001); criticism: Wentz and Schabel (1998); Kerr
(1998); for counter-arguments Singer (1999). BACK
41. National Academy of Sciences
(2000); see also Santer et al. (2000);
more recently, Santer et al. (2002); "claimed
inconsistencies between model predictions and satellite tropospheric temperature
data (and between the latter and surface data) may be an artifact of data
uncertainties," suggested Santer et al. (2003).
The why-didn't-I-think-of-that analysis by Fu et
al. (2004) showed that the microwave wavelengths supposed to measure
the mid-level troposphere had been contaminated by a contribution from
the higher stratosphere, which was rapidly cooling (as predicted by models).
Quotes: John Wallace, Schiermeier (2004b);
Kerr (2004b), p. 806. For further on recent
controversies see Stephen Schneider's site, http://stephenschneider.stanford.edu/Climate/Climate_Science/Contrarians.html#Contrarians
BACK
42. Buds: Menzel and Fabian
(1999); a more general biological indicator was the earlier arrival
of the seasonal dip in CO2 as plants took up carbon:
Keeling et al. (1996); snow and general discussion:
Easterling et al. (2000); oceans: Levitus
et al. (2000); oceans got some 30 times as much added heat as the
atmosphere: Levitus et al. (2001), updated
and improved by Levitus et al. (2005); Hansen
et al. (2005) with better models and data found a particularly striking
match between greenhouse effect computer model estimates and observed
ocean basin warming. BACK
43. Santer et al. (1996),
quote p. 39; see Stevens (1999), ch. 13. Tree
rings: see Fritts (1976); coral: Weber
and Woodhead (1972). BACK
44. Le Roy Ladurie (1967);
Lamb (1972-77); Fagan
(2000). BACK
45. Lachenbruch and Marshall
(1986). BACK
46. Reviews of boreholes: Pollack and Chapman (1993); Pollack
et al. (1998); Pollack and Huang (2000);
review of tropical ice: Thompson et al. (1993);
see Krajick (2002). BACK
47. Oerlemans (2005).
BACK
48. Mann et al.
(1999), p. 761, copyright © 1999 American Geophysical Union,
reproduced by permission. The first serious attack published in a peer-reviewed,
albeit obscure, journal (Climate Research) was Soon
and Baliunas (2003). Asked to respond, Mann and other top climate
experts gave strong reasons for regarding the criticism as groundless,
indeed based on grossly improper statistical methods, Mann
et al. (2003). The chief editor of Climate Research and four
other editors resigned, saying the peer-review process had been faulty,
see Monastersky (2003). The sloping dashed
line in the figure, indicating slight cooling over the past millennium,
did rely on data that were sparse and difficult to interpret. See Mann
et al. (2004), Jones and Mann (2004). The
possibility of abrupt shifts concealed in the uncertainty band was pointed
out by von Storch et al. (2004), but their
conclusion that the graph was faulty overall was refuted by Wahl
et al. (2006). The likelihood that the smoothing process concealed
large temperature shifts was asserted by Moberg
et al. (2005), disputed by Mann et al. (2005),
and McIntyre and McKitrick (2005). Jones and
Mann argued that better data and other lines of research confirmed, at
a minimum, the unprecedented nature of the modern rise. Their revised
global graph over two millenia is here. "Plausible":
National Research Council (2006). "Past
climate variations:" Keith Briffa quoted by Fred Pearce in New
Scientist, Feb. 18, 2006, p. 10. For further on recent controversies
see Stephen Schneider's site, http://stephenschneider.stanford.edu/Climate/Climate_Science/Contrarians.html#Contrarians
and the professionally-run blog realclimate.org.
BACK
49. IPCC (2001),
p. 6. BACK
copyright
© 2003-2006 Spencer Weart & American Institute of Physics
|