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. Government weather services began to record measurements more systematically during the 19th century. 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 for each particujlar region 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: since 1865 average temperatures had risen several degrees Fahrenheit (°F) 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 undertook a thorough and systematic effort to look for historical changes in the average temperature of the entire planet. One 19th-century German had already made an attempt at this, 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. But since the late 19th century meteorologists around the world had been meticulously compiling weather records, and had spent countless hours negotiating standards so the data from different countries and different years could be compared on the same basis. Callendar drew upon that massive international effort. After countless hours of sorting out data and penciling sums, he announced that the mean global temperature had definitely risen between 1890 and 1935, by close to half a degree Celsius (0.5°C, equal to 0.9°F).(4) <= International

Callendar's warming

CLICK FOR FULL IMAGE =>CO2 greenhouse

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 (CO 2 ) from burning fuel could cause a "greenhouse effect," Callendar said this was the cause of the warming. (For the details of the theory, click on the link in the righthand column from the essay on Simple models of climate. For scientists' views on the theory in Callendar's day, click on the link higher up to the essay on the CO2 greenhouse effect.) <= 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 in some region there was an unmistakable climate change, 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. As it happened, much 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 weather records. If not for this accident, people might have paid little 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 CO 2 emissions, another eminent climatologist wrote in 1949, "has rather broken down in the last few years" when temperatures in some regions fell. However, scarcely a year later he allowed that since 1850 glaciers had been in retreat, and noted that "Winter temperatures rose over a large part of the northern hemisphere."(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 CO 2 .(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;" 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 CO 2 .

=> 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



H.H. Lamb

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 (who had meanwhile been updating and improving his own global temperature history), Mitchell had trudged through all the exacting calculations, working out average temperatures for most of the globe, and got plausible results. He confirmed that global temperatures had risen until about 1940. But since then, he reported, 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 unambiguous.(14*)

=> Solar variation

Acknowledging that the increasing amount of CO 2 in the atmosphere should give a tendency for warming, Mitchell tentatively suggested that smoke from recent volcanic eruptions and perhaps cyclical changes in the Sun might partly account for the reversal. (Later studies confirmed that volcanoes, and possibly a decline in solar activity, probably did have some cooling effect around that time.) But he rightly held that "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 (including scientists) 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. In the early 1970s, wherever climate experts got together they debated whether the world was due to get warmer or cooler. 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 CO 2 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. A few experts worried that pollution from human sources, such as dust from overgrazed lands and haze from factories, was beginning to shade and cool the planet's surface. But most experts doubted we were putting out enough air pollution to seriously 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 seemed to be astronomically scheduled over the next few thousand years. Later and better calculations would make that tens of thousands of years, but at the time a few people speculated that we might even see substantial natural cooling within centuries.(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 CO 2 . 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"). And industrial haze might also have a cooling effect, perhaps reinforcing the natural long-term trend toward a new ice age. Nevertheless, they thought CO 2 "could conceivably" bring half a degree of warming by the end of the century.(27) The outspoken geochemist and oceanographer Wallace Broecker went farther. Referring to some recent data from Greenland ice cores, 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. Although it turned out he was wrong about the natural cycle, this was one of several occasions when Broecker's scientific instincts about general processes were better than his specific calculations. Introducing a new phrase, he asked, "Are we on the brink of a pronounced global warming?"(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

The tendency of some scientists in the early 1970s to suspect that the world was cooling now collapsed. Science journalists reported that climate scientists were openly divided, and those who expected warming were increasingly numerous. A good example is Hubert Lamb, the historian of climate who in the 1950s had called attention to climate changes without attempting to predict them. Generalizing from the unusually good historical records in his native England, Lamb had depicted a globally warm "Medieval Climatic Optimum" followed in the early modern period by a "Little Ice Age." During the chilly 1960s he was persuaded by the studies of natural cycles that a new ice age was likely to arrive over thousands of years. But after the hot English summer of 1976 he joined the emerging viewpoint that human greenhouse gas emissions would "become dominant over the natural climate fluctuations by about A.D. 2000."(31) => Public opinion

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.(31a) => Public opinion

Government officials and scientists wanted 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. Just storing the records was a formidable challenge. Already in 1966, "From storage rooms to hallways, punch card file cabinets containing the nation’s archive of climate data filled every conceivable space at the National Weather Records Center (NWRC)... There was concern that the NWRC building was in imminent danger of a structural collapse." Although computer memory storage technology improved with tremendous speed, the ever-increasing volume of data kept pace.(31b)

Around 1980 two groups undertook to work through the mass of numbers in all their grubby details, rejecting sets of uncertain data and tidying up the rest. To contrive an entirely fictional example, suppose in the 1910s there were only eight stations measuring temperatures across a million square miles of the Canadian Arctic, and six of them were in the southern half of the territory. How do you combine the numbers to get an average for that entire segment of the globe? If one of the stations was moved in 1915 from a riverbank to a higher point that was usually colder, how do you adjust? What if one of the stations gave inconsistent results in the winter, and you suspect that the fellow stationed there didn't care to go out to read the thermometer on really cold days, but made up the numbers? The final values for an average temperature in a region resulted from countless difficult decisions.

One of the groups that undertook the task 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 CO 2 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. Some of the cooling was probably due to natural variations, including changes in the Sun's output and a modest spate of volcanic eruptions such as the explosion of Mt. Agung in 1963. More significantly, a sharp increase in haze from pollution such as sulfate aerosol particles had blocked enough sunlight to temporarily cool the industrialized Northern Hemisphere, masking the greenhouse warming. After the 1960s, with pollution growing less rapidly while CO 2 continued to accumulate in the air, warming resumed in both hemispheres.(32a)

<= Aerosols

The temporary northern cooling had been bad luck for climate science. By feeding skepticism about the greenhouse effect, while provoking a few scientists (and rather more 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

So long as global pollution from smoke, smog and dust was increasing, its cooling effects would hold back some of the temperature rise. Furthermore, as a few scientists pointed out, the upper layer of the oceans must have been absorbing heat. This too was hiding the buildup of heat energy trapped by greenhouse gases in the air. For only ten percent of the heat added through the greenhouse effect went into heating up the atmosphere; nearly all the rest quickly sank into the oceans. However, Hansen's group and others calculated that these effects could delay atmospheric warming by no more than a few decades. His group boldly predicted that considering how fast CO 2 was accumulating, by the end of the 20th century "carbon dioxide warming should emerge from the noise level of natural climatic variability." An increasing number of other scientists using different calculations were coming to the same conclusion — the warming would show itself clearly sometime around 2000. (A few scientists had already said as much as far back as the 1950s.)(33*) <=> The oceans => Government

<=> Solar variation

= Milestone

The second important group analyzing global temperatures was the British government's Climatic Research Unit (CRU) at the University of East Anglia, founded by Lamb in 1971 and now led by Tom Wigley. 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 Northern Hemisphere 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 confirmed that there had been considerable warming from the late 19th century up to 1940, followed by some regional cooling in the Northern Hemisphere. Global conditions had been roughly level until 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*)

Tom Wigley (r.) with a later director of the CRU, Phil Jones (l.) in Villach, 1985.

(photoshopped) = Milestone

=> International

=> CO2 greenhouse

=> Public opinion

=> Simple models

Convincing confirmation came from Hansen and a collaborator. They too analyzed records going back a century, 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 since the late 19th century.(36)

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 carefully 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.(36a)

<= 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)

Knowledge of the global temperature record was becoming so important (and to some, controversial) that the work by the groups in New York and East Anglia no longer seemed enough. A new major effort to track the trends 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 began to tediously review the statistics for the world and especially the United States. Making their own decisions about how to combine data, they got results that inevitably differed in minor details from what the other groups reported. But there was no disagreement about the recent general trend. (In 2006 the Japan Meteorological Agency would provide yet a fourth independent analysis, confirming the others.)

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 (the 1980s now 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 1997 topped it. 1998 beat that in turn by a 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 — if they had, the politics of the matter might have been different. But looking at the world as a whole, in the late 1990s the great majority of experts at last agreed. Yes, 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 data measured far from cities — in particular, over the oceans. The global warming trend was no statistical error. Meanwhile, in urban areas whatever global warming the greenhouse effect might be causing did get a strong addition of heat, and the combination would significantly raise the mortality from heat waves.(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, according to an analysis by a group at the University of Alabama, Huntsville, 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.What began as a normal controversy among scientists about the best way to analyze data became politicized, as if this one set of observations could prove or disprove that the planet was warming.(40)

=> Public opinion

Link from below

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. <=> Models (GCMs)

The measurements indicating that middle layers of the atmosphere had not noticeably warmed were 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? While the skeptics persisted, most scientists believed that although the computer models were surely imperfect, the satellite data analysis was too ambiguous to pose a serious challenge to the consensus that global warming was underway. This hunch would be confirmed in 2004 when meticulous analysis of both satellite and balloon observations turned up systematic errors. The mid levels had in fact been warming.(41*) = Milestone

It was one of several cases where computer modelers had been unable to tweak their models until they matched data, not because the models were bad but because the observations were wrong. To be precise, the raw data were fine, but numbers are meaningless until they are processed; it was the complex analysis of the data that had gone astray. "This is the answer — I wish we had recognized it ourselves," said the chair of the 1999 Academy survey. In the public sphere, deniers of global warming continued to cite the satellites and other erroneous data; once an idea gets on the internet it can never be removed from circulation.

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. But the most fundamental indicator, as experts knew, was the temperature of the upper layers of the oceans — that was where nine-tenths of the heat energy entering the climate system wound up. Analysis of countless volumes of ship data found serious heating in recent decades.(42*) Overall, the 1990s were unquestionably the warmest decade since thermometers came into common use, and the trend was accelerating.

=> Simple models

<= The oceans

Link from below

Most climate scientists now took it for granted that greenhouse gases were the cause of the global warming, but critics pointed out that other things might be at work. 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.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 variations in the Sun's radiation or by random volcanic eruptions. Another influence was decades-long fluctuations in the atmosphere-ocean systems of the North Atlantic, Pacific, and Arctic regions, which drove gradual variations in regional weather patterns; these quasi-cyclical fluctuations had been suspected since the 1920s, but only started to become clear in the late 1990s. Until all the possible influences were sorted out, the cause of the warming since 1970 would remain controversial.

<= The oceans

However, there were "fingerprints" (or a “signature”) 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. Overall, minimum temperatures were rising three times faster than maximum temperatures (bad news for farmers, since warm nights hurt crops threatened by drought).(42a)

<= Simple models

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 as the melting of snow and ice exposed dark soil and water to sunlight and the heat of the air. Later studies showed this "arctic amplification" was further amplified by a more active circulation transporting heat and water vapor toward the poles, and perhaps additional forces.(42b) (The amplification would be less effective in Antarctica, with its colossal year-round ice cover at high altitude, and in fact warming was in fact warming was seen there only around the coasts and on the peninsula that projected beyond the ice sheet.) Arctic warming was indeed glaringly obvious to scientists as they watched trees take over mountain meadows in Sweden and the Arctic Ocean's ice pack grow spectacularly smaller and 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.

<=> Public opinion



Ecosystem changes

<= Sea rise & ice



A team of computer modelers at the Lawrence Livermore Lab in California, led by Benjamin Santer, predicted that greenhouse gases would cause a particular geographical pattern of temperature change. It was different from what might be caused by other external influences, such as solar variations. The maps of observed changes 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," the researchers concluded, "although many uncertainties remain."(43) Even before Santer's finding was published, it impressed the community of climate scientists. An important 1995 report by the world's leading experts (the Intergovernmental Panel on Climate Change, IPCC) offered the "fingerprint" as evidence that greenhouse warming was probably underway. By 2006, when the warming had progressed considerably farther and the computer models were much improved, their judgment was confirmed. A thorough analysis concluded that there was scarcely a 5% chance that anything but humans had brought the pattern of changes observed in many regions of the world.(43a)

Ben Santer => Models (GCMs) <=> International

<= Solar variation

Meanwhile a variety of new evidence suggested that the recent warming was exceptional even if one looked back many centuries — another unambiguous fingerprint of human influence. 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.(43b) As one example among many, by 2004 an international team had analyzed hundreds of thousands of weather observations recorded in 18th and 19th century ships' logs in a dozen languages. Whaling ships in particular might have the only record for vast stretches of the planet. Analyzing old records was tricky — for example, ocean temperatures measured with a thermometer in a canvas bucket of sea water had to be adjusted for the cooling that took place as the bucket was hauled aboard. It was thus necessary to dig out just how temperatures had been taken. The effort paid off in 2008 when a group reported that a switch of methods in 1945 had created a spurious drop in ocean temperature readings, exaggerating the global temperature dip of the 1950s.The labor of reconciling different types of measurements seemed endless, but the magnitude of the errors was gradually beaten down.(44)

For the distant past, tree rings in fossil wood were the most widely used measure — and therefore the most controversial. As early as the 1920s, a few scientists had used rings in ancient logs as a measure of past climates, claiming that the width of a ring varied with a season's rainfall. The studies were attacked as unreliable until the 1960s, when new research showed that the rings did track rainfall in dry regions, but elsewhere they varied according to other factors. In particular, temperature was the crucial factor for certain trees at high altitudes and high latitudes. For example, the venerable bristlecone pines of California survive for millenia on their bleak mountaintops like taoistic recluses in a Chinese painting, growing imperceptibly in close harmony with the changes of snow cover and sunlight. <=> Solar variation

Bristlecone pines

Calibration was difficult, however, for some trees (although not the bristlecones) began to act strangely in the late 20th century, thanks perhaps to acid rain and other pollution as well as global climate change. This "divergence problem" could be worked around by relying on earlier data, but the adjustments were tricky. The adjustments became a lightning-rod for critics who insisted that the tree experts were analyzing the data incorrectly (even, some exclaimed, dishonestly). Fortunately there were other climate proxies, and scientists worked to derive past temperatures entirely without the use of tree rings. Ingenious analysis of coral reefs, fossil pollen, layers in stalactites, and so forth engaged experts from a variety of obscure specialties. Unexpected sources of error turned up here too. But years of analysis by different and often rival groups produced increasingly reliable numbers, all pretty much in agreement with one another and with tree rings. The trees proved useful as a check, and for exploring climate change in times and places where nothing else was available.(44a)

A particularly telling independent proxy 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 Arctic 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 soil and rock layers around the planet. The averages gave a clear signal of a global warming accelerating in the 20th century. 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 exceeded anything seen for several thousand years. 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 => Sea rise & ice