The Latest Research on Climate Change Part One

THIS IS THE TEXT OF AN ESSAY IN THE WEB SITE “THE DISCOVERY OF GLOBAL WARMING” BY SPENCER WEART, HTTP://WWW.AIP.ORG/HISTORY/CLIMATE. JULY 2009. HYPERLINKS WITHIN THAT SITE ARE NOT INCLUDED IN THIS FILE.  FOR AN OVERVIEW SEE THE BOOK OF THE SAME TITLE (HARVARD UNIV. PRESS, REV. ED. 2008).  COPYRIGHT © 2003-2009 SPENCER WEART & AMERICAN INSTITUTE OF PHYSICS. 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, weather experts found that over the past couple of decades the trend had shifted to cooling. With a new awareness that climate could change in serious ways, many scientists predicted a continued cooling, perhaps a phase of a long natural cycle or perhaps caused by human pollution of the atmosphere with smog and dust. Others insisted that the effects of such pollution were temporary, and humanity’s emission of greenhouse gases would bring warming over the long run. This group’s views became predominant in the late 1970s. As global warming resumed it became clear that the cooling spell (mainly a Northern Hemisphere effect) had indeed been a temporary distraction. When the rise continued into the 21st century with unprecedented scope, scientists recognized that it signaled a profound change in the climate system.
"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. Callendar1
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.
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. In the 1930s, the press began to call 2 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
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 Kincer (1934), p. 62; “wie bei allen anderen Klimaschwankugen”: Scherhag (1937), p.1 263; similarly, “no evidence” of a permanent shift: George E. McEwen of Scripps, Science Newsletter (1940). Callendar (1938). Early attempt: Köppen (1873). On the “World Weather Records” see2 Le Treut et al. (2007), p. 101-102. Landsberg (1946), pp. 297-98.3
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.”1
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 global temperature. One 19th-century German had made an attempt at this in seeking a connection with sunspot cycles. 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 meteorologists around the world had meticulously compiled weather records, and Callendar drew upon that massive international effort. After countless hours of sorting out data and penciling sums, he 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). Callendar’s statistics gave him confidence to push ahead 2 with another and more audacious claim. Reviving an old theory that human emissions of carbon 2dioxide gas (CO ) from burning fuel could cause a “greenhouse effect,” Callendar said this was the cause of the warming. (For the old theory, follow the link 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 Carbon Dioxide Greenhouse Effect.)
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.” If during some decades there was an 3 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.
(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
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 Abarbanel and McCluskey (1950), p. 23, see New York Times, May 30, 1947 and1 August 7, 1952. Brooks (1949), p. 117.2 Willett (1949), p. 50.3 In particular, Lysgaard (1950); this was cited by several authors in Shapley (1953); see4 also Willett (1950); on the shift of views, see Lamb (1966b), 171-72, also ix, 1-2. Landsberg (1958); his analysis found an average 0.8°F rise, more around the Great5 Lakes. Landsberg (1960). Ahlmann (1952).6 Crary et al. (1955).7
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.)
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. The argument for warming caused by 1 2CO emissions, another eminent climatologist wrote in 1949, “has rather broken down in the last few years” when temperatures in some regions fell. In any case (as yet another authority 2 remarked), compared with the vast slow swings of ice ages, “the recent oscillations of climate have been relatively small.”3
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. An example was a careful study of U.S. 4 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 2thought it might be due either to variations in the Sun’s energy or to the rise of CO . Others 5 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. Another striking example was a report that in the Arctic “the ice is thinner at the present 6 than ever before in historic times,” and before long we might even see an open polar sea. Such 7 high-latitude effects were exactly what simple models suggested would result from the 2greenhouse effect warming of increased CO .
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 Lamb (1959), in Changing Climate (1966) p. 19.1 Mitchell was spurred by some Scandinavian studies showing a leveling off in the2 1950s—the Arctic was usually where trends showed up first. Mitchell (1961); see also Mitchell (1963), “rhythm” p. 180. In his independent calculations, Callendar (1961) found chiefly a temperature rise in the Arctic. For another and similar temperature curve, computed by the Main Geophysical Observatory in Leningrad (and attributed to volcanoes), see Budyko (1969), p. 612; an expert called the works of Mitchell, Callendar (1961) and Budyko “the first reasonably reliable estimates of large scale average temperatures,” Wigley et al. (1986), p. 278. One other attempt was Willett (1950). Mitchell (1961), pp. 249, 247.3
“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.1 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.
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 countless exacting calculations, working out average temperatures for most of the globe. 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 visible. 2
2Acknowledging that the increasing amount of CO 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 solar changes 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. The veteran science correspondent 3 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
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 For the North Atlantic Oscillation, see Fagan (2000), esp. pp. 207-08.1 Lamb (1997), p. 218.2 Landsberg (1967); quote: Landsberg (1970), p. 1273; on all this, see Mitchell (1991).3 Brief reviews of observations back to the 19th century include Mitchell (1953);4 Landsberg (1955); Landsberg (1970). Budyko (1962); others such as Wilson and Matthews (1971) pp. 60, 166-68 agreed the5 effect could be serious. e.g., Dronia (1967), removing urban heat effects found no net warming since the 19th6 century.
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.
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.) People 1 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. Landsberg returned to his earlier view that the climate 2 2was probably showing only transient fluctuations, not a rising trend. While pollution and CO 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. 3
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 a “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. Some pushed ahead to suggest that as human civilization used ever more 4 energy, in a century or so the direct output of heat could be great enough to disturb the entire global climate. If so, that would not happen soon, and for the moment the main consequences 5 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 in built-up areas. As the cities grew, so did their local heating, which might have given a spurious impression of global warming.6 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
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 Mitchell (1953); already in 1938 Callendar adjusted for the effect, while admitting that1 “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. Lamb (1977), pp. 709-10.2 Johnsen et al. (1970); Lamb (1977), pp. 529, 549.3 Emiliani (1966b).4
warming of remote Arctic regions. Yet the statistical difficulties were so complex that the global 1 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 old 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.2
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 begining 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. 3
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. In the early 1970s, a variety of measurements pinned down the nature and timing 4 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
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 Hays et al. (1976).1 Mitchell (1972), p. 445; GARP (1975), pp. 37, 43; they cite a Manabe computer model2 of 1971 and Mitchell (1973). He also suspected the natural cycle was scheduled to reverse within decades, adding to3 the rise. Broecker (1975). Salinger and Gunn (1975).4 Damon and Kunen (1976); a brief argument on turbidity reducing high-latitude5 temperatures is in Bryson (1973), p. 9; see also Damon and Kunen (1978).
years, but at the time a few people speculated that we might even see substantial natural cooling within centuries. Unless, that is, something intervened. 1
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.” 2Human industry would prevent an advance of the ice by blanketing the Earth with CO . 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 2thought CO “could conceivably” bring half a degree of warming by the end of the century. The 2 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. He asked, “Are we on the brink of a pronounced global warming?” 3
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. There were too few weather stations in the vast 4 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. 5
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 divided nearly equally among three opinions: some thought further cooling was likely, others suspected that moderate greenhouse warming would begin fairly soon, and most of the rest
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 National Defense University (1978); also published in Council on Environmental1 Quality (1980), ch. 17. Hansen et al. (1981), “misconception” p. 961, and Hansen, interview by Weart, Oct. and2 Nov. 2000, AIP. For summary and references see Hegerl et al. (2006), p. 673.3
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). 1
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.2
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 2CO 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 change certainly came from natural variations, probably including changes in the Sun’s output and a modest spate of volcanic eruptions. More significantly, a sharp increase in haze from pollution such as sulfate aerosol particles had indeed helped to temporarily cool the industrialized Northern Hemisphere. After the 1960s, with pollution growing less rapidly while 2CO continued to accumulate in the air, warming resumed in both hemispheres. 3
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.
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 Specifically they predicted the effect would rise above the 2 sigma level in the 1990s.1 Hansen et al. (1981), “emerge” p. 957; another scientist who compared temperature trends with a 2combination of CO , 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 (1982b); 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.  Ocean calculations: Hoffert et al. (1980); Hansen et al. (1984). 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. And in 1959 Bert Bolin said serious effects might be visible around then (see footnote in essay on “The Carbon Dioxide Greenhouse Effect”). On the other hand, in 1983 the editor of “Nature,” not a climate expert but no critic of greenhouse arguments, thought the effect would “become apparent only halfway through the next century” if not later, Maddox (1983). The news for 1981 was added in proof in mid-December. Jones et al. (1982). For2 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, http://www.agu.org/history/sv/temperature/bradleyinterview.html, copy at AIP.
Any greenhouse warming had been masked by chance fluctuations in solar activity, by pulses of volcanic aerosols, and by haze. 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 concealing the buildup of heat energy trapped by greenhouse gases, although as Hansen’s group and others calculated, it could delay atmospheric warming by no more than a few decades. 2Hansen’s group boldly predicted that considering how fast CO 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. (A few scientists had already said as much as far back as the 1950s.) 1
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. Returning to old records, in 2 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. Global conditions had been roughly level until the
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 Jones et al. (1986a); Jones et al. (1986b); a review is Wigley et al. (1986). See1 recollections of Raymond Bradley, http://www.agu.org/history/sv/temperature/bradleyinterview.html. Later analysis revealed that the dip had been less severe than their numbers showed, for a change in the way ocean temperatures were measured after 1945 had artificially lowered some numbers: Thompson et al. (2008). Hansen and Lebedeff (1987).2 Schneider (1992), p. 26; Other examples: MacCracken and Luther (1985a); Ramanathan3 (1988).
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. 1
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. 2
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. 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.