The Latest Research on Climate Change Part 2

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.3 After 1988
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.
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 There was strong U.S. warming 1976-2000, but only in the winter, not the summer1 warming that would have been noticeable. See IPCC (2001), p. 117; Hansen et al. (2001). Study of the U.S., the only place where sufficiently good records were available, showed2 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). Spencer and Christy (1990); Spencer and Christy (1992); Christy et al. (1997) with reply3 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
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 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. 1
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. 2
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. 3
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counter-arguments Singer (1999). National Academy of Sciences (2000); see also Santer et al. (2000); more recently,1 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). Quote: John Wallace, Schiermeier (2004b); see also Kerr (2004b). The apparent lack of warming in ballon (radiosonde) data was “an artifact of systematic reductions over time in the uncorrected error due to daytime solar heating of the instrument,” Sherwood et al. (2005). For a detailed discourse on various recent controversies see Stephen Schneider's site, http://stephenschneider.stanford.edu/Climate/Climate_Science/Contrarians.html#]Contrarians.
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 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 global warming consensus.
This hunch was confirmed in 2004 when meticulous analysis of both satellite and balloon observations turned up sysematic errors. The mid levels had in fact been warming. 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 data were wrong. “This is the answer—I wish we had recognized it ourselves,” said the chair of the 1999 Academy survey. Contrarians in the public sphere continued to cite the satellites and other erroneous data (once an idea gets on the internet it can never be removed from circulation). But scientists were now satisfied that warming was underway pretty much as the models had predicted.1
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
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 Buds: Menzel and Fabian (1999); a more general biological indicator was the earlier1 2arrival of the seasonal dip in CO as plants took up carbon: Keeling et al. (1996a); 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.
rise was found in recent decades, and the greenhouse effect was the only plausible cause. The 1 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.)
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.
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. 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 as the melting of snow and ice exposed dark soil and water. (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 ice pack grow thin. 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.
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 Santer et al. (1995), online at 1 http://wiki.nsdl.org/index.php/PALE:ClassicArticles/GlobalWarming/Article20. Santer et al. (1996), quote p. 39; see Stevens (1999), ch. 13. Allen et al. (2006).2
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 crude 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.” 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 1 “fingerprint” as evidence that greenhouse warming was probably 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. (By 2006, when the warming had progressed considerably farther and the computer models were much improved, his judgment was confirmed. A thorough analysis concluded that there was scarcely a 5% chance that anything but humans had brought the changes observed in many regions of the world.) 2
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.
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 example among many, by 2004 an international team had analyzed hundreds of thousands of weather observations recorded in a dozen languages in 18th and 19th century ships’ logs. 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 bucket of sea water had to be adjusted for the cooling that took place as the bucket was hauled aboard. The labor of reconciling different types of measurements seemed endless, but the magnitude of the errors was gradually beaten down. Other data came from physical analysis of ancient tree rings, coral reefs, stalactites and other ingenious proxy measures. Unexpected sources of error turned
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 Le Roy Ladurie (1967); Lamb (1972-77); Fagan (2000). Tree rings: see Fritts (1976);1 coral: Weber and Woodhead (1972). An overview is Somerville et al. (2007), p. 102. Lachenbruch and Marshall (1986).2 Reviews of boreholes: Pollack and Chapman (1993); Pollack et al. (1998); Pollack and3 Huang (2000); Pollack and Smerdon (2004). Review of tropical ice: Thompson et al. (1993); see Krajick (2002). J. Oerlemans, “Extracting a Climate Signal from 169 Glacier Records.” Science 3084 (2005): 675-77.
up here too, but years of analysis by different and often rival groups produced increasingly reliable numbers.1
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 Arctic air temperature by a few degrees since the last century, with the heat gradually seeping down into the earth. In a burst of 2 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 global warming over the last few centuries, 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 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.3
By 2005 glaciologists had gathered enough evidence to demonstrate that most of the world’s glaciers were in retreat. Glaciers that had existed since the last ice age were melting back, 4 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 strong impression on the public.
Three scientists, combining a variety of measures, made a graph of estimated temperatures averaged over the Northern Hemisphere over the past ten centuries. An apparent downward trend from a warmer Middle Ages ("Medieval Warm Period," roughly comparable to the 1950s) into a cooler "Little Ice Age" was abruptly interrupted by a steep rise in the 20th century. A gray shaded area showed the range of incomplete data, but most attention went to a dark solid line showing temperatures averaged over each half-century or so. The graph included modern temperature measurements, showing 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 was widely reprinted and made a strong impression. It was dubbed the “hockey stick” because it displayed a flat thousand-year trend
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 On Medieval climate see for example Lund et al. (2006). “Coral data for the tropical1 Pacific... suggest a ‘Medieval Cool Period’,” according to Mann et al. (2006). The Northern Hemisphere Medieval Warm Period and Little Ice Age were popularized by Brooks (1922) and Lamb, e.g., Lamb (1995), whose data and attention inevitably focussed on the North Atlantic region. Evidence that warm episodes were regional and “not strongly synchronous” was first assembled by Hughes and Diaz (1994). Mann et al. (1999), p. 761. For a historical overview see Monastersky (2006). The first2 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 slight cooling the curve showed over the past millennium did rely on data that were sparse and difficult to interpret. See
followed by a sharp upward turn. (For more on global temperatures before the 19th century, see the essay on Changing Sun, Changing Climate.)
The “hockey stick” graph was prominently featured in a report issued in 2001 by a consensus of experts (the Intergovernmental Panel on Climate Change). It 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 minority 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 at all comparable to the recent general warming. Like the temporary cooling of the 1960s, the famous Medieval Warm Period was probably seen mainly in parts of the Northern Hemisphere.1
In 2004 other teams 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. Indeed the way the popular press often displayed the “hockey stick” graphic, as a single, stout, level line hooking up at the end, gave a misleading impression of past stability. A main purpose of the original publication had been to establish the limits of uncertainty, but even if publications did show the broad gray band of shading, it was easy to overlook that it might conceal big climate shifts.
The National Academy of Sciences responded to the controversy by asking a panel to review all the evidence. In 2006 the panel announced that the main original conclusions held. The world had indeed grown warmer since the 1980s in a way that was without precedent, at least in the past four centuries for which a reliable record could be reconstructed. While earlier data were much less reliable, the panel found it “plausible” that the world was now hotter than at any time in the past millenium.(For more on the recent controversy see this footnote. ) 2
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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. Mann et al. (2008) narrowed the band of uncertainty with detailed reconstructions, finding a Northern Hemisphere medieval warm period, butwith post-1980s temperatures clearly higher even if they excluded data from tree rings (the main point where critics had attacked). For lack of Southern Hemisphere data they could not exclude a brief earlier period of comparable global warmth. “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 the professionally-run blog realclimate.org.
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. While attempts to reconstruct temperatures before the late nineteenth century remained controversial, the warming since then was now as certain a fact as anything in science. A few skeptics continued to seek confirmation of their views in data on air temperatures from weather stations and satellites. 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 did layers in ocean basins—which were gradually absorbing most of the heat energy—show a pattern of recent warming (see above). The pattern roughly matched what could be expected from greenhouse gas accumulation, and nothing else. Contrarians found a chance to question this too, when a paper was published in 2006 reporting that the oceans had cooled (although only in the past few years). However, the authors soon announced that they had made an embarrassing mistake in the way they had compared the data from older and newer instruments. It was just another example of the difficulties of interpreting science amid an uproar of politicized controversy.
Talk radio callers and right-leaning columnists continued to exclaim about an unusually cold winter or summer in this or that locality. They pointed out that some regions showed no warming at all, notably the massive Antarctic ice sheet. This was no surprise, but an effect predicted as far back as 1981 by Stephen Schneider and a collaborator. Noting that the Southern Hemisphere was mostly ocean, which would tend to take up heat and delay the rise of atmospheric temperature in the region, they had warned that people “may still be misled... in the decade A.D. 2000-2010” by cool weather there. (It turned out, however, that this and later computer studies were too conservative: in the 2000s regions around Antarctica began to show a bit of warming and significant loss of ice.)
More generally, the entire world-ocean was taking up heat, causing the rise of atmospheric temperature to lag by decades behind the planet’s uptake of extra solar energy from the greenhouse effect. Additional delay was introduced by the sluggish response of forests and tundra as they adjusted to the changing climate. The system had so much inertia that once the world had warmed to a given level, we were already committed to substantially more warming. Scientists
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 Schneider and Thompson (1981), quote p. 3145. Bryan et al. (1988) found that in1 Antarctica “there is no warming at the sea surface, and even a slight cooling over the 50-year duration of the experiment.” due to an increase of mixing of deeper waters in Circumpolar Ocean. This was further confirmed with a much better model, Manabe et al. (1991). Current observations of Antarctic sea ice cover etc. are in accord with current models: IPCC (2007b), pp. 616-17. For the history see Manabe and Stouffer (2007), pp. 386, 401. Bolin (2007) p. 158 remarks on the lag in temperature and policy. IPCC (2001), p. 6. The 2007 report saw even more evidence that it was “highly likely”2 that human activity was the main cause of warming. IPCC (2007b), section TS.4.
understood this by the end of the 20th century, but most policy-makers had yet to grasp the implications—another dangerous time-lag.1
Hoping to further impede policy changes, around 2008 contrarians began to popularize a new meaningless claim: the world had supposedly gotten no warmer in the decade since 1998. Indeed that had been an extraordinarily warm year, for a “super El Niño” event, the strongest of the century, had pumped some extra heat from the Pacific Ocean into the atmosphere. No year since had been hotter (although 2005 matched it). While such claims excited comment from some internet blogs, politicians and others who stuck by outdated convictions, the actual scientific literature gave scant attention to such short-term fluctuations. Researchers who took a ten-year average to smooth out the random variability found the planet was substantially hotter in the decade 1999-2008 than in the decade before, which was in turn hotter than the preceding decade, and so forth back to the 1970s. The natural variability of climate could make for a still longer pause in warming, like the two decades of fluctuations in the mid-20th century. If that happened it would give comfort only to those who ignored all the other data on changes in the climate system. Here as in many of their other claims, people committed to denying the gathering danger of global warming increasingly relied on a disingenous selection of very particular ways of looking at the evidence.
If you compared the climbing curve of late-20th-century temperatures with the curves produced by computer models that calculated the effects of the rise of greenhouse gases (with adjustments for volcanic eruptions, solar variations and aerosol pollution), the match was close indeed. Temperatures were soaring  much as scientists had been predicting, with increasing confidence, for half a century. Few could believe any longer that this was mere coincidence. By now the world’s community of experts had finally agreed, with little dissent, that it was highly likely that the strong global warming seen since the 1970s was in large part the work of humanity. In a 2007 consensus report, they went on to point to greenhouse warming as a likely cause of  the more frequent summer heat waves, warmer winters, dwindling glaciers, stronger rainstorms, and other changes in weather patterns that were already seen to be underway... as predicted. 2
Latest figures are available from the Hadley Centre for Climate Research, http://www.met-office.gov.uk/research/hadleycentre/obsdata/globaltemperature.html.
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