Variations on the "Composite Plus Scale" (CPS) method continued to be used to produce hemispheric or global mean temperature reconstructions. From 1998 this was complemented by Climate Field Reconstruction (CFR) methods which could show how climate patterns had developed over large spatial areas, making the reconstruction useful for investigating natural variability and long-term oscillations as well as for comparisons with patterns produced by climate models. The CFR method made more use of climate information embedded in remote proxies, but was more dependent than CPS on assumptions that relationships between proxy indicators and large-scale climate patterns remained stable over time. As part of his PhD research,
Michael E. Mann worked with seismologist
Jeffrey Park on developing statistical techniques to find long term oscillations of natural variability in the
instrumental temperature record of
global surface temperatures over the last 140 years; showed patterns relating to the
El Niño–Southern Oscillation, and found what was later termed the
Atlantic multidecadal oscillation. They then teamed up with
Raymond S. Bradley to use these techniques on the dataset from his study with the aim of finding long term oscillations of natural variability in global climate. The resulting reconstruction went back to 1400, and was published in November as . They were able to detect that the multiple proxies were varying in a coherent oscillatory way, indicating both the multidecadal pattern in the North Atlantic and a longer term oscillation of roughly 250 years in the surrounding region. Their study did not calibrate these proxy patterns against a quantitative temperature scale, and a new statistical approach was needed to find how they related to surface temperatures in order to reconstruct past temperature patterns.
Mann, Bradley and Hughes 1998 For his postdoctoral research Mann joined Bradley and tree ring specialist
Malcolm K. Hughes to develop a new statistical approach to reconstruct underlying spatial patterns of temperature variation combining diverse datasets of proxy information covering different periods across the globe, including a rich resource of tree ring networks for some areas and sparser proxies such as lake sediments, ice cores and corals, as well as some historical records. Their global reconstruction was a major breakthrough in evaluation of past climate dynamics, and the first
eigenvector-based climate field reconstruction (CFR) incorporating multiple climate proxy data sets of different types and lengths into a high-resolution global reconstruction. A balance was required over the whole globe, but most of the proxy data came from tree rings in the Northern
mid latitudes, largely in dense proxy networks. Since using all of the large numbers of tree ring records in would have overwhelmed the sparse proxies from the
polar regions and the
tropics, they used principal component analysis (PCA) to produce PC summaries representing these large datasets, and then treated each summary as a proxy record in their CFR analysis. Networks represented in this way included the North American tree ring network (NOAMER) and
Eurasia. The primary aim of CFR methods was to provide the spatially resolved reconstructions essential for coherent
geophysical understanding of how parts of the climate system varied and responded to
radiative forcing, so hemispheric averages were a secondary product. The CFR method could also be used to reconstruct Northern Hemisphere
mean temperatures, and the results closely resembled the earlier CPS reconstructions including . The (MBH98) multiproxy study on "Global-scale temperature patterns and climate forcing over the past six centuries" was submitted to the journal
Nature on 9 May 1997, accepted on 27 February 1998 and published on 23 April 1998. The paper announced a new statistical approach to find patterns of climate change in both time and global distribution, building on previous multiproxy reconstructions. The authors concluded that "Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) AD1400", and estimated empirically that greenhouse gases had become the dominant
climate forcing during the 20th century. In a review in the same issue,
Gabriele C. Hegerl described their method as "quite original and promising", which could help to verify model estimates of natural climate fluctuations and was "an important step towards reconstructing space–time records of historical temperature patterns".
Publicity and controversy on publication of MBH98 Release of the paper on 22 April 1998 was given exceptional media coverage, including questioning as to whether it proved that human influences were
responsible for climate change. Mann would only agree that it was "highly suggestive" of that inference. He said that "Our conclusion was that the warming of the past few decades appears to be closely tied to emission of greenhouse gases by humans and not any of the natural factors". Most proxy data are inherently imprecise, and Mann said "We do have error bars. They are somewhat sizable as one gets farther back in time, and there is reasonable uncertainty in any given year. There is quite a bit of work to be done in reducing these uncertainties." Climatologist
Tom Wigley welcomed the progress made in the study, but doubted if proxy data could ever be wholly convincing in detecting the human contribution to changing climate.
Phil Jones of the UEA
Climatic Research Unit told the
New York Times he was doubtful about adding the 150-year thermometer record to extend the proxy reconstruction, and compared this with putting together apples and oranges; Mann et al. said they used a comparison with the thermometer record to check that recent proxy data were valid. Jones thought the study would provide important comparisons with the findings of
climate modeling, which showed a "pretty reasonable" fit to proxy evidence. The study was disputed by contrarian
Pat Michaels with the claim that all of the warming took place between 1920 and 1935, before increased human greenhouse gas emissions. The
George C. Marshall Institute alleged that MBH98 was deceptive in only going back to 1400, and so not covering the
Medieval Warm Period which predated industrial greenhouse gas emissions. The same criticisms were made by
Willie Soon and
Sallie Baliunas.
Pollack, Huang and Shen, Jones et al. 1998 In October 1998 the borehole reconstruction published by Pollack, Huang and Shen gave independent support to the conclusion that 20th century warmth was exceptional for the past 500 years. Jones,
Keith Briffa,
Tim P. Barnett and
Simon Tett had independently produced a "Composite Plus Scale" (CPS) reconstruction extending back for a thousand years, comparing tree ring, coral layer, and glacial proxy records, but not specifically estimating uncertainties. was submitted to
The Holocene on 16 October 1997; their revised manuscript was accepted on 3 February and published in May 1998. As Bradley recalls, Mann's initial view was that there was too little information and too much uncertainty to go back so far, but Bradley said "Why don't we try to use the same approach we used in Nature, and see if we could push it back a bit further?" Within a few weeks, Mann responded that to his surprise, "There is a certain amount of skill. We can actually say something, although there are large uncertainties."
Mann, Bradley and Hughes 1999 In considering the 1998 Jones et al. reconstruction which went back a thousand years, Mann, Bradley and Hughes reviewed their own research and reexamined 24 proxy records which extended back before 1400. Mann carried out a series of statistical
sensitivity tests, removing each proxy in turn to see the effect its removal had on the result. He found that certain proxies were critical to the reliability of the reconstruction, particularly one tree ring dataset collected by Gordon Jacoby and
Rosanne D'Arrigo in a part of North America Bradley's earlier research had identified as a key region. This dataset only extended back to 1400, and though another proxy dataset from the same region (in the
International Tree-Ring Data Bank) went further back and should have given reliable proxies for earlier periods, validation tests only supported their reconstruction after 1400. To find out why, Mann compared the two datasets and found that they tracked each other closely from 1400 to 1800, then diverged until around 1900 when they again tracked each other. He found a likely reason in the "
fertilisation effect" affecting tree rings as identified by Graybill and Idso, with the effect ending once levels had increased to the point where warmth again became the key factor controlling tree growth at high altitude. Mann used comparisons with other tree ring data from the region to produce a corrected version of this dataset. Their reconstruction using this corrected dataset passed the validation tests for the extended period, but they were cautious about the increased uncertainties. The Mann, Bradley and Hughes reconstruction covering 1,000 years (MBH99) was submitted in October 1998 to
Geophysical Research Letters which published it in March 1999 with the cautious title
Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations to emphasise the increasing uncertainty involved in reconstructions of the period before 1400 when fewer proxies were available. A
University of Massachusetts Amherst news release dated 3 March 1999 announced publication in the 15 March issue of
Geophysical Research Letters, "strongly suggesting that the 1990s were the warmest decade of the millennium, with 1998 the warmest year so far." Bradley was quoted as saying "Temperatures in the latter half of the 20th century were unprecedented", while Mann said "As you go back farther in time, the data becomes sketchier. One can't quite pin things down as well, but, our results do reveal that significant changes have occurred, and temperatures in the latter 20th century have been exceptionally warm compared to the preceding 900 years. Though substantial uncertainties exist in the estimates, these are nonetheless startling revelations." While the reconstruction supported theories of a relatively warm medieval period, Hughes said "even the warmer intervals in the reconstruction pale in comparison with mid-to-late 20th-century temperatures." The
New York Times report had a colored version of the graph, distinguishing the instrumental record from the proxy evidence and emphasising the increasing range of possible error in earlier times, which MBH said would "preclude, as yet, any definitive conclusions" about climate before 1400. The reconstruction found significant variability around a long-term cooling trend of –0.02 °C per century, as expected from
orbital forcing, interrupted in the 20th century by rapid warming which stood out from the whole period, with the 1990s "the warmest decade, and 1998 the warmest year, at moderately high levels of confidence." This was illustrated by the
time series line graph Figure 2(a) which showed their reconstruction from AD 1000 to 1980 as a thin line, wavering around a thicker dark 40-year smoothed line. This curve followed a downward trend (shown as a thin dot-dashed line) from a
Medieval Warm Period (about as warm as the 1950s) down to a cooler
Little Ice Age before rising sharply in the 20th century. Thermometer data shown with a dotted line overlapped the reconstruction for a calibration period from 1902 to 1980, then continued sharply up to 1998. A shaded area showed uncertainties to two standard error limits, in medieval times rising almost as high as recent temperatures. When Mann gave a talk about the study to the
National Oceanic and Atmospheric Administration's
Geophysical Fluid Dynamics Laboratory,
Jerry Mahlman nicknamed the graph the "hockey stick", With work progressing on the next IPCC report,
Chris Folland told researchers on 22 September 1999 that a figure showing temperature changes over the millennium "is a clear favourite for the policy makers' summary". Two graphs competed: Jones et al. (1998) and MBH99. In November, Jones produced a simplified figure for the cover of the short annual
World Meteorological Organization report, which lacks the status of the more important IPCC reports. Two fifty-year smoothed curves going back to 1000 were shown, from MBH99 and Jones et al. (1998), with a third curve to 1400 from Briffa's new paper, combined with modern temperature data bringing the lines up to 1999: in 2010 the lack of a clarity about this change of data was
criticised as misleading. Briffa's paper as published in the January 2000 issue of
Quaternary Science Reviews showed the unusual warmth of the last century, but cautioned that the impact of human activities on tree growth made it subtly difficult to isolate a clear climate message. In February 2000
Thomas J. Crowley and
Thomas S. Lowery's reconstruction incorporated data not used previously. It reached the conclusion that peak Medieval warmth only occurred during two or three short periods of 20 to 30 years, with temperatures around 1950s levels, refuting claims that 20th century warming was not unusual. An analysis by Crowley published in July 2000 compared simulations from an energy balance climate model with reconstructed mean annual temperatures from MBH99 and Crowley & Lowery (2000). While earlier reconstructed temperature variations were consistent with volcanic and
solar irradiation changes plus residual variability, very large 20th-century warming closely agreed with the predicted effects of greenhouse gas emissions. Reviewing twenty years of progress in palaeoclimatology, Jones noted the reconstructions by Jones et al. (1998), MBH99, Briffa (2000) and Crowley & Lowery (2000) showing good agreement using different methods, but cautioned that use of many of the same proxy series meant that they were not independent, and more work was needed.
IPCC Third Assessment Report, 2001 showing the IPCC fig. 2.20 hockey stick graph at a climate conference in 2005 The Working Group 1 (WG1) part of the
IPCC Third Assessment Report (TAR) included a subsection on multi-proxy synthesis of recent temperature change. This noted five earlier large-scale palaeoclimate reconstructions, then discussed the reconstruction going back to 1400 AD and its extension back to 1000 AD in (MBH99), while emphasising the substantial uncertainties in the earlier period. The MBH99 conclusion that the 1990s were likely to have been the warmest decade, and 1998 the warmest year, of the past millennium in the Northern Hemisphere, with "likely" defined as "66-90% chance", was supported by reconstructions by and by using different data and methods. The reconstruction covering the past 500 years gave independent support for this conclusion, which was compared against the independent (extra-tropical, warm-season) tree-ring density NH temperature reconstruction of . Its Figure 2.21 showed smoothed curves from the MBH99, Jones et al. and Briffa reconstructions, together with modern thermometer data as a red line and the grey shaded 95% confidence range from MBH99. Above it, figure 2.20 was adapted from MBH99. This iconic graph adapted from MBH99 was featured prominently in the WG1 Summary for Policymakers under a graph of the instrumental temperature record for the past 140 years. The text stated that it was "likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year" in the past 1,000 years. Versions of these graphs also featured less prominently in the short
Synthesis Report Summary for Policymakers, which included a sentence stating that "The increase in surface temperature over the 20th century for the Northern Hemisphere is likely to have been greater than that for any other century in the last thousand years", and the
Synthesis Report - Questions. The Working Group 1 scientific basis report was agreed unanimously by all member government representatives in January 2001 at a meeting held in
Shanghai, China. A large poster of the IPCC illustration based on the MBH99 graph formed the backdrop when Sir
John T. Houghton, as co-chair of the working group, presented the report in an announcement shown on television, leading to wide publicity. ==Scientific debates==