Hockey Stick Graph (global Temperature)
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Hockey stick graphs present the global or hemispherical
mean A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...
temperature record of the past 500 to 2000 years as shown by quantitative climate reconstructions based on climate proxy records. These reconstructions have consistently shown a slow long term cooling trend changing into relatively rapid warming in the 20th century, with the
instrumental temperature record Global surface temperature (GST) is the average temperature of Earth's surface. More precisely, it is the weighted average of the temperatures over the ocean and land. The former is also called sea surface temperature and the latter is calle ...
by 2000 exceeding earlier temperatures. The term ''hockey stick graph'' was popularized by the climatologist
Jerry Mahlman Jerry Mahlman (February 21, 1940 – November 28, 2012) was an American meteorologist and climatologist. Biography Mahlman was born on February 21, 1940, in Crawford, Nebraska, and received his undergraduate degree from Chadron State College in ...
, to describe the pattern shown by the (MBH99) reconstruction, envisaging a graph that is relatively flat with a downward trend to 1900 as forming an
ice hockey stick An ice hockey stick is a piece of equipment used in ice hockey to shoot, pass, and carry the hockey puck, puck across the ice. Ice hockey sticks are approximately 150–200 cm long, composed of a long, slender shaft with a flat extension ...
's "shaft" followed by a sharp, steady increase corresponding to the "blade" portion. The reconstructions have featured in
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to "provide governments at all levels with scientific information that they can use to develop climate policies". The World Met ...
(IPCC) reports as evidence of
global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...
. Arguments over the reconstructions have been taken up by fossil fuel industry funded lobbying groups attempting to cast doubt on climate science.
Paleoclimatology Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...
dates back to the 19th century, and the concept of examining
varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. Of the many rhythmites in the geological record ...
s in lake beds and tree rings to track local climatic changes was suggested in the 1930s. In the 1960s, Hubert Lamb generalised from
historical documents Historical documents are original documents that contain important historical information about a person, place, or event and can thus serve as primary sources as important ingredients of the historical methodology. Significant historical docume ...
and temperature records of central England to propose a
Medieval Warm Period The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about . Climate proxy records show peak warmth occu ...
from around 900 to 1300, followed by
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Mat ...
. This was the basis of a "schematic diagram" featured in the
IPCC First Assessment Report The First Assessment Report (FAR) of the Intergovernmental Panel on Climate Change (IPCC) was completed in 1990. It served as the basis of the United Nations Framework Convention on Climate Change (UNFCCC). This report had effects not only on the ...
of 1990 beside cautions that the medieval warming might not have been global. The use of indicators to get quantitative estimates of the
temperature record Global surface temperature (GST) is the average temperature of Earth's surface. More precisely, it is the weighted average of the temperatures over the ocean and land. The former is also called sea surface temperature and the latter is called ...
of past centuries was developed, and by the late 1990s a number of competing teams of climatologists found indications that recent warming was exceptional. introduced the "Composite Plus Scaling" (CPS) method which, as of 2009, was still being used by most large-scale reconstructions. Their study was featured in the
IPCC Second Assessment Report The Second Assessment Report (SAR) of the Intergovernmental Panel on Climate Change (IPCC), published in 1995, is an assessment of the then available scientific and socio-economic information on climate change. The report was split into four parts ...
of 1995. In 1998 Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes developed new statistical techniques to produce (MBH98), the first
eigenvector In linear algebra, an eigenvector ( ) or characteristic vector is a vector that has its direction unchanged (or reversed) by a given linear transformation. More precisely, an eigenvector \mathbf v of a linear transformation T is scaled by ...
-based climate field reconstruction (CFR). This showed global patterns of annual surface temperature, and included a graph of average hemispheric temperatures back to 1400 with shading emphasising that uncertainties (to two
standard error The standard error (SE) of a statistic (usually an estimator of a parameter, like the average or mean) is the standard deviation of its sampling distribution or an estimate of that standard deviation. In other words, it is the standard deviati ...
limits) were much greater in earlier centuries. independently produced a CPS reconstruction extending back for a thousand years, and (MBH99) used the MBH98 methodology to extend their study back to 1000. A version of the MBH99 graph was featured prominently in the 2001 IPCC Third Assessment Report (TAR), which also drew on Jones et al. 1998 and three other reconstructions to support the conclusion that, in the Northern Hemisphere, the 1990s was likely to have been the warmest decade and 1998 the warmest year during the past 1,000 years. The graph became a focus of dispute for those opposed to the strengthening
scientific consensus Scientific consensus is the generally held judgment, position, and opinion of the majority or the supermajority of scientists in a particular field of study at any particular time. Consensus is achieved through scholarly communication at confer ...
that late 20th century warmth was exceptional."Part three: Hockey stick graph took pride of place in IPCC report, despite doubts"
In 2003, as lobbying over the 1997
Kyoto Protocol The was an international treaty which extended the 1992 United Nations Framework Convention on Climate Change (UNFCCC) that commits state parties to reduce greenhouse gas emissions, based on the scientific consensus that global warming is oc ...
intensified, a paper claiming greater medieval warmth was quickly dismissed by scientists in the Soon and Baliunas controversy. Later in 2003, Stephen McIntyre and Ross McKitrick published disputing the data used in MBH98 paper. In 2004 Hans von Storch published criticism of the statistical techniques as tending to underplay variations in earlier parts of the graph, though this was disputed and he later accepted that the effect was very small. In 2005 McIntyre and McKitrick published criticisms of the
principal components analysis Principal component analysis (PCA) is a Linear map, linear dimensionality reduction technique with applications in exploratory data analysis, visualization and Data Preprocessing, data preprocessing. The data is linear map, linearly transformed ...
methodology as used in MBH98 and MBH99. Their analysis was subsequently disputed by published papers including and which pointed to errors in the McIntyre and McKitrick methodology. Political disputes led to the formation of a panel of scientists convened by the
United States National Research Council The National Academies of Sciences, Engineering, and Medicine (NASEM), also known as the National Academies, is a congressionally chartered organization that serves as the collective scientific national academy of the United States. The name i ...
, their North Report in 2006 supported Mann's findings with some qualifications, including agreeing that there were some statistical failings but these had little effect on the result.
"Part four: Climate change debate overheated after sceptics grasped 'hockey stick'"
More than two dozen reconstructions, using various statistical methods and combinations of proxy records, support the broad consensus shown in the original 1998 hockey-stick graph, with variations in how flat the pre-20th century "shaft" appears.. The 2007
IPCC Fourth Assessment Report ''Climate Change 2007'', the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC), was published in 2007 and is the fourth in a series of reports intended to assess scientific, technical and soci ...
cited 14 reconstructions, 10 of which covered 1,000 years or longer, to support its strengthened conclusion that it was likely that Northern Hemisphere temperatures during the 20th century were the highest in at least the past 1,300 years. Further reconstructions, including Mann et al. 2008 and , have supported these general conclusions.


Origins: the first paleoclimate reconstructions

Paleoclimatology Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...
influenced the 19th century physicists
John Tyndall John Tyndall (; 2 August 1820 – 4 December 1893) was an Irish physicist. His scientific fame arose in the 1850s from his study of diamagnetism. Later he made discoveries in the realms of infrared radiation and the physical properties of air ...
and
Svante Arrhenius Svante August Arrhenius ( , ; 19 February 1859 – 2 October 1927) was a Swedish scientist. Originally a physicist, but often referred to as a chemist, Arrhenius was one of the founders of the science of physical chemistry. In 1903, he received ...
who found the
greenhouse gas Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...
effect of
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
() in the atmosphere to explain how past
ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and g ...
s had ended. From 1919 to 1923,
Alfred Wegener Alfred Lothar Wegener (; ; 1 November 1880 – November 1930) was a German climatologist, geologist, geophysicist, meteorologist, and polar researcher. During his lifetime he was primarily known for his achievements in meteorology and ...
did pioneering work on reconstructing the climate of past eras in collaboration with
Milutin Milanković Milutin Milanković (sometimes Anglicisation of names, anglicised as Milutin Milankovitch; sr-Cyrl, Милутин Миланковић, ; 28 May 1879 – 12 December 1958) was a Serbian mathematician, astronomer, climatologist, geophysics, geo ...
, publishing ''Die Klimate der geologischen Vorzeit'' ("The Climates of the Geological Past") together with Wladimir Köppen, in 1924. In the 1930s Guy Stewart Callendar compiled temperature records to look for changes. Wilmot H. Bradley showed that annual
varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. Of the many rhythmites in the geological record ...
s in lake beds showed climate cycles, and A. E. Douglass found that tree rings could track past climatic changes but these were thought to only show random variations in the local region. It was only in the 1960s that accurate use of tree rings as
climate proxies Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorolog ...
for reconstructions was pioneered by Harold C. Fritts. In 1965 Hubert Lamb, a pioneer of
historical climatology Historical climatology is the study of historical changes in climate and their effect on civilization from the emergence of homininis to the present day. It is concerned with the reconstruction of weather and climate and their effect on historical ...
, generalised from temperature records of central England by using historical, botanical and archeological evidence to popularise the idea of a
Medieval Warm Period The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about . Climate proxy records show peak warmth occu ...
from around 900 to 1300, followed by a cold epoch culminating between 1550 and 1700..; In 1972 he became the founding director of the
Climatic Research Unit The Climatic Research Unit (CRU) is a component of the University of East Anglia and is one of the leading institutions concerned with the study of natural and anthropogenic climate change. With a staff of some thirty research scientists and s ...
(CRU) in the
University of East Anglia The University of East Anglia (UEA) is a Public university, public research university in Norwich, England. Established in 1963 on a campus university, campus west of the city centre, the university has four faculties and twenty-six schools of ...
(UEA), which aimed to improve knowledge of climate history in both the recent and far distant past, monitor current changes in global climate, identify processes causing changes at different timescales, and review the possibility of advising about future trends in climate. During the cold years of the 1960s, Lamb had anticipated that natural cycles were likely to lead over thousands of years to a future ice age, but after 1976 he supported the emerging view that greenhouse gas emissions caused by humanity would cause detectable
global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...
"by about A.D. 2000". The first quantitative reconstruction of Northern Hemisphere (NH) annual mean temperatures was published in 1979 by Brian Groveman and Helmut Landsberg. They used "a short-cut method" based on their earlier paper which showed that 9 instrumental stations could adequately represent an extensive gridded instrumental series, and reconstructed temperatures from 1579 to 1880 on the basis of their compilation of 20 time-series. These records were largely instrumental but also included some proxy records including two tree-ring series. Their method used nested multiple regression to allow for records covering different periods, and produced measures of uncertainty. The reconstruction showed a cool period extending beyond the Maunder Minimum, and warmer temperatures in the 20th century. After this around a decade elapsed before Gordon Jacoby and Rosanne D'Arrigo produced the next quantitative NH reconstruction, published in 1989. This was the first based entirely on non-instrumental records, and used tree rings. They reconstructed northern hemisphere annual temperatures since 1671 on the basis of boreal North American tree ring data from 11 distinct regions. From this, they concluded that recent warming was anomalous over the 300-year period, and went as far as speculating that these results supported the hypothesis that recent warming had human causes.


IPCC First Assessment Report, 1990, supplement, 1992

Publicity over the concerns of scientists about the implications of global warming led to increasing public and political interest, and the
Reagan administration Ronald Reagan's tenure as the 40th president of the United States began with his first inauguration on January 20, 1981, and ended on January 20, 1989. Reagan, a Republican from California, took office following his landslide victory over ...
, concerned in part about the political impact of scientific findings, successfully lobbied for the 1988 formation of the
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to "provide governments at all levels with scientific information that they can use to develop climate policies". The World Met ...
to produce reports subject to detailed approval by government delegates. The
IPCC First Assessment Report The First Assessment Report (FAR) of the Intergovernmental Panel on Climate Change (IPCC) was completed in 1990. It served as the basis of the United Nations Framework Convention on Climate Change (UNFCCC). This report had effects not only on the ...
in 1990 noted evidence that
Holocene climatic optimum The Holocene Climate Optimum (HCO) was a warm period in the first half of the Holocene epoch, that occurred in the interval roughly 9,500 to 5,500 years BP, with a thermal maximum around 8000 years BP. It has also been known by many other names ...
around 5,000-6,000 years ago had been warmer than the present (at least in summer) and that in some areas there had been exceptional warmth during "a shorter Medieval Warm Period (which may not have been global)", the "Medieval Climatic Optimum" from the "late tenth to early thirteenth centuries (about AD 950-1250)", followed by a cooler period of the
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Mat ...
which ended only in the middle to late nineteenth century. The report discussed the difficulties with proxy data, "mainly pollen remains, lake varves and ocean sediments, insect and animal remains, glacier termini" but considered tree ring data was "not yet sufficiently easy to assess nor sufficiently integrated with indications from other data to be used in this report." A "schematic diagram" of global temperature variations over the last thousand years. has been traced to a graph based loosely on Lamb's 1965 paper, nominally representing central England, modified by Lamb in 1982.
Mike Hulme Michael Hulme (born 23 July 1960) is Professor of Human Geography in the Department of Geography at the University of Cambridge, and also a Fellow of Pembroke College, Cambridge. He was formerly professor of Climate and Culture at King's Colleg ...
describes this schematic diagram as "Lamb's sketch on the back of an envelope", a "rather dodgy bit of hand-waving". In , a working group of climatologists including Raymond S. Bradley, Malcolm K. Hughes, Jean Jouzel, Wibjörn Karlén, Jonathan Overpeck and
Tom Wigley Tom Michael Lampe Wigley is a climate scientist at the University of Adelaide. He is also affiliated with the University Corporation for Atmospheric Research (UCAR, Boulder, CO). He was named a fellow of the American Association for the Advanc ...
proposed a project to improve understanding of natural climatic variations over the last two thousand years so that their effect could be allowed for when evaluating human contributions to climate change. Climate proxy temperature data was needed at seasonal or annual resolution covering a wide geographical area to provide a framework for testing the part climate forcings had played in past variations, look for cycles in climate, and find if debated climatic events such as the Little
Ice Age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and g ...
and
Medieval In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of World history (field), global history. It began with the fall of the West ...
Warm Period were global. Reconstructions were to be made of key climate systems, starting with three climatically sensitive regions: the Asian
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...
region, the
El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is a global climate phenomenon that emerges from variation in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cyc ...
region and the Atlantic region. Areas where more data was needed were to be identified, and there was a need for improved data exchange with computer-based archiving and translation to give researchers access to worldwide paleoclimate information. The IPCC supplementary report, 1992, reviewed progress on various proxies. These included a study of 1,000 years of tree ring data from Tasmania which, like similar studies, did not allow for possible overestimate of warming due to increased levels having a fertilisation effect on tree growth. It noted the suggestion of Bradley et al. 1991 that instrumental records in specific areas could be combined with paleoclimate data for increased detail back to the 18th century.


Composite plus scaling (CPS) reconstructions


Bradley and Jones 1993

Archives of
climate proxies Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorolog ...
were developed: in 1993 Raymond S. Bradley and Phil Jones composited historical records, tree-rings and ice cores for the Northern Hemisphere from 1400 up to the 1970s to produce a decadal reconstruction. Like later reconstructions including the MBH "hockey stick" studies, the reconstruction indicated a slow cooling trend followed by an exceptional temperature rise in the 20th century.. Their study also used the modern instrumental temperature record to evaluate how well the regions covered by proxies represented the northern hemisphere average, and compared the instrumental record with the proxy reconstruction over the same period. It concluded that the "Little Ice Age" period was complex, with evidence suggesting the influence of volcanic eruptions. It showed that temperatures since the 1920s were higher than earlier in the 500-year period, an indication of other factors which could most probably be attributed to human caused changes increasing levels of
greenhouse gas Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...
es.. This paper introduced the "Composite Plus Scaling" (CPS) method which was subsequently used by most large-scale climate reconstructions of hemispheric or global average temperatures. In this method, also known as "Composite Plus Scale", selected climate proxy records were
standardized Standardization (American English) or standardisation (British English) is the process of implementing and developing technical standards based on the consensus of different parties that include firms, users, interest groups, standards organiza ...
before being
average In colloquial, ordinary language, an average is a single number or value that best represents a set of data. The type of average taken as most typically representative of a list of numbers is the arithmetic mean the sum of the numbers divided by ...
d (composited), and then centred and scaled to provide a quantitative estimate of the target temperature series for the climate of the region or hemisphere over time. This method was implemented in various ways, including different selection processes for the proxy records, and averaging could be unweighted, or could be weighted in relation to an assessment of reliability or of area represented. There were also different ways of finding the scaling coefficient used to scale the proxy records to the instrumental temperature record.. John A. Eddy had earlier tried to relate the rarity of
sunspot Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...
s during the Maunder Minimum to Lamb's estimates of past climate, but had insufficient information to produce a quantitative assessment. The problem was reexamined by Bradley in collaboration with solar physicists Judith Lean and Juerg Beer, using the findings of . The paper confirmed that the drop in solar output appeared to have caused a temperature drop of almost 0.5 °C during the
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Mat ...
, and increased solar output might explain the rise in early 20th century temperatures. A reconstruction of Arctic temperatures over four centuries by reached similar conclusions, but both these studies came up against the limitations of the climate reconstructions at that time which only resolved temperature fluctuations on a decadal basis rather than showing individual years, and produced a single time series so did not show a spatial pattern of relative temperatures for different regions.


IPCC Second Assessment Report

The
IPCC Second Assessment Report The Second Assessment Report (SAR) of the Intergovernmental Panel on Climate Change (IPCC), published in 1995, is an assessment of the then available scientific and socio-economic information on climate change. The report was split into four parts ...
(SAR) of 1996 featured Figure 3.20 showing the decadal summer temperature reconstruction for the northern hemisphere, overlaid with a 50-year smoothed curve and with a separate curve plotting instrumental thermometer data from the 1850s onwards. It stated that in this record, warming since the late 19th century was unprecedented. The section proposed that "The data from the last 1000 years are the most useful for determining the scales of natural climate variability". Recent studies including the 1994 reconstruction by Hughes and Diaz questioned how widespread the Medieval Warm Period had been at any one time, thus it was not possible "to conclude that global temperatures in the Medieval Warm Period were comparable to the warm decades of the late 20th century." The SAR concluded, "it appears that the 20th century has been at least as warm as any century since at least 1400 AD. In at least some areas, the recent period appears to be warmer than has been the case for a thousand or more years"., pp. 173–176, fig 3.20; . Tim Barnett of the
Scripps Institution of Oceanography Scripps Institution of Oceanography (SIO) is the center for oceanography and Earth science at the University of California, San Diego. Its main campus is located in La Jolla, with additional facilities in Point Loma. Founded in 1903 and incorpo ...
was working towards the next IPCC assessment with Phil Jones, and in 1996 told journalist
Fred Pearce Fred Pearce (born 30 December 1951) is an English science writer and public speaker based in London. He reports on the environment, popular science and development issues. He specialises in global environmental issues, including water and clim ...
"What we hope is that the current patterns of temperature change prove distinctive, quite different from the patterns of natural variability in the past". A divergence problem affecting some tree ring proxies after 1960 had been identified in
Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
by and . Tree ring specialist
Keith Briffa Keith Raphael Briffa (27 December 1952 – 29 October 2017) was a climatologist and deputy director of the Climatic Research Unit. He authored or co-authored over 130 scholarly articles, chapters and books. In his professional work, he focused on ...
's February 1998 study showed that this problem was more widespread at high northern latitudes, and warned that it had to be taken into account to avoid overestimating past temperatures.


Climate field reconstruction (CFR) methods; MBH 1998 and 1999

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. Related rigorous statistical methods had been developed for tree ring data, with Harold C. Fritts publishing a 1991 study and a 1991 book showing methodology and examples of how to produces maps showing climate developments in North America over time. These
methods Method (, methodos, from μετά/meta "in pursuit or quest of" + ὁδός/hodos "a method, system; a way or manner" of doing, saying, etc.), literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In re ...
had been used for regional reconstructions of temperatures, and other aspects such as rainfall. 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 Global surface temperature (GST) is the average temperature of Earth's surface. More precisely, it is the weighted average of the temperatures over the ocean and land. The former is also called sea surface temperature and the latter is calle ...
of
global surface temperature Global surface temperature (GST) is the average temperature of Earth's surface. More precisely, it is the weighted average of the temperatures over the ocean and land. The former is also called sea surface temperature and the latter is called ...
s over the last 140 years; showed patterns relating to the
El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is a global climate phenomenon that emerges from variation in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cyc ...
, and found what was later termed the
Atlantic multidecadal oscillation The Atlantic Multidecadal Oscillation (AMO), also known as Atlantic Multidecadal Variability (AMV), is the theorized variability of the sea surface temperature (SST) of the North Atlantic Ocean on the timescale of several decades. While there ...
. 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 In linear algebra, an eigenvector ( ) or characteristic vector is a vector that has its direction unchanged (or reversed) by a given linear transformation. More precisely, an eigenvector \mathbf v of a linear transformation T is scaled by ...
-based climate field reconstruction (CFR) incorporating multiple climate proxy data sets of different types and lengths into a high-resolution global reconstruction. To relate this data to measured temperatures, they used
principal component analysis Principal component analysis (PCA) is a linear dimensionality reduction technique with applications in exploratory data analysis, visualization and data preprocessing. The data is linearly transformed onto a new coordinate system such that th ...
(PCA) to find the leading patterns, or principal components, of
instrumental temperature record Global surface temperature (GST) is the average temperature of Earth's surface. More precisely, it is the weighted average of the temperatures over the ocean and land. The former is also called sea surface temperature and the latter is calle ...
s during the calibration period from 1902 to 1980. Their method was based on separate multiple regressions between each proxy record (or summary) and all of the leading principal components of the instrumental record. The
least squares The method of least squares is a mathematical optimization technique that aims to determine the best fit function by minimizing the sum of the squares of the differences between the observed values and the predicted values of the model. The me ...
simultaneous solution of these multiple regressions used
covariance In probability theory and statistics, covariance is a measure of the joint variability of two random variables. The sign of the covariance, therefore, shows the tendency in the linear relationship between the variables. If greater values of one ...
between the proxy records. The results were then used to reconstruct large-scale patterns over time in the spatial field of interest (defined as the
empirical orthogonal functions In statistics and signal processing, the method of empirical orthogonal function (EOF) analysis is a decomposition of a signal or data set in terms of orthogonal basis functions which are determined from the data. The term is also interchangeable ...
, or EOFs) using both local relationships of the proxies to climate and distant climate
teleconnection Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of kilometers). The most emblematic teleconnection is that linking sea-level pressure at Tahiti and Darwin, Aust ...
s. Temperature records for almost 50 years prior to 1902 were analysed using PCA for the important step of validation calculations, which showed that the reconstructions were statistically meaningful, or skillful. 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 region The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitu ...
s and the
tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
, 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 Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...
. The primary aim of CFR methods was to provide the spatially resolved reconstructions essential for coherent
geophysical Geophysics () is a subject of natural science concerned with the physical processes and properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct investigations acros ...
understanding of how parts of the climate system varied and responded to
radiative forcing Radiative forcing (or climate forcing) is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of greenhouse gases ...
, so hemispheric averages were a secondary product. The CFR method could also be used to reconstruct Northern Hemisphere
mean A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...
temperatures, and the results closely resembled the earlier CPS reconstructions including . Mann describes this as the least scientifically interesting thing they could do with the rich spatial patterns, but also the aspect that got the most attention. Their original draft ended in 1980 as most reconstructions only went that far, but an anonymous
peer review Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work (:wiktionary:peer#Etymology 2, peers). It functions as a form of self-regulation by qualified members of a profession within the ...
er of the paper suggested that the curve of instrumental temperature records should be shown up to the present to include the considerable warming that had taken place between 1980 and 1998. The (MBH98) multiproxy study on "Global-scale temperature patterns and climate forcing over the past six centuries" was submitted to the journal ''
Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'' 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 Radiative forcing (or climate forcing) is a concept used to quantify a change to the Earth's energy budget, balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentration ...
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 Tom Michael Lampe Wigley is a climate scientist at the University of Adelaide. He is also affiliated with the University Corporation for Atmospheric Research (UCAR, Boulder, CO). He was named a fellow of the American Association for the Advanc ...
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 The Climatic Research Unit (CRU) is a component of the University of East Anglia and is one of the leading institutions concerned with the study of natural and anthropogenic climate change. With a staff of some thirty research scientists and s ...
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 model Numerical climate models (or climate system models) are mathematical models that can simulate the interactions of important drivers of climate. These drivers are the atmosphere, oceans, land surface and ice. Scientists use climate models to st ...
ing, which showed a "pretty reasonable" fit to proxy evidence. A commentary on MBH98 by Jones was published in ''
Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
'' on 24 April 1998. He noted that it used almost all the available long term proxy climate series, "and if the new multivariate method of relating these series to the instrumental data is as good as the paper claims, it should be statistically reliable." He discussed some of the difficulties, and emphasised that "Each paleoclimatic discipline has to come to terms with its own limitations and must unreservedly admit to problems, warts and all." 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 The George C. Marshall Institute (GMI) was a nonprofit conservative think tank in the United States. It was established in 1984 with a focus on science and public policy issues and had an initial focus in defense policy. Starting in the late 1980 ...
alleged that MBH98 was deceptive in only going back to 1400, and so not covering the
Medieval Warm Period The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about . Climate proxy records show peak warmth occu ...
which predated industrial greenhouse gas emissions. The same criticisms were made by
Willie Soon Willie Wei-Hock Soon (born September 30, 1965) is a Malaysian astrophysicist and aerospace engineer who was long employed as a part-time externally funded researcher at the Solar and Stellar Physics (SSP) Division of the Harvard–Smithsonian Ce ...
and
Sallie Baliunas Sallie Louise Baliunas (born February 23, 1953) is a retired astrophysicist. She formerly worked at the Harvard-Smithsonian Center for Astrophysics, Center for Astrophysics Harvard & Smithsonian and was the Deputy Director of the Mount Wilson Ob ...
.


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 Keith Raphael Briffa (27 December 1952 – 29 October 2017) was a climatologist and deputy director of the Climatic Research Unit. He authored or co-authored over 130 scholarly articles, chapters and books. In his professional work, he focused on ...
, Tim P. Barnett and
Simon Tett Simon Tett is a climatologist at the University of Edinburgh who was formerly with the Hadley Centre The Met Office Hadley Centre — named in honour of George Hadley — is one of the United Kingdom's leading centres for the study of scienti ...
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 ''The Holocene'' is a peer-reviewed scientific journal that covers research in the field of environmental studies, in particular environmental change over the last years, particularly the interface between the long Quaternary record and the natu ...
'' 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 The International Tree-Ring Data Bank (ITRDB) is a data repository for tree ring measurements that has been maintained since 1990 by the United States' National Oceanic and Atmospheric Administration Paleoclimatology Program and World Data Center fo ...
) 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 ''Geophysical Research Letters'' is a biweekly peer-reviewed scientific journal of geoscience published by the American Geophysical Union that was established in 1974. The editor-in-chief iKristopher Karnauskas Aims and scope The journal aims for ...
'' 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 The University of Massachusetts Amherst (UMass Amherst) is a public land-grant research university in Amherst, Massachusetts, United States. It is the flagship campus of the University of Massachusetts system and was founded in 1863 as the ...
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 ''The New York Times'' (''NYT'') is an American daily newspaper based in New York City. ''The New York Times'' covers domestic, national, and international news, and publishes opinion pieces, investigative reports, and reviews. As one of ...
'' 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 Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the Earth's orbit around the Sun (see Milankovitch cycles). These orbital changes modify the total amount of sunlight reaching the Earth by up to ...
, 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 In mathematics, a time series is a series of data points indexed (or listed or graphed) in time order. Most commonly, a time series is a sequence taken at successive equally spaced points in time. Thus it is a sequence of discrete-time data. ...
line graph In the mathematics, mathematical discipline of graph theory, the line graph of an undirected graph is another graph that represents the adjacencies between edge (graph theory), edges of . is constructed in the following way: for each edge i ...
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 The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about . Climate proxy records show peak warmth occu ...
(about as warm as the 1950s) down to a cooler
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Mat ...
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 The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with Weather forecasting, forecasting weather, monitoring oceanic and atmospheric conditions, Hydrography, charting the seas, ...
's
Geophysical Fluid Dynamics Laboratory The Geophysical Fluid Dynamics Laboratory (GFDL) is a laboratory in the National Oceanic and Atmospheric Administration (NOAA) Office of Oceanic and Atmospheric Research (OAR). The current director is Venkatachalam Ramaswamy. It is one of seven ...
,
Jerry Mahlman Jerry Mahlman (February 21, 1940 – November 28, 2012) was an American meteorologist and climatologist. Biography Mahlman was born on February 21, 1940, in Crawford, Nebraska, and received his undergraduate degree from Chadron State College in ...
nicknamed the graph the "hockey stick", with the slow cooling trend the "stick", and the anomalous 20th century warming the "blade".


Critique and independent reconstructions

Briffa and Tim Osborn critically examined MBH99 in a May 1999 detailed study of the uncertainties of various proxies. They raised questions later adopted by critics of Mann's work, including the point that
bristlecone pine The term bristlecone pine covers three species of pine tree (family Pinaceae, genus ''Pinus'', subsection ''Balfourianae''). All three species are long-lived and highly resilient to harsh weather and bad soils. One of the three species, ''Pinus ...
s from the Western U.S. could have been affected by pollution such as rising levels as well as temperature. The temperature curve was supported by other studies, but most of these shared the limited well dated proxy evidence then available, and so few were truly independent. The uncertainties in earlier times rose as high as those in the reconstruction at 1980, but did not reach the temperatures of later thermometer data. They concluded that although the 20th century was almost certainly the warmest of the millennium, the amount of anthropogenic warming remains uncertain.". 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 The World Meteorological Organization (WMO) is a List of specialized agencies of the United Nations, specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology an ...
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 ''Quaternary Science Reviews'' is a peer-reviewed scientific journal covering quaternary science. It was established in 1982 by Pergamon Press and is currently published by Elsevier. The editor-in-chief is C.V. Murray Wallace (University of Wollo ...
'' 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 Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W ...
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

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 .
2.3.2.2 Multi-proxy synthesis of recent temperature change
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. Figure 5 in WG1 Technical Summary B (as shown to the right) repeated this figure without the linear trend line declining from AD 1000 to 1850. 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 Shanghai, Shanghainese: , Standard Chinese pronunciation: is a direct-administered municipality and the most populous urban area in China. The city is located on the Chinese shoreline on the southern estuary of the Yangtze River, with the ...
, 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. (BBC News)


Scientific debates

The borehole temperature reconstruction covering the past five centuries supported the conclusion that 20th century warming was exceptional. In a perspective commenting on MBH99,
Wallace Smith Broecker Wallace "Wally" Smith Broecker (November 29, 1931 – February 18, 2019) was an American geochemist. He was the Newberry Professor in the Department of Earth and Environmental Sciences at Columbia University, a scientist at Columbia's Lamont–D ...
argued that the Medieval Warm Period (MWP) was global. He attributed recent warming to a roughly 1500-year cycle which he suggested related to episodic changes in the Atlantic's conveyor circulation. A March 2002 tree ring reconstruction by Jan Esper et al. noted the debate, and Broecker's criticism that MBH99 did not show a clear MWP. They concluded that the MWP was likely to have been widespread in the extratropical northern hemisphere, and seemed to have approached late 20th century temperatures at times. In an interview, Mann said the study did not contradict MBH as it dealt only with extratropical land areas, and stopped before the late 20th century. He reported that Edward R. Cook, a co-author on the paper, had confirmed agreement with these points, and a later paper by Cook, Esper and D'Arrigo reconsidered the earlier paper's conclusions along these lines..
Lonnie Thompson Lonnie Thompson (born July 1, 1948), is an American paleoclimatologist and university professor in the School of Earth Sciences at Ohio State University. He has achieved global recognition for his drilling and analysis of ice cores from ice caps ...
published a paper on "Tropical Glacier and Ice Core Evidence of Climate Change" in January 2003, featuring Figure 7 showing graphs based on ice cores closely resembling a graph based on the MBH99 reconstruction, combined with thermometer readings from Jones et al. 1999..


RegEM climate field reconstruction

In March 2001 Tapio Schneider published his regularized expectation–maximization (RegEM) technique for analysis of incomplete climate data. The original MBH98 and MBH99 papers avoided undue representation of large numbers of tree ring proxies by using a
principal component analysis Principal component analysis (PCA) is a linear dimensionality reduction technique with applications in exploratory data analysis, visualization and data preprocessing. The data is linearly transformed onto a new coordinate system such that th ...
step to summarise these proxy networks, but from 2001 Mann stopped using this method and introduced a multivariate Climate Field Reconstruction (CFR) technique based on the RegEM method which did not require this PCA step. In May 2002 Mann and Scott Rutherford published a paper on testing methods of climate reconstruction which discussed this technique. By adding artificial noise to actual temperature records or to model simulations they produced synthetic datasets which they called "pseudoproxies". When the reconstruction procedure was used with these pseudoproxies, the result was then compared with the original record or simulation to see how closely it had been reconstructed. The paper discussed the issue that regression methods of reconstruction tended to underestimate the amplitude of variation.;


Controversy after IPCC Third Assessment Report

While the IPCC Third Assessment Report (TAR) drew on five reconstructions to support its conclusion that recent Northern Hemisphere temperatures were the warmest in the past 1,000 years, it gave particular prominence to an IPCC illustration based on the MBH99 paper. The hockey stick graph was subsequently seen by mass media and the public as central to the IPCC case for global warming, which had actually been based on other unrelated evidence. From an expert viewpoint the graph was, like all newly published science, preliminary and uncertain, but it was widely used to publicise the issue of global warming, and it was targeted by those opposing ratification of the
Kyoto Protocol The was an international treaty which extended the 1992 United Nations Framework Convention on Climate Change (UNFCCC) that commits state parties to reduce greenhouse gas emissions, based on the scientific consensus that global warming is oc ...
on global warming. A
literature review A literature review is an overview of previously published works on a particular topic. The term can refer to a full scholarly paper or a section of a scholarly work such as books or articles. Either way, a literature review provides the rese ...
by
Willie Soon Willie Wei-Hock Soon (born September 30, 1965) is a Malaysian astrophysicist and aerospace engineer who was long employed as a part-time externally funded researcher at the Solar and Stellar Physics (SSP) Division of the Harvard–Smithsonian Ce ...
and
Sallie Baliunas Sallie Louise Baliunas (born February 23, 1953) is a retired astrophysicist. She formerly worked at the Harvard-Smithsonian Center for Astrophysics, Center for Astrophysics Harvard & Smithsonian and was the Deputy Director of the Mount Wilson Ob ...
, published in the relatively obscure journal ''
Climate Research Climatology (from Greek , ''klima'', "slope"; and , ''-logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. Climate concerns the atmospheric ...
'' on 31 January 2003, used data from previous papers to argue that the Medieval Warm Period had been warmer than the 20th century, and that recent warming was not unusual. In March they published an extended paper in ''
Energy & Environment ''Energy & Environment'' is an academic journal "covering the direct and indirect environmental impacts of energy acquisition, transport, production and use".
'', with additional authors.. The Bush administration's
Council on Environmental Quality The Council on Environmental Quality (CEQ) is a division of the Executive Office of the President of the United States, Executive Office of the President that coordinates federal Natural environment, environmental efforts in the United States ...
chief of staff
Philip Cooney Philip A. Cooney (born July 16, 1959) is a former member of the administration of United States President George W. Bush. Before being appointed to work in the Council on Environmental Quality, he was a lawyer and lobbyist for the American Petr ...
inserted references to the papers in the draft first
Environmental Protection Agency Environmental Protection Agency may refer to the following government organizations: * Environmental Protection Agency (Queensland), Australia * Environmental Protection Agency (Ghana) * Environmental Protection Agency (Ireland) * Environmenta ...
''Report on the Environment'', and removed all references to reconstructions showing world temperatures rising over the last 1,000 years. In the Soon and Baliunas controversy, two scientists cited in the papers said that their work was misrepresented, and the ''Climate Research'' paper was criticised by many other scientists, including several of the journal's editors. On 8 July ''
Eos In ancient Greek mythology and Ancient Greek religion, religion, Eos (; Ionic Greek, Ionic and Homeric Greek ''Ēṓs'', Attic Greek, Attic ''Héōs'', "dawn", or ; Aeolic Greek, Aeolic ''Aúōs'', Doric Greek, Doric ''Āṓs'') is the go ...
'' featured a detailed rebuttal of both papers by 13 scientists including Mann and Jones, presenting strong evidence that Soon and Baliunas had used improper statistical methods. Responding to the controversy, the publisher of ''Climate Research'' upgraded Hans von Storch from editor to editor in chief, but von Storch decided that the Soon and Baliunas paper was seriously flawed and should not have been published as it was. He proposed a new editorial system, and though the publisher of ''Climate Research'' agreed that the paper should not have been published uncorrected, he rejected von Storch's proposals to improve the editorial process, and von Storch with three other board members resigned.. Senator
James M. Inhofe James Mountain Inhofe (; ; November 17, 1934 – July 9, 2024) was an American politician who served as a United States senator from Oklahoma from 1994 to 2023. A member of the Republican Party, he was the longest serving U.S. senator from Okl ...
stated his belief that "manmade global warming is the greatest hoax ever perpetrated on the American people", (NYT). and a hearing of the
United States Senate Committee on Environment and Public Works The United States Senate Committee on Environment and Public Works is responsible for legislation and oversight of the natural and built environment and for studying matters concerning environmental protection and resource conservation and util ...
which he convened on 29 July 2003 heard the news of the resignations. (hearing). Stephen McIntyre downloaded datasets for MBH99, and obtained MBH98 datasets by request to Mann in April 2003. At the suggestion of
Sonja Boehmer-Christiansen Sonja Boehmer-Christiansen (born 1942) is an Emeritus Reader in the Department of Geography at the University of Hull in Kingston upon Hull England, where she taught environmental policy, management and politics. She was editor of the journal ...
, editor of the
social science Social science (often rendered in the plural as the social sciences) is one of the branches of science, devoted to the study of societies and the relationships among members within those societies. The term was formerly used to refer to the ...
journal ''
Energy & Environment ''Energy & Environment'' is an academic journal "covering the direct and indirect environmental impacts of energy acquisition, transport, production and use".
'', McIntyre wrote an article with the assistance of
University of Guelph The University of Guelph (abbreviated U of G) is a comprehensive Public university, public research university in Guelph, Ontario, Canada. It was established in 1964 after the amalgamation of Ontario Agricultural College (1874), the MacDonald I ...
economics professor Ross McKitrick, which ''Energy & Environment'' published on 27 October 2003. The paper (MM03) said that the (MBH98) "hockey stick" shape was "primarily an artefact of poor data handling and use of obsolete proxy records.". Their criticism was comprehensively refuted by ,, (p. 19, n1 in pdf). which showed errors in the methods used by McIntyre and McKitrick.
Sec. 6.6.1.1: What Do Reconstructions Based on Palaeoclimatic Proxies Show?

p. 466
.
The statistical methods used in the MBH reconstruction were questioned in a 2004 paper by Hans von Storch with a team including Eduardo Zorita, which said that the methodology used to average the data and the wide uncertainties might have hidden abrupt climate changes, possibly as large as the 20th century spike in measured temperatures. They used the pseudoproxy method which Mann and Rutherford had developed in 2002, and like them found that regression methods of reconstruction tended to underestimate the amplitude of variation, a problem covered by the wide error bars in MBH99. It was a reasonable critique of nearly all the reconstructions at that time, but MBH were singled out. Other researchers subsequently found that the von Storch paper had an undisclosed additional step which, by detrending data before estimating statistical relationships, had removed the main pattern of variation. The von Storch et al. view that the graph was defective overall was refuted by Wahl, Ritson and Ammann (2006).. In 2004 McIntyre and McKitrick tried unsuccessfully to get an extended analysis of the hockey stick into the journal ''
Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
''., At this stage ''Nature'' contacted Mann, Bradley, and Hughes, about minor errors in the online supplement to MBH98. In a
corrigendum An erratum or corrigendum (: errata, corrigenda) (comes from ) is a correction of a published text. Generally, publishers issue an erratum for a production error (i.e., an error introduced during the publishing process) and a corrigendum for an a ...
published on 1 July 2004 they acknowledged that McIntyre and McKitrick had pointed out errors in proxy data that had been included as supplementary information, and supplied a full corrected listing of the data. They included a documented archive of all the data used in MBH98, and expanded details of their methods. They stated that "None of these errors affect our previously published results." The McIntyre and McKitrick comment was accepted for publication by ''
Geophysical Research Letters ''Geophysical Research Letters'' is a biweekly peer-reviewed scientific journal of geoscience published by the American Geophysical Union that was established in 1974. The editor-in-chief iKristopher Karnauskas Aims and scope The journal aims for ...
''. (MM05) reported a technical statistical error in the (MBH98) method, which they said would produce hockey stick shapes from random data. This claim was given widespread publicity and political spin. Scientists found that the issues raised by McIntyre and McKitrick were minor and did not affect the main conclusions of MBH98 or . Mann himself had already stopped using the criticised statistical method in 2001, when he changed over to the RegEM climate field reconstruction method. To balance dense networks of tree-ring proxies against sparse proxy temperature records such as lake sediments, ice cores or corals, MBH 1998 (and 1999) used
principal component analysis Principal component analysis (PCA) is a linear dimensionality reduction technique with applications in exploratory data analysis, visualization and data preprocessing. The data is linearly transformed onto a new coordinate system such that th ...
(PCA) to find the leading patterns of variation (PC1, PC2, PC3 etc.), with an objective method establish how many significant principal components should be kept so that the patterns put together characterized the original dataset. McIntyre and McKitrick highlighted the effect of centering over the 1902–1980 period rather than the whole 1400–1980 period which would have changed the order of principal components so that the warming pattern of high altitude tree ring data was demoted from PC1 to PC4,. but instead of recalculating the objective selection rule which increased the number of significant PCs from two to five, they only kept PC1 and PC2. This removed the significant 20th century warming pattern of PC4, discarding data that produced the "hockey stick" shape, Subsequent investigation showed that the "hockey stick" shape remained with the correct selection rule. The MM05 paper claimed that 1902–1980 centering would produce hockey stick shapes from "persistent red noise", but their methods exaggerated the effect. Tests of the MBH98 methodology on pseudoproxies formed with noise varying from
red noise In science, Brownian noise, also known as Brown noise or red noise, is the type of signal noise produced by Brownian motion, hence its alternative name of random walk noise. The term "Brown noise" does not come from the color, but after Rober ...
to
white noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used with this or similar meanings in many scientific and technical disciplines, i ...
found that this effect caused only very small differences which were within the uncertainty range and had no significance for the final reconstruction.. McIntyre and McKitrick's code selected 100 simulations with the highest "hockey stick index" from the 10,000 simulations they had carried out, and their illustrations were taken from this pre-selected 1%. On 23 June 2005, Rep.
Joe Barton Joseph Linus Barton (born September 15, 1949) is an American politician. A member of the Republican Party (United States), Republican Party, he represented in the United States House of Representatives, U.S. House of Representatives from 1985 t ...
, chairman of the
House Committee on Energy and Commerce A house is a single-unit residential building. It may range in complexity from a rudimentary hut to a complex structure of wood, masonry, concrete or other material, outfitted with plumbing, electrical, and heating, ventilation, and air condi ...
wrote joint letters with
Ed Whitfield Wayne Edward Whitfield (born May 25, 1943) is an American politician and attorney who served as the U.S. representative of from January 1995, until his resignation in September 2016. He is a member of the Republican Party, and the first to re ...
, Chairman of the Subcommittee on Oversight and Investigations, referring to the publicity and demanding full records on climate research, as well as personal information about their finances and careers, from the three scientists
Mann Mann may refer to: Arts, entertainment and media * ''Mann'' (film), a 1999 Indian Hindi-language romantic drama * Mann (chess), a variant chess piece * ''Mann'' (magazine), a Norwegian magazine * Mann (rapper), Dijon Shariff Thames (born 19 ...
,
Bradley Bradley may refer to: People * Bradley (given name) * Bradley (surname) Places In the United Kingdom In England: * Bradley, Cheshire * Bradley, Derbyshire * Bradley (house), a manor in Kingsteignton, Devon * Bradley, Gloucestershire * ...
and Hughes.
Hunting Witches"
Sherwood Boehlert Sherwood Louis Boehlert (September 28, 1936September 20, 2021) was an American politician from New York. He represented a large swath of central New York in the United States House of Representatives from 1983 until 2007. Boehlert, a Republica ...
, chairman of the
House Science Committee The Committee on Science, Space, and Technology is a committee of the United States House of Representatives. It has jurisdiction over non-defense federal scientific research and development. More specifically, the committee has complete jurisdic ...
, told his fellow Republican Joe Barton it was a "misguided and illegitimate investigation" apparently aimed at intimidating scientists. The
U.S. National Academy of Sciences The National Academy of Sciences (NAS) is a United States nonprofit, non-governmental organization. NAS is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of Engineering (NAE) and the Natio ...
(NAS) president Ralph J. Cicerone proposed that the NAS should appoint an independent panel to investigate. Barton dismissed this offer, (Washington Post). but following Boehlert's November 2005 request, the National Academy of Science arranged for its National Research Council to set up a special committee chaired by Gerald North, to investigate and report. (NYT). The North Report went through a rigorous review process, and was published on 22 June 2006.. It concluded "with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries", justified by consistent evidence from a wide variety of geographically diverse proxies, but "Less confidence can be placed in large-scale surface temperature reconstructions for the period from 900 to 1600". It broadly agreed with the basic findings of the original MBH studies which had subsequently been supported by other reconstructions and proxy records, while emphasising uncertainties over earlier periods. The contested principal component analysis methodology had a small tendency to bias results so was not recommended, but it had little influence on the final reconstructions, and other methods produced similar results.. Barton's staffer contacted statistician
Edward Wegman Edward Wegman is an American statistician and was a professor of statistics at George Mason University (GMU) until his retirement in 2018. He holds a Ph.D. in mathematical statistics and is a Fellow of the American Statistical Association, a Senio ...
who produced the
Wegman report The Wegman Report (officially called the Ad Hoc Committee Report on the 'Hockey Stick' Global Climate Reconstruction) was prepared in 2006 by three statisticians led by Edward Wegman at the request of Rep. Joe Barton of the United States House Comm ...
with his graduate student Yasmin H. Said, and statistician David W. Scott, all statisticians with no expertise in
climatology Climatology (from Greek , ''klima'', "slope"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. Climate concerns the atmospher ...
or other
physical sciences Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together is called the "physical sciences". Definition ...
. The Wegman report was announced on 14 July 2006 in the ''
Wall Street Journal ''The Wall Street Journal'' (''WSJ''), also referred to simply as the ''Journal,'' is an American newspaper based in New York City. The newspaper provides extensive coverage of news, especially business and finance. It operates on a subscriptio ...
'',
Hockey Stick Hokum
and discussed at hearings of the
United States House Energy Subcommittee on Oversight and Investigations The U.S. House Energy Subcommittee on Oversight and Investigations is a subcommittee within the House Committee on Energy and Commerce. Jurisdiction Responsibility for oversight of agencies, departments, and programs within the jurisdiction of th ...
on 19 July 2006, and 27 July 2006.. The report was not properly peer reviewed. It reiterated McIntyre and McKitrick's claims on statistical failings in the MBH studies, but did not quantify whether correcting these points had any significant effect. It included a
social network analysis Social network analysis (SNA) is the process of investigating social structures through the use of networks and graph theory. It characterizes networked structures in terms of ''nodes'' (individual actors, people, or things within the network) ...
to allege a lack of independent peer review of Mann's work: this analysis has been discredited by expert opinion and found to have issues of plagiarism...


Reconstructions 2003–2006

Using various high-resolution proxies including tree rings, ice cores and sediments, Mann and Jones published reconstructions in August 2003 which indicated that "late 20th century warmth is unprecedented for at least roughly the past two millennia for the Northern Hemisphere. Conclusions for the Southern Hemisphere and global mean temperature are limited by the sparseness of available proxy data in the Southern Hemisphere at present." They concluded that "To the extent that a 'Medieval' interval of moderately warmer conditions can be defined from about AD 800 – 1400, any hemispheric warmth during that interval is dwarfed in magnitude by late 20th century warmth.". Borehole climate reconstructions in a paper by Pollack and Smerdon, published in June 2004, supported estimates of a surface warming of around over the period from 1500 to 2000.. In a study published in November 2004 Edward R. Cook, Jan Esper and Rosanne D'Arrigo re-examined their 2002 paper, and now supported MBH. They concluded that "annual temperatures up to AD 2000 over extra-tropical NH land areas have probably exceeded by about 0.3 °C the warmest previous interval over the past 1162 years". A study by Anders Moberg et al. published on 10 February 2005 used a wavelet transform technique to reconstruct Northern Hemisphere temperatures over the last 2,000 years, combining low-resolution proxy data such as lake and ocean sediments for century-scale or longer changes, with tree ring proxies only used for annual to decadal resolution. They found there had been a peak of temperatures around AD 1000 to 1100 similar to those reached in the years before 1990,. and supported the basic conclusion of MBH99 by stating "We find no evidence for any earlier periods in the last two millennia with warmer conditions than the post-1990 period". (graph). At the end of April 2005 ''Science'' published a reconstruction by J. Oerlemans based on glacier length records from different parts of the world, and found consistent independent evidence for the period from 1600 to 1990 supporting other reconstructions regarding magnitude and timing of global warming.. On 28 February 2006 was accepted for publication, and an "in press" copy was made available on the internet. Two more reconstructions were published, using different methodologies and supporting the main conclusions of MBH. Rosanne D'Arrigo, Rob Wilson and Gordon Jacoby suggested that medieval temperatures had been almost 0.7 °C cooler than the late 20th century but less homogenous,. Osborn and Briffa found the spatial extent of recent warmth more significant than that during the medieval warm period... They were followed in April by a third reconstruction led by Gabriele C. Hegerl..


IPCC Fourth Assessment Report, 2007

The
IPCC Fourth Assessment Report ''Climate Change 2007'', the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC), was published in 2007 and is the fourth in a series of reports intended to assess scientific, technical and soci ...
(AR4) published in 2007 included a chapter on Paleoclimate, with a section on the last 2,000 years. This featured a graph showing 12 proxy based temperature reconstructions, including the three highlighted in the IPCC Third Assessment Report (TAR); as before, and had both been calibrated by newer studies. In addition, analysis of the
Medieval Warm Period The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about . Climate proxy records show peak warmth occu ...
cited reconstructions by (as cited in the TAR) and . Ten of these 14 reconstructions covered 1,000 years or longer. Most reconstructions shared some data series, particularly tree ring data, but newer reconstructions used additional data and covered a wider area, using a variety of statistical methods. The section discussed the divergence problem affecting certain tree ring data.
Section 6.6: The Last 2,000 Years
.
It concluded that "The weight of current multi-proxy evidence, therefore, suggests greater 20th-century warmth, in comparison with temperature levels of the previous 400 years, than was shown in the TAR. On the evidence of the previous and four new reconstructions that reach back more than 1 kyr, it is likely that the 20th century was the warmest in at least the past 1.3 kyr." The SPM statement in the IPCC TAR of 2001 had been 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. The AR4 SPM statement was that "Average Northern Hemisphere temperatures during the second half of the 20th century were ''very likely'' higher than during any other 50-year period in the last 500 years and ''likely'' the highest in at least the past 1,300 years. Some recent studies indicate greater variability in Northern Hemisphere temperatures than suggested in the TAR, particularly finding that cooler periods existed in the 12th to 14th, 17th and 19th centuries. Warmer periods prior to the 20th century are within the uncertainty range given in the TAR."


Mann et al., 2008 and 2009

Further reconstructions were published, using additional proxies and different methodology. and compared and evaluated the various statistical approaches. In July 2008 Huang, Pollack and Shen published a suite of borehole reconstructions covering 20,000 years. They showed warm episodes in the mid-Holocene and the Medieval period, a little ice age and 20th century warming reaching temperatures higher than Medieval Warm Period peak temperatures in any of the reconstructions: they described this finding as consistent with the IPCC AR4 conclusions. In a paper published by PNAS on 9 September 2008, Mann and colleagues produced updated reconstructions of Earth surface temperature for the past two millennia. This reconstruction used a more diverse dataset that was significantly larger than the original tree-ring study, at more than 1,200 proxy records. They used two complementary methods, both of which showed a similar "hockey stick" graph with recent increases in northern hemisphere surface temperature are anomalous relative to at least the past 1300 years. Mann said, "Ten years ago, the availability of data became quite sparse by the time you got back to 1,000 AD, and what we had then was weighted towards tree-ring data; but now you can go back 1,300 years without using tree-ring data at all and still get a verifiable conclusion." In a PNAS response, McIntyre and McKitrick said that they perceived a number of problems, including that Mann ''et al'' used some data with the axes upside down. . Mann et al. replied that McIntyre and McKitrick "raise no valid issues regarding our paper" and the "claim that 'upside down' data were used is bizarre", as the methods "are insensitive to the sign of predictors." They also said that excluding the contentious datasets has little effect on the result. A study of the changing
climate of the Arctic The climate of the Arctic is characterized by long, cold winters and short, cool summers. There is a large amount of variability in climate across the Arctic, but all regions experience extremes of Solar irradiance, solar radiation in both summ ...
over the last 2,000 years, by an international consortium led by Darrell Kaufman of
Northern Arizona University Northern Arizona University (NAU) is a public research university based in Flagstaff, Arizona, United States. Founded in 1899, it was the third and final university established in the Arizona Territory. It is one of the three universities gove ...
, was published on 4 September 2009. They examined sediment core records from 14 Arctic lakes, supported by tree ring and ice core records. Their findings showed a long term cooling trend consistent with cycles in the Earth's orbit which would be expected to continue for a further 4,000 years but had been reversed in the 20th century by a sudden rise attributed to greenhouse gas emissions. The decline had continued through the Medieval period and the Little Ice Age. The most recent decade, 1999–2008, was the warmest of the period, and four of the five warmest decades occurred between 1950 and 2000.
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it, with more than 150 Nobel Pri ...
described the graph as largely replicating "the so-called 'hockey stick,' a previous reconstruction". Further support for the "hockey stick" graph came from a new method of analysis using
Bayesian statistics Bayesian statistics ( or ) is a theory in the field of statistics based on the Bayesian interpretation of probability, where probability expresses a ''degree of belief'' in an event. The degree of belief may be based on prior knowledge about ...
developed by Martin Tingley and Peter Huybers of Harvard University, which produced the same basic shape, albeit with more variability in the past, and found the 1990s to have been the warmest decade in the 600-year period the study covered.


2010 onwards

A 2,000 year extratropical Northern Hemisphere reconstruction by Ljungqvist published by ''
Geografiska Annaler ''Geografiska Annaler'' is a scientific journal published by the Swedish Society for Anthropology and Geography in Stockholm, Sweden. The journal is founded in 1919. Since 1965 the journal is published in two series A and B. Series A deals with ar ...
'' in September 2010 drew on additional proxy evidence to show both a
Roman Warm Period The Roman Warm Period, or Roman Climatic Optimum, was a period of unusually-warm weather in Europe and the North Atlantic that ran from approximately 250 BC to AD 400. Theophrastus (371 – c. 287 BC) wrote that date trees could grow in Greece i ...
and a
Medieval Warm Period The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about . Climate proxy records show peak warmth occu ...
with decadal mean temperatures reaching or exceeding the reference 1961–1990 mean temperature level. Instrumental records of the period 1990–2010 were possibly above any temperature in the reconstruction period, though this did not appear in the proxy records. They concluded that their "reconstruction agrees well with the reconstructions by Moberg ''et al.'' (2005) and Mann ''et al.'' (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. ad 300–800, despite significant differences in both data coverage and methodology." A 2010 opinion piece by David Frank, Jan Esper, Eduardo Zorita and Rob Wilson () noted that by then over two dozen large-scale climate reconstructions had been published, showing a broad consensus that there had been exceptional 20th century warming after earlier climatic phases, notably the Medieval Warm Period and
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Mat ...
. There were still issues of large-scale natural variability to be resolved, especially for the lowest frequency variations, and they called for further research to improve expert assessment of proxies and to develop reconstruction methods explicitly allowing for structural uncertainties in the process. As several studies had noted, regression-based reconstruction methods tended to underestimate low-frequency variability. Bo Christiansen designed a new method (LOC) to overcome this problem, and with Ljungqvist used LOC to produce a 1,000 year reconstruction published in 2011. This showed more low frequency variability and a colder Little Ice Age than previous studies. They then extended the LOC reconstruction back using selected proxies which had a documented relation to temperature and passed a screening procedure. This 2,000 year reconstruction, published in 2012, again showed more variability than earlier reconstructions. It found a homogenous Little Ice Age from 1580 to 1720 showing colder conditions in all areas, and a well defined but possibly less homogenous Medieval Warm Period peak around 950–1050, reaching or slightly exceeding mid 20th century temperatures as indicated by previous studies including Mann et al. 2008 and 2009. used a larger network of proxies than previous studies, including use low-resolution proxy data with as few as two data points per century, to produce a reconstruction showing centennial patterns of temperature variability in space and time for northern hemisphere land areas over the last 1,200 years. At this broad scale, they found widespread warmth from the 9th to 11th centuries approximating to the 20th century
mean A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...
, with dominant cooling from the 16th to 18th centuries. The greatest warming occurred from the 19th to the 20th centuries, and they noted that instrumental records of recent decades were much warmer than the 20th century mean. Their spatial reconstruction showed similarities to the climate field reconstruction, though the different resolution meant these were not directly comparable. The results were robust, even when significant numbers of proxies were removed. used seafloor and lake bed sediment proxies, which were completely independent of those used in earlier studies, to reconstruct global temperatures over the past 11,300 years, covering the entire
Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
, and showing over the last 1,000 years confirmation of the original MBH99 hockey stick graph. Temperatures had slowly risen from the last ice age to reach a level which lasted from 10,000 to 5,000 years ago, then in line with
Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after the Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he pr ...
had begun a slow decline, interrupted by a small rise during the Medieval Warm Period, to the Little Ice Age. That decline had then been interrupted by a uniquely rapid rise in the 20th century to temperatures which were already the warmest for at least 4,000 years, within the range of uncertainties of the highest temperatures in the whole period, and on current estimates were likely to exceed those temperatures by 2100.


See also

*
Description of the Medieval Warm Period and Little Ice Age in IPCC reports The description of the Medieval Warm Period and Little Ice Age in IPCC reports has changed since the first report in 1990 as scientific understanding of the temperature record of the past 1000 years has improved. The Medieval Warm Period (MWP) and ...
*
Global warming controversy There are past and present public debates over certain aspects of climate change: how much has occurred in modern times, what causes it, what its effects will be, and what action should be taken to curb it now or later, and so forth. In the sc ...
*
Temperature record of the past 1000 years The temperature record of the last 2,000 years is reconstructed using data from climate proxy records in conjunction with the modern instrumental temperature record which only covers the last 170 years at a global scale. Large-scale reconstructi ...


Citations


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{{DEFAULTSORT:Hockey Stick graph Historical climatology