Hockey stick diagram
The hockey stick diagram is named after the shape of the hockey stick and is based on a scientific study published in 1999 by Michael E. Mann , Raymond S. Bradley, and Malcolm K. Hughes on global warming . It became known through the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). By showing the temperature history of the last millennium in the northern hemisphere , it served to illustrate the statements made in this report.
The diagram was often used for illustration purposes in press reports on the third assessment report and on global warming issues, and the assessment report itself also contained a representation of the diagram. Although global warming was already widely documented at this point in time, since the diagram clearly illustrates this, it became an important target for climate change deniers .
After 2003, a controversy arose over the statistical basis of the diagram and the conclusions to be drawn from it. An assessment of the creation of the diagram as well as the controversy is given in the fourth assessment report . A panel of experts from the National Research Council (NRC) came to the conclusion in 2006 that it is plausible to conclude that the last decades of the 20th century in the northern hemisphere were the warmest period in the past millennium.
The basic form and statement of the hockey stick diagram - a gradual cooling down to the beginning of industrialization, followed by an unusually rapid warming, with the warmest decades in the present - has meanwhile been confirmed many times by the work of other authors and on the basis of other climate practices .
To create the diagram, a large amount of available climate data from the last few centuries was summarized, including measurement data from weather stations , but also certain indirect climate data (so-called proxies ) from sediments , drill core investigations of the polar ice or data from the pine tree chronology (tree ring data). The result was a diagram that showed a relatively uniform temperature profile over a long period of time and documented a significant increase in temperature from the 20th century . The resemblance of this curve to the shape of an ice or grass hockey stick quickly led to its memorable name.
The chart received widespread media attention and was significant in that it was an obvious illustration of the rise in global mean temperature. In this way, it supports the now extremely likely assumption of man-made global warming within the last few centuries, the cause of which is seen in particular in the burning of fossil fuels and the resulting increase in the proportion of greenhouse gases in the atmosphere .
In the fourth assessment report of the IPCC from 2007, the following is written about the temperature increase shown in the diagram:
“The TAR pointed to the 'exceptional warmth of the late 20th century, relative to the past 1,000 years'. Subsequent evidence has strengthened this conclusion. It is very likely that average Northern Hemisphere temperatures during the second half of the 20th century were higher than for any other 50-year period in the last 500 years. It is also likely that this 50-year period was the warmest Northern Hemisphere period in the last 1.3 kyr, and that this warmth was more widespread than during any other 50-year period in the last 1.3 kyr. ""
“The TAR (= third assessment report, editor's note) pointed to the 'extraordinary warmth in the late 20th century compared to the last 1,000 years'. Subsequent evidence has reinforced this finding. It is very likely that the mean temperatures in the northern hemisphere during the second half of the 20th century were higher than any other 50-year period in the past 500 years. It is also likely that this 50-year period was the warmest in the last 1.3 millennia and that this warming was considerably more extensive than any other 50-year period in the last 1.3 millennia. "
Since its publication in the Third Assessment Report of the IPCC , the statistical methodology of the hockey stick diagram and the associated implications have been discussed frequently and controversially in public. The study by Mann et al. Developed particularly in circles that deny climate change . on a main topic. In the following years, the paper was checked several times on a scientific basis using various reconstruction methods without any obvious errors being found. Up to and including 2013, more than a dozen follow-up studies with different statistical approaches had been published and came to similar results.
In contrast, Steven McIntyre and Ross McKitrick of the Fraser Institute , a think tank funded by the petroleum industry , fundamentally criticized the statistical method used to obtain the hockey stick diagram.
Errors in the computer-based evaluation of the basic data on which the characteristic diagram is based appeared. In particular, the averaging routines used from program libraries would only be able to deliver correct results from 1902 onwards, due to the implementation . Attempts to check the program using the Monte Carlo method with several input data sets would also show that these, too, might mutate into the familiar hockey stick shape. In addition, there would be software errors that would falsify the results. In the opinion of McIntyre and McKitrick, it is therefore a statistical artifact . However, this view is now considered refuted.
While up to the present the position has been taken, especially outside of science, that the diagram is based on false foundations, on the other hand, most critics have been criticized for hasty assessments and immature analysis methods. The main reason why the discussions about the technical details were so intense was that the allegation of the incorrectness of the graphics, especially in the public debate, was often used as alleged evidence against man-made global warming. In the meantime, more recent climatic reconstructions of the past 1,000 years have provided a picture comparable to the hockey stick diagram. These current graphs are largely consistent with the original hockey stick diagram and are within the scope of the Mann et al. specified error limits .
The hockey stick diagram has been checked several times for errors from various sides. The results of Mann et al. have thus been confirmed several times.
Study by the National Research Council
At the request of the US House of Representatives , the National Research Council (NRC) put together a twelve-person panel of experts that brought together all the scientific findings on temperature development over the past 2,000 years, including examining Mann's work on the hockey stick diagram. In June 2006 the NRC published a 155-page report on the subject.
The NRC report states that Michael Mann's research has been supported by a variety of evidence since it was published. Based on this evidence, the experts at the NRC believe it is plausible that the last decades of the 20th century in the northern hemisphere were the warmest period in the past millennium. The certainty of statements about previous temperature curves differs, however, depending on the period of time one is looking at. It can be said with great certainty that the average global surface temperature during the last decades of the 20th century was higher than in any comparable period during the previous four centuries. For the period before 1600, large-scale temperature reconstructions were still (at the time of the report) associated with uncertainties that were difficult to quantify, but would nevertheless make an important contribution to climate research. The available climate proxies would suggest that temperatures in many, but not all, of the individual locations over the past 25 years have been higher than in any comparably long period between the years 900 and 1600.
Following a suggestion in the NRC report, in 2006 Mann, Bradley and Hughes repeated their climate reconstruction of the past 2,000 years using a greatly increased set of climate proxies and several validated , independent methods. The result, published in 2008, confirmed the anomaly for at least the past 1,300 years. If tree ring data is also taken into account, this can also apply to the last 1,700 years with severe restrictions. Practically all comparable work on climate reconstruction over these periods of time partly fall back on the same chronologies and are therefore not completely independent of one another. However, if one takes individual proxy rows - such as the controversial tree rings - out of the investigation, the result does not change significantly. All reconstructions of the climate that are obtained from proxies are fraught with some uncertainties.
In a short correspondence in the journal Nature on August 10, 2006, Bradley, Hughes and Mann affirmed that in their 1998 publication they had pointed out the great uncertainties and contradictions of the data before 1400, which were concrete conclusions about the temperatures in the period before 1400 prevent. In doing so, they reacted to statements by Gerald North from the NRC expert panel, who had criticized that they had not communicated certain uncertainties optimally.
In July 2006, Edward Wegman , Yasmin H. Said, and David W. Scott published a report for the United States House Committee on Energy and Commerce on recent research into the Hockey Stick Diagram publications. Often referred to as the "Wegman Report," commissioned by Republican committee chair Joe Barton , Mann's publications were described as "somewhat obscure and incomplete," and criticisms of McIntyre and McKitrick's publications were shared. Furthermore, a network of 43 authors of paleoclimatic studies with direct connections to Mann (e.g. by co-authorship in publications) was described which, from the point of view of critics, would make the previously cited “independent studies” appear in a not too independent light. The conclusion of the Wegman Report was that the thesis that 1998 should have been the warmest year, but the 1990s the warmest decade in the last 1,000 years, is not confirmed. Richard Kerr of the criticized science magazine Science , for his part, criticized Joe Barton's demeanor at a hearing later held before the Senate committee on the subject.
Massive criticism of the Wegman report arose since the end of 2010, when three well-known experts raised serious allegations of plagiarism against the authors of the report. The physicist Paul Ginsparg from Cornell University described the extent of plagiarism in the Wegman report as "quite shocking". His own preliminary assessment was " guilty as charged ". Plagiarism, etc. a. from Wikipedia , were also found in a technical article in which the report authors Edward Wegman and Yasmin H. Said again criticized climate researchers. The article was withdrawn by the affected journal Computational Statistics & Data Analysis . After Raymond Bradley filed a complaint against Wegman , two commissions to investigate the incidents were set up at Wegman's University, George Mason University . These came, unanimously, to different results. While one did not identify any wrongdoing, the other noted the presence of plagiarism, for which Edward Wegman, as team leader, was responsible. Wegman received a letter of reprimand . Andrew Gelman , a statistics professor at Columbia University , raised further allegations of plagiarism against Wegman, but these were no longer the subject of university investigations.
The network analysis of the Wegman report was examined by the bioinformatician Katharina Zweig as a bad example “gone wrong” in her textbook on network analysis. The analysis of the report was not suitable to examine the hypothesis of Wegman and his co-authors.
Controversy renewed after the 2009 hacker incident
In connection with the hacking incident at the University of East Anglia's climate research center and the e-mails stolen from it, Mann's research was re-examined by Pennsylvania State University . To this end, a commission was convened, made up of five professors from other departments of the university. After completing the investigation, the commission came to the unanimous conclusion that Mann had not been guilty of any scientific misconduct. After an initial investigation had been completed, a three-member university committee had previously rejected allegations that Mann had withheld data or deleted sensitive e-mails.
In an investigation report by the Inspector General of the National Science Foundation (NSF), the results of the university investigation were essentially confirmed. With regard to the allegation of data manipulation, the NSF carried out its own investigation, in which critics of Mann's studies were also heard. Finally, the NSF also stated that there was no evidence of scientific misconduct by Mann.
More hockey stick-like reconstructions
After the first publications by Mann, Bradley and Hughes on the hockey stick diagram around the year 2000, there were numerous other temperature reconstructions - the American geologist and environmental consultant G. Thomas Farmer and the Australian communication scientist John Cook write of a “hockey team “- which confirm the basic statement: After a gradual cooling over a minimum period of several hundred to several thousand years, industrialization began a rapid warming that was unusual for the period. Global temperatures are now probably the highest in at least two thousand years.
For example, temperature reconstructions carried out by the Pages2k consortium in 2012 and 2019 for the past 2000 years confirmed the correctness of the basic results of Mann et al. The Pages2k reconstructions are based on data obtained from ice cores , tree rings and lake sediments, among other things . Not only is the current global temperature very likely higher than it has been for at least two thousand years, the rate of increase is likely to be unprecedented over the period.
A study published in Science in 2013, which reconstructed the climate evolution of the entire Holocene - the last 11,700 years - also supports the results of Mann, Bradley and Hughes. The climate data in this study is based on an evaluation of 73 locations around the world and 80 percent come from drill cores from deep-sea sediments.
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