Hockey stick graph
The hockey stick graph is a visual representation of historical average temperatures in the Northern Hemisphere, characterized by a long period of stability followed by a sharp increase in temperatures around 1900. Its shape resembles a hockey stick, with the vertical spike symbolizing the recent rise in temperatures attributed to industrial activity and potentially anthropogenic causes. First published in 1998 by researchers Michael E. Mann, Raymond S. Bradley, and Malcolm K. Hughes, the graph became a focal point in discussions about climate change, particularly regarding the debate over whether current temperatures are historically abnormal.
The graph's methodology involved a multiproxy reconstruction that combined various temperature indicators, including tree rings and lake sediments. However, its accuracy has been contested by critics, particularly Steven McIntyre and Ross McKitrick, who claimed that the underlying data contained significant errors and biases. This controversy led to multiple high-level reviews, which yielded mixed conclusions about the graph's validity, further fueling the debate among climate change skeptics and researchers.
Despite the scrutiny, the hockey stick graph remains significant in the climate change discourse, influencing public perception and scientific methodologies related to temperature reconstructions. Its role highlights the complexities of interpreting historical climate data and the ongoing challenges faced by the scientific community in conveying their findings effectively.
Hockey stick graph
The hockey stick graph, named for its shape, represents a thousand-year period of essentially stable average temperatures in the Northern Hemisphere, followed by an upward spike around 1900. The graph, which suggests the impact of industrial activity on global warming, has been cited to support arguments that global warming has anthropogenic causes, but its accuracy has been questioned; it has thus been both prominent and controversial.
Background
The “hockey stick” is a nickname given to a dramatic graph of historic average temperatures in the Northern Hemisphere. The graph became a prominent—and controversial—symbol in the debate over whether recent global average temperatures are historically abnormal and thus more likely to have been caused by human activities. The graph gets its nickname from its shape, which represents a flat, thousand-year period of stability (the stick) followed by a sudden upward spike (the blade). The nickname is generally attributed to Jerry Mahlman, head of the Geophysical Fluid Dynamics Laboratory, part of the National Oceanographic and Atmospheric Administration.
The hockey stick graph was first published in Nature magazine in 1998 by a research team of Michael E. Mann, Raymond S. Bradley, and Malcolm K. Hughes and was featured prominently in the 2001 Second Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC). Mann and Bradley were researchers with the University of Massachusetts, while Hughes was a specialist in dendrochronology (tree-ring dating), working at the University of Arizona in Tucson. The scientists engaged in a type of historical temperature reconstruction generally referred to as “multiproxy” reconstruction. That is, they combined a broad set of measured temperatures with estimates based on several proxy temperature indicators, such as tree-ring growth, fossilized leaf stomata, boreholes, lake sediments, and tree pollen.
The graph published in Nature was a radical departure from previous temperature reconstructions, which had depicted a period of significant warmth (equal to or greater than current temperatures) during the Medieval Warm Period, which lasted from the tenth to the fourteenth century. Controversy over the hockey stick graph arose when two Canadian researchers—Steven McIntyre, a policy analyst with a background in mathematics and mineral exploration, and Ross McKitrick, an economist at the University of Guelph—sought to examine the underlying data and programming used by Mann and his colleagues in creating the graph.
The Criticism
McIntyre and McKitrick examined the data and attempted to reproduce the hockey stick model using the same methodology Mann’s team had employed. They then published an article in the European journal Energy and Environment asserting that the earlier team’s data set contained significant errors, omissions, and duplicated data. They also claimed that the computer program used to analyze the data gave undue importance to a single proxy series of dubious value. In later analyses, published outside of the scientific literature, McIntyre and McKitrick asserted that the analytical program had a built-in bias that would produce a hockey-stick shape even if it was analyzing completely random data. A continuing bone of contention was the resistance that Mann’s group demonstrated to sharing its data or providing sufficient information about its computer program to facilitate others’ efforts to reproduce the hockey stick.
Mann’s Response
The claims made by McIntyre and McKitrick have been sharply rejected by Mann, Bradley, Hughes, and other members of the academic community. In addition to questioning McIntyre and McKitrick’s ability to understand the development of multiproxy temperature reconstructions, Mann and other paleoclimatologists argue that what McIntyre and McKitrick found were only trivial data errors that did not alter the fundamental shape of the graph. Claims regarding bias in the analytical program have also been disputed and attributed to improper mathematical analysis techniques, as well as to failures properly to use the program.
Third-Party Referees
The controversy over the hockey stick graph led several high-level review panels to be convened. One such panel was created by the National Academy of Sciences’ National Research Council at the request of U.S. representative Sherwood Bohlert, chair of the House Committee on Science. It was led by climate modeler Gerald North. Another committee, assembled at the request of two other Republican members of the House, was directed by statistician Edward Wegman, head of the National Academy of Sciences’ Committee on Applied and Theoretical Statistics. Both of these committees upheld many of the findings of McIntyre and McKitrick, although the North panel was slightly less critical of the hockey stick model than was the Wegman panel.
Context
Despite the findings of the two review panels, the hockey stick controversy continued to rage with considerable rancor. It was conducted primarily via claims and counterclaims about various elements of the debate on blogs such as that of Steven McIntyre (www.climateaudit.org) and Real Climate, a collective blog operated by a group of climate researchers (www.realclimate.org). Areas of contention involved assumptions regarding the accuracy and global representativeness of individual proxy data sets.
The hockey stick controversy affected public opinion and the methodology of climate reconstruction, as well as the representation of such reconstructions in scientific publications. It became a rallying point for climate change skeptics, especially those doubting that greenhouse gas emissions and other anthropogenic factors play a role in global warming. These skeptics saw the controversy as evidence that climate researchers were intentionally exaggerating recent warming by erasing warmth from prior centuries. They produced a huge number of publications proclaiming the “breaking” of the hockey stick and, with it, the refutation of the idea that recent temperatures are historically abnormal.
Many believe that the controversy over the hockey stick also led to changes in the way that the IPCC chose to represent historical temperature reconstructions in its Fourth Assessment Report on the science of climate change. Rather than relying on a single historical climate reconstruction, the report presented the results of an ensemble of reconstructions. Some of these reconstructions included the Medieval Warm Period; others did not.
Key Concepts
- Medieval Warm Period: a period of warmer-than-average temperatures between the tenth and fourteenth centuries
- multiproxy reconstruction: a method of estimating prehistoric climate conditions using a combination of proxy indicators
- paleoclimatology: the study of prehistoric climate conditions
- proxy indicators: tree rings, fossils, and other artifacts that provide indirect evidence of past temperatures
Bibliography
Intergovernmental Panel on Climate Change. Climate Change, 2001—The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Edited by J. T. Houghton et al. New York: Cambridge University Press, 2001.
‗‗‗‗‗‗‗. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.
Kerr, Richard A. “Politicians Attack, but Evidence for Global Warming Does Not Melt.” Science 313, no. 5786 (July 28, 2006): 421.
Mann, Michael. "Beyond the Hockey Stick: What We're MIssing When We Talk About Climate Change." TIME, 25 Oct. 2023, time.com/6328017/climate-change-hockey-stick/. Accessed 18 Dec. 2024.
McIntyre, Steven, and Ross McKitrick. “Corrections to the Mann et al. (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series.” Energy and Environment 14, no. 6 (November 1, 2003): 751-772.
National Academy of Sciences. Surface Temperature Reconstructions for the Last Two Thousand Years. Washington, D.C.: National Academies Press, 2006.
Wegman, Edward, et al. Ad Hoc Committee Report on the “Hockey Stick” Global Climate Reconstruction. Washington, D.C.: House Committee on Energy and Commerce, 2006.