Hockey Stick Controversy

Introduction

In 1998, a paper authored by Michael E. Mann, Raymond S. Bradley, and Malcolm K. Hughes was published in the journal Nature. The authors reconstructed average air temperatures in the Northern Hemisphere from 1500 AD to the present using a number of different kinds of data. They included in their article a graph of these average Northern Hemisphere temperatures. The graph showed that, despite much up-and-down variability, the temperature rise seen in recent decades is unprecedented over the time period considered. The authors also concluded that human-released greenhouse gases, especially carbon dioxide, were almost certainly the cause of the recent global warming. In 1999, Mann published a version of the graph going back 1,000 years. The temperature curve of both graphs is relatively flat for most of its length, then shoots up suddenly at the right-hand end (marking the recent past to the present), resembling in outline a hockey stick laid on its side.

Since its publication, the paper has been attacked by greenhouse skeptics, and its methods have been criticized by various mathematicians. However, the National Academy of Sciences and the Intergovernmental Panel on Climate Change (IPCC) have both asserted that the graph is, despite uncertainties, an essentially correct picture of global temperature over at least the last 1,000 to 1,300 years.

Historical Background and Scientific Foundations

Mann, Bradley, and Hughes are neither the first nor the last scientists to reconstruct past temperature proxy data, that is, from measurements of phenomena such as tree-ring thicknesses that show temperature indirectly. Proxy data are needed for periods longer than about 150 years ago because no precise weather measurements were recorded before that time.

In their 1998 paper, Mann and his colleagues cite data from multiproxy (many-proxy) studies using ice cores, glacial melting, written historical records, tree rings, coral layers, and other sources of information about past climate. Such work had been done by other scientists before, but Mann, Bradley, and Hughes were the first to assemble records into a single chart spanning such a long time period. A version of the graph appeared in the Third Assessment Report of the IPCC in 2001, the United Nations-based organization's summary of the state of scientific knowledge about climate change.

Scientific papers criticizing the graph began appearing in 2003. A paper by W. Soon and S. Baliunas argued that a warm interval in the Middle Ages called the Medieval Warm Epoch was roughly equivalent to today's warming; S. McIntyre and R. McKitrick stated that they could not reproduce the hockey stick graph using Mann et al.'s original multiproxy data. In 2004, Hans von Storch argued in Science that Mann's group had underestimated the amount of variation in past climate by a factor of two, which would tend to make it less likely that today's rising temperatures are truly unusual. In 2005, McIntyre and McKitrick claimed that they could produce the hockey-stick graph shape by using Mann et al.'s mathematical methods on noise (random data). If true, this would tend to show that the hockey stick graph is meaningless.

In 2005, the U.S. Congress asked the National Research Council's Board on Atmospheric Sciences and Climate, part of the National Academy of Sciences, to assemble a committee to examine the question of global temperatures over the last 2,000 years—essentially, to decide whether the hockey stick graph is a valid picture of climate change. A Committee on Surface Temperature Reconstructions for the Last 2,000 Years, containing 16 scientists, was formed. In 2006, the scientists released a book-length report on surface-temperature reconstruction. In it, they painstakingly reviewed all categories of proxy and instrumental (direct-measurement) data on atmospheric surface temperature.

The committee concluded that the hockey stick graph is essentially valid. “It can be said with a high level of confidence,” the committee wrote, “that global mean temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries.” However, it also noted that as one goes further back in time, uncertainty increases. Therefore, they said, less confidence can be placed in currently available temperature reconstructions from 900 AD to 1600 AD, and very little confidence can be placed in reconstructions of temperature before 900 AD.

Regarding the work of Mann et al., the committee wrote that their basic conclusion—namely that the late twentieth-century warming in the Northern Hemisphere was unprecedented over at least the last 1,000 years— “has subsequently been supported by an array of evidence,” although “not all individual proxy records indicate that the recent warmth is unprecedented.”

Various critics were not satisfied, however. At the request of Congressman Joe Barton (R-Texas), a group of National Academy of Sciences mathematicians specializing in statistics was formed to investigate the mathematical methods used by Mann and his colleagues. The group's 2006 report stated that Mann and his coauthors's mathematical methods were defective and that criticisms of those methods by McIntyre and McKitrick were valid. Climatologists argued in response that the report did not touch the many other reconstructions of climate history that agree in outline with the hockey stick graph, and that making the group's suggested corrections to the math used by Mann et al. leaves the shape of the hockey stick graph unchanged.

In early 2007 the IPCC, a United Nations group of hundreds of scientists tasked with pulling together the views of climate and weather scientists from around the world, released a report called Climate Change 2007: The Physical Science Basis. Chapter 6 of the 1,000-page survey examines the hockey stick controversy and related science in detail. The earlier criticisms of Soon, Baliunas, McIntyre, McKitrick, and others were dismissed as having been adequately answered by other scientists. The report published a graph overlaying a dozen reconstructions of average Northern Hemisphere air surface temperatures for the last 1,300 years. (Data for the Southern Hemisphere are harder to get.) The reconstructions all agree that the rise in temperatures seen in the twentieth century is unprecedented over the time period considered.

WORDS TO KNOW

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

INSTRUMENTAL DATA: Measurements of physical phenomena such as temperature, windspeed, and the like that are made directly using instruments so that numbers with well-defined units (e.g., degrees Celsius) can be assigned to the measurements.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

MEDIEVAL WARM EPOCH: Interval from 1000 to 1300 AD during which some parts of the Northern Hemisphere (e.g., Europe) were warmer than during the Little Ice Age, which followed. Global climate, contrary to claims made by some climate skeptics, was not as warm during the Medieval Warm Period as it has been since about 1950.

PROXY DATA: Information (data) about past climate obtained indirectly from various long-lasting physical traces left by climate or weather, such as oxygen isotopes in ancient ice layers, tree-rings, coral reef layers, and more. “Proxy” means a thing that represents something else: in this case, directly measurable quantities stand for or represent ancient climate conditions.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk. Tree-ring thickness provides proxy data about climate conditions: most trees put on thicker rings in warm, wet conditions than in cool, dry conditions.

Paralleling the conclusions of the U.S. National Research Council, the IPCC asserted that it was likely (at least 66% probable) that the twentieth century was the warmest century in the last 1,300 years, and very likely (at least 90% probable) that the second half of the twentieth century was the warmest 50-year period in the last 500 years. Uncertainty increases for the more distant past, the report emphasized, making it difficult to say whether the warmth of individual recent record-breaking years such as 1998 and 2005 is unprecedented over the last millennium.

Impacts and Issues

In 2006, Nature described the hockey stick graph as “probably the most politicized graph in science.” It has been debated in scientific journals, newspaper editorials, and Congressional committees, and was featured in Al Gore's popular documentary on climate change, An Inconvenient Truth. It has been reviled by greenhouse skeptics, and the Senate Committee on Environment and Public Works called it “discredited” and “broken” in a 2006 press release.

A common misunderstanding about the hockey stick graph is that it is the product of a single team of researchers, namely Mann and colleagues. In reality, more than a dozen other researchers and climate-simulation teams have produced similar-looking reconstructions of climate. All proxy data sets agree that the recent global warming is unprecedented for the last millennium, and all simulations of climate history find that the warming of the late twentieth and early twenty-first century cannot be explained without the forcing effect of human-released greenhouse gases. The IPCC has found that the graph—or, rather, the family of graphs produced by many researchers that have a similar shape—are essentially valid, though more uncertain in more remote centuries.

It is also sometimes thought that the idea that global warming is real and human-caused depends on the validity of the hockey stick graph—that if the hockey stick falls, human-caused global warming falls with it. That is not the case as scores of lines of scientific evidence, as reviewed in Climate Change 2007: The Physical Science Basis, converge to the conclusion that global warming is real and primarily human-caused.

Scientific research on reconstructing the atmospheric temperature history of the last 1,000 to 2,000 years continues. Scientific debate about levels of uncertainty and about many details of how proxy data are used to reconstruct temperature history also continue. However, the scientific mainstream view today is that the hockey stick graph is probably a correct outline of the temperature history of the last 1,000 or more years.

Statements that the hockey stick graph is a hoax, or that it has been definitely discredited or abandoned by climate scientists, are at odds with the IPCC's extended treatment of the subject. The majority of climate scientists today affirm that recent global warming is both unprecedented in recent centuries and human-caused. Uncertainty about these assertions has decreased significantly since 2001.

See Also Climate Change Skeptics; IPCC Climate Change 2007 Report; Media Influences: An Inconvenient Truth; Paleoclimatology.

BIBLIOGRAPHY

Books

North, Gerald R., et al. Surface Temperature Reconstructions for the Last 2,000 Years. Washington, DC: National Academy of Sciences, 2006.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Brumfiel, Geoff. “Academy Affirms Hockey-Stick Graph.” Nature 441 (2006): 1032-1033.

Kerr, Richard A. “Politicians Attack, But Evidence for Global Warming Doesn't Wilt.” Science 313 (2006): 421.

Kerr, Richard A. “Yes, It's Been Getting Warmer in Here Since the CO₂ Began to Rise.” Science 312 (2006): 1854.

Mann, Michael E., et al. “Global-Scale Temperature Patterns and Climate Forcing Over the Past Six Centuries.” Nature 392 (1998): 779-787.

McIntyre, S., and R. McKitrick. “Corrections to the Mann et al. (1998) Proxy Database and Northern Hemispheric Average Temperature Series.” Annual Review of Energy and the Environment 14 (2003): 751-771.

Web Sites

“Majority Press Release: AP Incorrectly Claims Scientists Praise Gore's Movie.” U.S. Senate Committee on Environment and Public Works, June 27, 2006. <http://www.epw.senate.gov/pressitem.cfm?party=rep&id=257909> (accessed October 8, 2007).

“Temperature Variations in Past Centuries and the So-Called ‘Hockey Stick.’” RealClimate, June 27, 2006. <http://www.realclimate.org/index.php/archives/2004/12/temperaturevariations-in-past-centuries-and-the-so-called-hockey-stick> (accessed October 8, 2007).

Larry Gilman