The Hockey Stick and Climate Wars

By Michael E. Mann

In 2001, the Intergovernmental Panel on Climate Change prominently featured the “Hockey Stick,” a chart showing global temperature data over the past one thousand years and how worldwide human activity since the industrial age had raised CO2 levels. The chart became a central icon in the consequent “climate wars.” Michael E. Mann, lead author of the original paper in which the Hockey Stick first appeared, shares the real story of the science and politics behind this controversy.

On the morning of November 17, 2009, I awoke to learn that my private e-mail correspondence with fellow scientists had been hacked from a climate research center at the University of East Anglia in the United Kingdom and selectively posted on the Internet for all to see. Words and phrases had been cherry-picked from the thousands of e-mail messages, removed from their original context, and strung together in ways designed to malign me, my colleagues, and climate research itself. Sound bites intended to imply impropriety on our part were quickly disseminated over the Internet. Through a coordinated public relations campaign, groups affiliated with the fossil fuel industry and other climate change critics helped catapult these sound bites onto the pages of leading newspapers and onto television screens around the world. A cartoon video ridiculing and falsely accusing me of “hiding the decline” in global temperature was released on YouTube and advertised through a sponsored link that appeared with any Google search of my name. The video eventually even made its way onto the CBS Nightly News. Pundits dubbed the wider issue of the hacked e-mails “climate-
gate,” and numerous investigations were launched. Though our work was subsequently vindicated time and again, the whole episode was a humiliating one – unlike anything I’d ever imagined happening. I had known that climate change critics were willing to do just about anything to try to discredit climate scientists like myself. But I was horrified by what they now had stooped to.

In their efforts to inform the public about climate change and its potentially disastrous consequences, scientists have run up against powerful vested interests that either deny that such change is occurring or, if it is, that human activity plays much, if any role in it.

My thoughts turned to an event from a decade earlier. In August 1999, I attended a meeting in Arusha, Tanzania, as a lead author for an upcoming report of the Intergovernmental Panel on Climate Change (IPCC). From my hotel room, I could see one of the world’s great wonders, Mount Kilimanjaro, with its magnificent ice cap lying just degrees from the equator. The ice cap, by the end of the twentieth century, had already shrunk to just a third of the area it covered in 1936 when Ernest Hemingway wrote “The Snows of Kilimanjaro,” but it was majestic all the same.

After the meeting, I joined a daylong expedition to see one of the world’s greatest displays of nature: Serengeti National Park. Here, zebras, giraffes, elephants, water buffalo, hippos, wildebeests, baboons, warthogs, gazelles, and ostriches wander among some of the world’s most dangerous predators: lions, leopards, and cheetahs. Among the most striking and curious scenes I saw that day were groups of zebras standing back to back, forming a continuous wall of vertical stripes. “Why do they do this?” an IPCC colleague asked the tour guide. “To confuse the lions,” he explained. Predators, in what I call the “Serengeti strategy,” look for the most vulnerable animals at the edge of a herd. But they have difficulty picking out an individual zebra to attack when it is seamlessly incorporated into the larger group, lost in this case in a continuous wall of stripes. Only later would I understand the profound lesson this scene from nature had to offer me and my fellow climate scientists in the years to come.

For more than two decades, in their efforts to inform the public about climate change and its potentially disastrous consequences, scientists have run up against powerful vested interests that either deny that such change is occurring or, if it is, that human activity plays much, if any role in it. I have been privileged to be part of this scientific effort and, indeed, at times singled out in the ensuing conflict. My story is that of a once-aspiring theoretical physicist, driven by a curiosity about the natural world, who wound up as a central object of attack in what some have characterized as the most highly funded, most carefully orchestrated assault on science the world has known.

Why did the Hockey Stick stir such passion among the deniers of human-caused climate change? Perhaps because it so graphically addressed the critical question of whether there was truly anything unusual about modern global warming.

The extent of this assault was recently described by John M. Broder of the New York Times: “[T]he fossil fuel industries have for decades waged a concerted campaign to raise doubts about the science of global warming and to undermine policies devised to address it,” and they have “created and lavishly financed institutes to produce anti-global warming studies, paid for rallies and Web sites to question the science, and generated scores of economic analyses that purport to show that policies to reduce emissions of climate-altering gases will have a devastatingeffect on jobs and the overall economy.”1

A central figure in this controversy has been the “hockey stick,” a simple, easy-to-understand graph my colleagues and I constructed to depict changes in the Earth’s temperature back to a.d. 1000 (see Figure 1). The graph was featured in the high-profile “Summary for Policy Makers” of the 2001 report of the Intergovernmental Panel on Climate Change, and it quickly became an icon in the debate over human-caused (anthropogenic) climate change.




The hockey stick’s prominence in the climate change debate would secure its status as a principal bête noir for those who denied the importance or even the existence of climate change. Climate change deniers went on to wage a public – and very personal – assault against my coauthors and me in the hope that somehow they might discredit all climate science, the fruits of the labor of thousands of scientists from around the world, by discrediting us and our work. The Serengeti strategy writ large.

Why did our work stir such passion among the deniers of human-caused climate change? Perhaps because it so graphically addressed the critical question of whether there was truly anything unusual about modern global warming. Instrumental records from around the globe indicate that the Earth has warmed by almost one degree Celsius (about 1.5oF) over the past century. That may seem a small amount, but it is already noticeable in glacier retreat, rising sea level, increasingly frequent heatwaves, and more intense hurricanes, among many other phenomena. If the trend continues, the warming will have large and in some cases horrific consequences for the world’s agricultural production, seacoast settlements, ocean health, and biodiversity.

Few records of thermometer readings reach further back in time than a century, however. Is it possible then, that we have simply managed to catch a glimpse, through the myopic window of modern observations, of what is in reality a larger natural cycle of periodic cooling and warming? Is it possible that in the decades to come, the average temperature will, by itself, stabilize or even begin to drop?

The field of paleoclimatology seeks to address such questions by placing modern evidence of climate change in a longer-term context. Paleoclimatologists make use, among other things, of the indirect evidence provided by so-called proxy climate data – natural archives of information that record, either physically, chemically, or biologically, some attribute of the climate back in time. Scientists have found, for example, that the shifting ratio of oxygen isotopes in the frozen layers of ice cores, some of which date back thousands of years, can be used to estimate temperature changes back in time at the site of the core.2 Some proxy records, such as the layers contained in deep ocean sediments, record only the coarsest changes, such as the coming and going of the major ice ages over eons. Other proxy records, such as tree rings, ice cores, corals, and lake sediments, can potentially tell us about climate conditions such as temperature, rainfall, or wind patterns for a single year or season. By using an array of such records, we can establish a year-by-year chronology of the climate changes of past centuries. In the late 1990s, my coauthors and I published an attempt to use such paleoclimate proxy data to obtain a quantitative assessment of how the Earth’s surface temperature had varied in past centuries. We published our original findings, which spanned the past six hundred years, in 19983, and the following year we were able to extend the analysis back over the entire past millennium4. The picture that emerged was a wiggly curve documenting past temperature changes over the entire Northern Hemisphere (the hemisphere with the most data) and indicating a sharp rise in temperature over the past century. The graph we drew looked fuzzy because for each point on the curve it included the margin of error, reflecting the simple fact that ice cores, corals, tree rings, and other proxies are useful but rather imperfect thermometers. Despite the uncertainties, my coauthors and I were able to draw certain important conclusions. We deduced that there had been a decline in temperature from the period running from the eleventh century through the fourteenth–a period sometimes referred to as the medieval warm period–into the colder Little Ice Age of the fifteenth to nineteenth centuries. Think of this as the shaft of a hockey stick laid on its back. This long-term gradual decline was followed by an abrupt upturn in temperatures over the past century. Think of this as the blade. In the original draft, our reconstruction ended in 1980, as relatively few long-term proxy records had been updated since the early 1980s. Yet much of the observed warming had actually taken place since then. An anonymous reviewer suggested that we bring the curve up to date by including the observational temperature data, that is, modern thermometer measurements available through to the present. That led to a sharpened blade. The warmth at the end of the record rose well above that of any period of the past millennium, even taking into account the increasing margin of error as one goes back in time.

Thus was born the hockey stick–though the term itself was actually coined later by a colleague in Princeton5. It didn’t take long for the hockey stick to become a central icon in the climate change debate. It told an easily understood story with a simple picture: that a sharp and highly unusual rise in atmospheric warming was occurring on Earth. Furthermore, that rise seemed to coincide with dramatic changes in human activity heralded by industrialization and increased use of fossilfuels. The controversy that the hockey stick would ultimately generate, however, had little to do with the depicted temperature rise in and of itself. Rather, it was a result of the perceived threat this simple graph represented to those who are opposed to governmental regulations or other social restraints aimed at protecting our environment and the long-term prospects for the health of our planet. In my book The Hockey Stick and Climate Wars (Columbia University Press 2012), I attempt to tell the real story behind the hockey stick. I reflect on the emphasis, and indeed at times the overemphasis, that players on both sides of the climate change debate have often placed on this work. I explore the controversy associated with the hockey stick, some of it real, much of it specious. I use my own story, more than anything else, as a vehicle for exploring broader issues regarding the role of skepticism in science, the uneasy relationship between science and politics, and the dangers that arise when special interests and those who do their bidding attempt to skew the discourse over policy-relevant areas of science. In short, I attempt to use the hockey stick to cut through the fog of disinformation that has been generated by the campaign to deny the reality of climate change. It is my intent, in so doing, to reveal the very real threat to our future that lies behind it.

*Excerpted from The Hockey Stick and Climate Wars, by Michael E. Mann. Copyright © 2012 Michael E. Mann. Used by arraignment with Columbia University Press. All rights reserved.


About the author

Michael E. Mann is a member of the Penn State University faculty, holding joint positions in the Departments of Meteorology and Geosciences and the Earth and Environmental Systems Institute (EESI) He is also director of the Penn State Earth System Science Center (ESSC). With Lee Kump, he co-authored the book Dire Predictions: Understanding Global Warming, and is a cofounder and avid contributor to the award-winning science website, His book, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines was published earlier this year. Along with other scientists who participated in the Intergovernmental Panel on Climate Change, he jointly received the Nobel Peace Prize in 2007.



1. John M. Broder, “Climate Change Doubt Is Tea Party Article of Faith,” New York Times, October 20, 2010.

2. This paleothermometer is imprecise, however, as other factors, such as the source region of the moist air that produced the snow, can also have an impact on these ratios.

3. M. E. Mann, R. S. Bradley, and, M. K. Hughes, “Global-Scale Temperature Patterns and Climate Forcing Over the Past Six Centuries,” Nature, 392(1998): 779–787.

4. M. E. Mann, R. S. Bradley, and M. K. Hughes, “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations,” Geophysical Research Letters, 26 (1999): 759–762.

5. The colleague was Jerry Mahlman, director of NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey.

The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of The World Financial Review.