Michael E. Mann

Michael E. Mann
Institutions	Pennsylvania State University; University of Pennsylvania
Nationality	American
Known for	Hockey stick graph; Paleoclimate reconstruction; Public engagement
Alma mater	University of California, Berkeley; Yale University
Notable works	The Hockey Stick and the Climate Wars
Occupation	Climate scientist
Field	Paleoclimatology; Climate dynamics
Wikidata	Q633081

Michael E. Mann (born 1965) is an American climate scientist known for pioneering studies of past climate change and for his high-profile role in the public understanding of global warming. In the late 1990s he coauthored groundbreaking temperature reconstructions of the Northern Hemisphere, whose shape became known as the “hockey stick.” This graph showed a roughly level climate for centuries followed by a sharp 20th-century rise, emphasizing that recent warming is unprecedented in the past millennium. Mann’s work in paleoclimate (the study of Earth’s ancient climate) and climate modeling, combined with his efforts as a communicator and in policy debates, have made him a prominent – and sometimes controversial – voice in the field.

Early Life and Education

Mann was born in 1965 in Amherst, Massachusetts, the son of a university mathematics professor. He showed an early aptitude for mathematics and science; in high school he won science fair contests and taught himself programming. In 1984 he entered the University of California, Berkeley, where he earned a bachelor’s degree (with honors) in applied mathematics and physics in 1989. Originally he intended to pursue theoretical physics, but during his senior year he became fascinated by climate and Earth systems.

After UC Berkeley, Mann went to Yale University for graduate study. He earned an M.S. in physics (1991) and continued toward a doctorate in geology and geophysics. Influenced by his advisor Barry Saltzman, he shifted to climate research. At Yale he studied how the ocean and atmosphere interact, applying physics and statistics to climate data. For example, while still a student he and colleagues identified a long (50–80 year) natural temperature oscillation in the Atlantic Ocean, now part of what is called the Atlantic Multidecadal Oscillation (AMO). Mann completed his Ph.D. in 1998 with a dissertation on low-frequency variability in the climate system. His thesis work, which won a Yale earth-sciences dissertation prize, laid the groundwork for his later studies of century-scale climate patterns.

Career and Major Contributions

After Yale, Mann did a postdoctoral fellowship (1996–1998) at the University of Massachusetts Amherst, working with climate scientists Raymond Bradley and Malcolm Hughes. There he helped develop advanced statistical methods to reconstruct past temperatures from proxy records – natural archives that contain climate information (for example, tree-ring widths, coral growth bands, ice core layers and lake sediments). Mann and colleagues mapped how these proxy records co-varied over space and time and related them to the modern instrumental thermometer record.

In 1998 Mann was lead author of a Nature paper that reconstructed Northern Hemisphere temperatures over the past 600 years, using a network of proxies. A year later (1999) he and the same team published in Geophysical Research Letters a reconstruction extending back a millennium. These studies applied sophisticated data analysis (including principal component analysis) to filter the noisy proxy data and produce estimates of past annual temperatures. The resulting time series showed little long-term trend from 1000 to about 1900 (with the Medieval Warm Period and Little Ice Age fluctuations), followed by a steep rise in the 20th century. When plotted, the curve resembled a hockey stick – a long flat “handle” with a sharp upturned “blade” at the end. This hockey-stick graph quickly became iconic. It gained wide attention when the United Nations IPCC featured it prominently in 2001 (during the Third Assessment Report) and when it appeared in popular media, including Al Gore’s 2006 film An Inconvenient Truth. Mann’s team emphasized that the sudden 20th-century upturn was almost certainly due to human-induced greenhouse gases.

Beyond the hockey stick studies, Mann contributed to many areas of climate science. He was a lead author of the IPCC’s Observed Climate Variability and Change chapter in 2001, helping to integrate paleoclimate evidence into the assessment of global warming. He co-published additional millennial temperature reconstructions (for example, with Phil Jones in 2003) that extended his analysis and confirmed that late-20th-century warmth stands out even against the last two millennia. Mann also investigated natural climate oscillations like El Niño and the AMO, finding that while these patterns exist, they cannot explain the overall warming trend. Later work by Mann’s group examined how major volcanic eruptions and solar variations affected climate, how hurricanes respond to warming, and how well computer climate models match past data (for example, showing that tree-ring records can underestimate rapid cooling after volcanoes). Throughout, he focused on detection and attribution – determining which climate changes are human-driven versus natural variability – and refining the statistical tools of paleoclimate reconstruction.

Research Methods

Mann’s paleoclimate reconstructions rely on combining many proxy records to infer past temperatures. A proxy is a natural recorder of climate conditions; for instance, trees often grow wider rings in warmer years, and the ratio of certain oxygen isotopes in ice cores tracks past temperatures. Each proxy on its own is an indirect thermometer and is noisy, but by collecting large numbers of proxies one can identify coherent signals. Mann’s approach was to gather proxies from around the Northern Hemisphere, calibrate each series against the overlapping period of precise thermometer data, and then merge them statistically into maps of past temperature patterns.

Technically, this involves methods like principal component analysis (PCA) and more advanced data-assimilation algorithms. PCA is a statistical procedure that reduces many proxy time series into a few principal patterns, capturing the dominant modes of variability. Mann initially used PCA to summarize networks of tree-ring records so that one group of similar proxies would not dominate the analysis. Later (around 2001) he adopted a “climate field reconstruction” (CFR) method using a regularized expectation–maximization (RegEM) algorithm. RegEM is a way to fill in missing data in the proxy network by iteratively estimating values from the statistically learned relationships among records. These techniques allowed Mann to integrate diverse proxy types (trees, corals, ice cores, sediments) despite gaps or uneven coverage, producing a best estimate of hemispheric temperature for each year.

To validate his methods, Mann and colleagues also created synthetic data tests called pseudoproxies. They would take known temperature records, add random noise, and see if their statistical procedure could reconstruct the original signal. In general, these experiments showed that their algorithms could reliably recover past temperature trends, lending confidence to the actual reconstructions. Of course, all reconstructions have uncertainties: Mann’s papers always include error bars reflecting the sparseness of data the further back in time they go. Overall, the core idea in Mann’s work is that by averaging over many imperfect proxies and using statistical models, one can tease out the climate signal from ancient data that individually would be too noisy or incomplete.

Influence and Public Engagement

Mann’s research had a profound influence on both climate science and the public understanding of global warming. Scientifically, his results provided strong evidence that modern warming is anomalous and likely driven by human activities. This bolstered the consensus in the IPCC and elsewhere about anthropogenic climate change. By the mid-2000s, multiple independent studies using different methods had produced reconstructions similar to Mann’s “hockey stick,” confirming its broad validity. The IPCC cited these findings repeatedly, and they helped make the argument for urgency in reducing greenhouse gas emissions. Indeed, Mann’s contributions were part of the body of work that led to the IPCC (and Al Gore) winning the 2007 Nobel Peace Prize under the climate change banner.

Beyond academia, Mann became a well-known public communicator. In 2004 he co-founded RealClimate, a commentary website run by working climate scientists that explains climate research for the media and public. The site was recognized by Time magazine and Scientific American as an important online resource. Mann has written several popular books: Dire Predictions (2008, with Lee Kump) is a visually rich introduction to climate change science; The Hockey Stick and the Climate Wars (2012) recounts his scientific journey and the political battles over the hockey-stick graph; and The Madhouse Effect (2016, with cartoonist Tom Toles) satirically examines climate-change denial. He also co-authored a children’s climate book (The Tantrum That Saved the World, 2022) and wrote The New Climate War (2021) analyzing modern disinformation strategies.

Mann is a frequent interviewee on climate issues and has testified to U.S. Congress on multiple occasions. His media presence – on television, in newspapers, and on social media – has made him one of the most high-profile climate scientists. He has received numerous honors reflecting his impact: in 2002 Scientific American named him a “visionary” in science, the American Geophysical Union awarded him a fellowship, and the European Geosciences Union presented him with the Oeschger Medal (2012) for outstanding climate research. He also earned prizes for science communication, such as the AAAS Award for Public Engagement (2018). In academia, Mann served at University of Virginia (1999–2005) and then spent 17 years at Pennsylvania State University, where he became Distinguished Professor and directed the Earth System Science Center. In 2022 he moved to the University of Pennsylvania as Presidential Distinguished Professor of Earth and Environmental Science, and now also directs Penn’s Center for Science, Sustainability and the Media. Throughout his career, Mann has authored over 200 peer-reviewed papers, making him one of the most cited climate researchers. Through teaching and mentorship, he has trained many students and postdocs who continue work on climate data and modeling.

Critiques and Controversies

Mann’s prominence has drawn intense scrutiny and criticism, mostly from climate skeptics and political opponents rather than the scientific community. Some statistical critics (notably on the blog Climate Audit) argued that the methods used in the original hockey-stick papers – for example, the PCA step – could bias the reconstruction to produce a long flat line. This debate led to formal reviews. In 2006 the U.S. National Research Council convened an independent panel (the “North Report”) to assess late-Holocene temperature reconstructions. The panel found that while Mann’s specific PCA technique might slightly influence the shape, the overall conclusion was robust: independent analyses using different methods and data also show that recent decades are the warmest in roughly a millennium. In other words, even if the original paper’s details were fine-tuned, the “hockey stick” result largely stands. The panel recommended using alternative methods in future work, a recommendation Mann himself heeded.

Another major flap occurred in late 2009, when hackers stole thousands of e-mail messages and private documents from Britain’s Climatic Research Unit (CRU), including some correspondence involving Mann. Selective quoting of these emails by climate-change deniers gave rise to the “Climategate” controversy. Some media outlets falsely claimed that scientists had admitted to “fudging” data or ‘hiding the decline’ in temperature. In reality, the phrases in question were taken out of context; scientists often use informal language (like calling a calculation a “trick”) without sinister intent. A series of official investigations – at the University of East Anglia, the University of Pennsylvania, NOAA, the EPA’s inspector general, and others – ultimately cleared Mann and his colleagues of any wrongdoing. For example, Penn State commissioned independent panels in 2010 that found no evidence of data fabrication or misconduct, only minor criticisms about email etiquette. The EPA examined allegations from industry groups and found the claims unsupported. In summary, while Climategate temporarily fueled skepticism, it did not change the science: multiple peer-reviewed studies and reconstructions remain consistent that modern warming is real.

Beyond the science itself, Mann has been embroiled in legal and political battles. In 2010 Virginia’s Attorney General Ken Cuccinelli (then running for governor) issued a subpoena demand for Mann’s early research records from the University of Virginia, accusing Mann of possible fraud. The demand was widely seen as a political attack; universities and scientists decried it as intimidation. Ultimately a court blocked Cuccinelli’s request, ruling that the AG lacked proper grounds to seize a scientist’s private files. Cuccinelli’s effort was repeatedly rebuffed to the point that he dropped it. More recently, Mann has pursued defamation suits of his own. In 2019 he sued a blogger associated with the Competitive Enterprise Institute and a well-known commentator (Mark Steyn) who published a provocative article comparing Mann to a convicted pedophile for “molesting” climate data. In early 2024, a D.C. jury agreed that such statements were slanderous and awarded Mann over $1 million in damages. (A judge later reduced that award to $5,000 on appeal, and Mann himself was ordered to pay some legal fees—cases are ongoing.) These court battles highlight a tense issue: Mann argues that false personal attacks on scientists are harmful and should not be protected speech, while critics frame it as a free-speech issue.

Some observers have also critiqued Mann’s style. His defenders praise him for courageously countering misinformation; others worry that his combative rhetoric fuels polarization. In interviews Mann has acknowledged that the climate debate has “gotten ugly” but says it is necessary to stand up to smear campaigns. He contends that the era demands scientists be active in communication rather than academic isolation. Colleagues generally regard Mann as a rigorous researcher, even if his public persona is more forceful than that of many scientists. Importantly, no credible scientific critique has overturned Mann’s core results. In fact, newer reconstructions (using more data and refined methods) still show a hockey-stick-like outcome. Mann himself notes that dozens of independent studies produce similar large-scale patterns of past climate.

Legacy

Michael Mann’s legacy lies in both his scientific contributions and his role in shaping climate discourse. The essence of Mann’s key scientific finding – that recent global warming is unusually large in the context of the last millennium – is now embedded in textbooks and consensus reports. Today many reconstructions (from tree rings, ocean sediments, borehole temperatures, etc.) corroborate that finding. In this sense, Mann did not “invent” the hockey stick so much as bring it to broad attention with a new methodology. His name has become synonymous with climate reconstructions.

Politically and culturally, Mann came to symbolize the clash over climate change. The hockey-stick graph became an icon for climate action advocates, while Mann himself was cast as a villain by deniers. His struggles illustrate how scientific findings can become politicized. In turn, Mann’s experiences have influenced how science is communicated. For example, many climate scientists now take steps to keep their data public and respond rapidly to misinformation, partly due to lessons from that era. Mann’s co-founding of RealClimate helped pioneer scientists’ direct engagement with media. In a broader sense, his legacy is the normalization (for better or worse) of scientists as public figures in policy debates.

Mann’s ongoing work continues to influence climate science. He remains an active researcher – publishing on climate analysis, attribution studies, and data reconstruction – and he involves new generations of students in these topics. He has also pivoted to studying the social aspects of climate science: how to communicate effectively, how disinformation spreads, and what motivates climate action. His recent book The New Climate War (2021) investigates how fossil-fuel interests have shifted tactics in the climate debate, and Our Fragile Moment (2023) looks at humanity’s prospects on a long timescale. These works add to his impact beyond paleoclimate.

In sum, Mann’s career exemplifies a scientist who bridged deep climate research with public outreach. He played a key role in framing the conversation about how unusual recent warming is. His direct approach has won support from many in the scientific community, even as it has drawn ire from opponents. As climate models and data improve, Mann’s early insights continue to be validated. He stands as a prominent example of a climate researcher turned activist-communicator, and his legacy will be judged both by his scientific papers and by how he helped shape public awareness of climate change.

Selected Works: (Papers and books by Mann.

Mann, M. E., Bradley, R. S., & Hughes, M. K. (1999). Northern hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations. Geophysical Research Letters 26, 759–762. (Seminal millennium-temperature reconstruction with annual resolution.)
Mann, M. E., Bradley, R. S., & Hughes, M. K. (1998). Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392, 779–787. (Key 600-year climate reconstruction.)
Dire Predictions: Understanding Climate Change (M. E. Mann and L. Kump, 2008). (A visual guide to modern climate science.)
The Hockey Stick and the Climate Wars: Dispatches from the Front Lines (M. E. Mann, 2012). (Memoir on the hockey-stick graph and ensuing debate.)
The Madhouse Effect: How Climate Change Denial Is Threatening Our Planet, Destroying Our Politics, and Driving Us Crazy (M. E. Mann & T. Toles, 2016). (Illustrated book on climate misinformation.)
The New Climate War: The Fight to Take Back Our Planet (M. E. Mann, 2021). (Analysis of modern climate politics and how to address it.)

Timeline:.

1965 – Born in Amherst, Massachusetts.
1989 – Graduated UC Berkeley (A.B. in applied math & physics).
1998 – Completed Ph.D. at Yale in geology & geophysics; published first multi-century climate reconstruction (Nature paper).
1999 – Extended reconstruction to 1000 years (GRL paper); hockey-stick graph gains attention.
2001 – Lead author on the IPCC Third Assessment Report (Working Group I, Observations chapter).
2005 – Joined Pennsylvania State University as associate professor of Meteorology.
2007 – IPCC (with which Mann was involved) awarded Nobel Peace Prize.
2009 – “Climategate” email hack brings scrutiny to climate scientists.
2010 – Multiple investigations (at universities and government agencies) clear Mann of any misconduct in Climategate.
2012 – Publishes The Hockey Stick and the Climate Wars.
2016 – Publishes The Madhouse Effect (with cartoonist Tom Toles).
2018 – Elected Fellow of AAAS and receives climate communication awards.
2020 – Elected to the U.S. National Academy of Sciences.
2022 – Moves to University of Pennsylvania as Presidential Distinguished Professor (Earth & Environmental Science) and Director of a new science-media center.
2024 – Jury awards him $1M in damages in a defamation lawsuit (later reduced on appeal) after being compared to a child molester by a columnist.

Through these years, Mann has continued both his scientific research and his engagement with climate policy and communication, making him a central figure in modern discussions of global warming.