Michael E. Mann (born 1965) is an American climatologist and geophysicist, currently director of the Earth System Science Center at Pennsylvania State University, who has contributed to the scientific understanding of historic climate change based on the temperature record of the past thousand years. He has pioneered techniques to find patterns in past climate change, and to isolate climate signals from “noisy data”.
As lead author of a paper produced in 1998 with co-authors Raymond S. Bradley and Malcolm K. Hughes, Mann introduced innovative statistical techniques to find regional variations in a hemispherical climate reconstruction covering the past 600 years. In 1999 the same team used these techniques to produce a reconstruction over the past 1,000 years (MBH99) which was dubbed the “hockey stick graph” because of its shape. He was one of 8 lead authors of the “Observed Climate Variability and Change” chapter of the Intergovernmental Panel on Climate Change (IPCC) Third Scientific Assessment Report published in 2001. A graph based on the MBH99 paper was highlighted in several parts of the report, and was given wide publicity. The IPCC acknowledged that his work, along with that of the many other lead authors and review editors, contributed to the award of the 2007 Nobel Peace Prize, which was won jointly by the IPCC and Al Gore.
He was organizing committee chair for the National Academy of Sciences Frontiers of Science in 2003 and has received a number of honors and awards including selection by Scientific American as one of the fifty leading visionaries in science and technology in 2002. In 2012 he was inducted as a Fellow of the American Geophysical Union and was awarded the Hans Oeschger Medal of the European Geosciences Union. In 2013 he was elected a Fellow of the American Meteorological Society, and awarded the status of distinguished professor in Penn State’s College of Earth and Mineral Sciences.
Mann is author of more than 160 peer-reviewed and edited publications, and has published two books: Dire Predictions: Understanding Global Warming in 2008 and The Hockey Stick and the Climate Wars: Dispatches from the Front Lines, published in early 2012. In 2013 the European Geosciences Union described his publication record as “outstanding for a scientist of his relatively young age”. He is also a co-founder and contributor to the climatology blog RealClimate.
From 1996–1998, after defending his PhD thesis at Yale, Mann carried out paleoclimatology research at the University of Massachusetts Amherst funded by a United States Department of Energy postdoctoral fellowship. He collaborated with Raymond S. Bradley and Bradley’s colleague Malcolm K. Hughes, a Professor of Dendrochronology at the University of Arizona, with the aim of developing and applying an improved statistical approach to climate proxy reconstructions. He taught a course in Data Analysis and Climate Change in 1997 and became a Research Assistant Professor the following year.
The first truly quantitative reconstruction of Northern Hemisphere temperatures had been published in 1993 by Bradley and Phil Jones, but it and subsequent reconstructions compiled averages for decades, covering the whole hemisphere. Mann wanted temperatures of individual years showing differences between regions, to find spatial patterns showing natural oscillations and the effect of events such as volcanic eruptions. Sophisticated statistical methods had already been applied to dendroclimatology, but to get wider geographical coverage these tree ring records had to be related to sparser proxies such as ice cores, corals and lake sediments. To avoid giving too much weight to the more numerous tree data, Mann, Bradley and Hughes used the statistical procedure of principal component analysis to represent these larger datasets in terms of a small number of representative series and compare them to the sparser proxy records. The same procedure was also used to represent key information in the instrumental temperature record for comparison with the proxy series, enabling validation of the reconstruction. They chose the period 1902–1980 for calibration, leaving the previous 50 years of instrumental data for validation. This showed that the statistical reconstructions were only skillful (statistically meaningful) back to 1400.
Their study highlighted interesting findings, such as confirming anecdotal evidence that there had been a strong El Niño in 1791, and finding that in 1816 the “Year Without a Summer” in Eurasia and much of North America had been offset by warmer than usual temperatures in Labrador and the Middle East. It was also an advance on earlier reconstructions in that it went back further, showed individual years, and showed uncertainty with error bars.” Global-scale temperature patterns and climate forcing over the past six centuries” (MBH98) was published on April 23, 1998 in the journal Nature. In it, “Spatially resolved global reconstructions of annual surface temperature patterns” were related to “changes in greenhouse-gas concentrations, solar irradiance, and volcanic aerosols” leading to the conclusion that “each of these factors has contributed to the climate variability of the past 400 years, with greenhouse gases emerging as the dominant forcing during the twentieth century. Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) AD 1400. The last point received most media attention. Mann was surprised by the extent of coverage which may have been due to chance release of the paper on Earth Day in an unusually warm year. In a CNN interview, John Roberts repeatedly asked him if it proved that humans were responsible for global warming, to which he would go no further than that it was “highly suggestive” of that inference.
In May 1998, Jones, Briffa and colleagues published a reconstruction going back a thousand years, but not specifically estimating uncertainties. As Bradley recalls, Mann’s initial reaction to the paper was “Look at this. This is rubbish. You can’t do this. There isn’t enough information. There’s too much uncertainty.” Bradley suggested using the MBH98 methodology to go further back. Within a few weeks, Mann responded that to his surprise, “There is a certain amount of skill. We can actually say something, although there are large uncertainties.” Mann carried out a series of statistical sensitivity tests on 24 long term datasets, in which he statistically “censored” each proxy in turn to see the effect its removal had on the result. He found that a dataset which would otherwise have been reliable diverged from 1800 until around 1900, suggesting that it had been affected for that time by the CO2 “fertilisation effect“. Using this dataset corrected in comparisons with other tree series, their reconstruction passed the validation tests for the extended period, but they were cautious about the increased uncertainties involved.
The Mann, Bradley and Hughes reconstruction covering 1,000 years (MBH99) was published by Geophysical Research Letters in March 1999 with the cautious title Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations. Mann said that “As you go back farther in time, the data becomes sketchier. One can’t quite pin things down as well, but, our results do reveal that significant changes have occurred, and temperatures in the latter 20th century have been exceptionally warm compared to the preceding 900 years. Though substantial uncertainties exist in the estimates, these are nonetheless startling revelations.” When Mann gave a talk about the study to the National Oceanic and Atmospheric Administration‘s Geophysical Fluid Dynamics Laboratory, climatologist Jerry D. Mahlman nicknamed the graph the “hockey stick”
By Phil Plait
Dr. Michael Mann is a climate scientist of some repute: He has been researching our planet’s climate for nearly 25 years, and was one of the co-authors of the paper that presented the world with the famed “hockey stick diagram”, showing unprecedented and rapid warming of our Earth over the past century or so.
This has made him a primary target of a vast amount of vitriol and attacks by climate change deniers, from the Attorney General of Virginia (and Republican nominee for Governor) Ken Cuccinelli, to a not-so-small cohort of far-right Congresscritters. He was a central figure in the ridiculous “climategate” nontroversy (or what I like to also call a manufactroversy), and has generally suffered the slings and arrows of a legion of reality-deniers.
Since the hockey stick paper in 1998, there have been a number of proxy studies analyzing a variety of different sources including corals, stalagmites, tree rings, boreholes and ice cores. They all confirm the original hockey stick conclusion: the 20th century is the warmest in the last 1000 years and that warming was most dramatic after 1920.