The Hockey Games

Nearly everyone accepts the data from ice cores that show a long period—8000 years or so—of relatively stable concentrations of carbon dioxide, methane and nitrous oxide in Earth’s atmosphere, followed by a rapid increase to today’s elevated values beginning about 150 years ago.  For CO2 it is clear that fossil fuel combustion and deforestation have led to the rise in concentration. For nitrous oxide, the rise is most likely due to the use of nitrogen fertilizer in agricultural systems worldwide.  Attribution of the rise in methane is less clear, but a variety of human activities have played a role.

Unfortunately, there is no ice-core record of the change in global temperature and the widespread use of thermometers to record temperature began only about 200 years ago.  We must derive a record of changes in Earth’s temperature from indirect sources—what climate scientists call proxies.  A variety of these are available, including tree rings, measures of glacial retreat, and anecdotal observations, such as the “ice-out” time in rivers and lakes.

About 20 years ago, Michael Mann, now at Penn State University, compiled a record of the past 1000 years of Earth’s temperature from tree rings.  It, too, shows a long period of relatively stable (even slightly declining) temperature, followed by a rapid increase beginning about 150 years ago.  While correlation does not prove causation, the resemblance of Mann’s temperature record to the record of changing concentrations of “greenhouse” gases was striking.

But, with its publication, Mann’s temperature record drew a rancorous response from conservative, climate-change denying folks.  Even now, whenever, I show a picture from Mann’s publication—which looks like a hockey stick—I can expect a flurry of criticism from climate change deniers in the audience.

This continuing disbelief that the Earth’s temperature has risen sharply from relatively stable levels during most of the past 1000 years is surprising. In 2006, The National Academy of Science brought together a panel of experts, including a well known climate-change denier John Christy, to evaluate Mann’s work.  Their conclusion: “it can be said with a high degree of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries.”  In other words, Mann was right.

The unprecedented rise in temperature during the past 150 years is consistent with a variety of proxy measures published since the National Academy report, including the timing of bird migration and the springtime blooming of plants.  The Academy put less confidence on the reconstructed record of temperature before 1600, but none of the proxy data extending from 1000 to 1600 A.D. changed the basic conclusions about recent, rapid global warming, as derived from tree rings.

It is time to silence the critics of the Hockey Stick. Certainly, tree rings are not perfect proxies of past temperature, but when the tree-ring records agree with a variety of other records, we can have confidence in what they say. The disruption of thoughtful evaluations of Earth’s past temperature by those who wish to deny the reality of the ongoing climate change serves no useful purpose, any more than a denial of satellite pictures of our planet will make the Earth flat.



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Karl, T.R., A. Arguez, B. Huang, J.H. Lawrimore, J.R. McMahon, M.J. Menne, T.C. Peterson, R.S. Voss, H-M. Zhang. 2015.  Possible artifacts of data biases in the recent global surface warming hiatus.  Science 348: 1469-1472.

Mann, M.E., R.S. Bradley, and M.K. Hughes. 1999.  Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Miller-Rushing, A.J., R.B. Primack, D. Primack, and S. Mukunda. 2006.  Photographs and herbarium speciments as tools to document phenological changes in response to global warming. American Journal of Botany 93(11): 1667–1674

National Research Council. 2006.  Surface temperature reconstructions for the last 2,000 years.  National Academy Press, Washington.

Schlesinger, W.H. 2011.  Climate Change.  Interpretation 65(4):378-390.

Vitale, J. and W.H. Schlesinger.  2011.  Historical analysis of the spring arrival of migratory birds to Dutchess County, New York:  A 123-year record.  Northeastern Naturalist 18:335-346.

2 thoughts on “The Hockey Games

  1. From:


    Subject: Hockey sticks

    Date: Tue, 21 Jun 2016 06:54:05 -0700


    Thanks again for another great contribution. I look forward to all your articles. Your Hockey Stick article re global warming takes me back to the extended resistance to the cigarette smoking cancer false consciousness which we embraced for so long. However we get there, it seems we are attached to a culture of morbid rationalism and false consciousness at just about every important turning point we come upon in contemporary US history.

    If you are not already familiar with Jules Henry, I know you will enjoy his Culture Against Man, about 1957.

    Tom Baughman


  2. Bill:

    I spoke with an astronomy professor recently who indicated that were it not for the current warming trend, the Earth should have begun a cooling period. Here is the question: If we were to somehow reverse the human element of the warming process, would there in fact be a cooling period, and if so how much? As a follow-up question, has the current warming trend been responsible for the end of the 500 year mini-cooling period that may have contributed to the failure of the early Norse settlements in North America?

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