Questions Surrounding the "Hockey Stick" Temperature Studies: Implications for Climate Change Assessments

July 19, 2006

by H. Sterling Burnett, Ph.D.

Testimony before the
House Energy and Commerce Oversight and Investigations Subcommittee

Mr. Chairman and members of the Subcommittee, I applaud you for holding today's hearing to investigate the so-called "hockey stick," which claims to depict temperature trends over the last thousand years. On behalf of the National Center for Policy Analysis, a leader in promoting private alternatives to government regulation and control, I hope to offer some insight to bring clarity to the confusing climate change rhetoric.

The "hockey stick" describes an image used by the United Nations' Intergovernmental Panel on Climate Change (IPCC) that shows a relatively flat temperature variation from A.D. 1000 to 1900, and a dramatic temperature increase from 1900 to 2000. The conclusion is that human industrial development over the past 100 years has caused a dramatic and unprecedented rise in temperatures across the globe.

However, several independent studies call into question the "hockey stick" conclusions, including the recent report by Dr. Edward Wegman, a prominent statistics professor at George Mason University and witness before today's Subcommittee hearing. Today there are serious questions about the "hockey stick" and the reliability of the conclusions that many have used to propose environmental policies to control global temperature.

Natural warming and cooling cycles

Omitted from the "hockey stick" picture is the widely recognized Medieval Warm Period from about A.D. 800 to 1400, as well as the Little Ice Age from 1600 to 1850. Willie Soon, Sallie L. Baliunas and David R. Legates published a 2003 paper in Energy and the Environment that demonstrated:

  • In such widely disparate regions as Argentina, Chile, southern Peru, southern Africa and northern China, records indicate a marked warming trend at the beginning of the last millennium followed by extreme cold during the middle centuries.
  • Historical proxies for temperature-such as tree rings, ice cores and bore holes-in New Zealand, Australia and California also confirm widespread, significant warming and cooling trends.

Furthermore, Stephen McIntyre and Ross McKitrick also shed light on methodological problems associated with the "hockey stick." They contend that some data sources were unjustifiably truncated or extrapolated, calculations were done incorrectly, data sets were associated with incorrect geographical locations, and some of the data was simply obsolete. In a 2004 study published in Geophysical Research Letters, David Chapman, Marshall Bartlett and Robert Harris showed that the "hockey stick" unjustifiably excluded some data from bore-hole samples, which resulted in a "selective and inappropriate presentation" of results.

More recently, and contrary to what was reported in much of the mainstream mass media, the National Academy of Sciences (NAS) confirmed many of the criticisms leveled against the "hockey stick" picture. In a July 2006 report, the NAS concluded that "[e]ven less confidence can be placed in the original conclusions [of the "hockey stick"] by Mann et al. (1999) that 'the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium' because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales" (emphasis added). Although the overall conclusions of the NAS study suggested a warming trend over the past 400 years, this is certainly consistent with our current understanding of the climate change that is occurring after the Little Ice Age from A.D. 1600 to 1850. Far from corroborating the "hockey stick" image of climate change, the NAS report concludes that these "temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 ... and a relatively cold period (or "Little Ice Age") centered around 1700" (emphasis added).

Adding Dr. Wegman's report to our body of knowledge only further diminishes the validity of the "hockey stick" picture of global warming. Needless to say, evidence strongly suggests that the "hockey stick" is not a faithful representation of the true temperature variations over the last 1,000 years. Instead, it appears that the earth experiences routine warming and cooling trends over time.

Climate models and climate change

Climate models are often used to investigate possible causes of climate change and the simulation of that change over time. However, current modeling techniques are limited in several ways. We simply do not have a complete understanding of the earth's climate system, nor can we always accurately describe our knowledge in mathematical terms. Computers have added tremendous power to our ability to create climate models, but computers are still unable to always reproduce the atmospheric phenomena that we observe. Complex interconnections exist in nature that are difficult to replicate in a computer program with any degree of accuracy. As accurate climate change predictors, climate models have many limitations and weaknesses and should not form the basis for policy decisions involving climate change.

For instance, the lowest predicted global temperature measurement in the numerous models is nearly three times more than the temperature rise measured by ground-based thermometers. Current global climate modeling techniques are simply not able to show the actual amount of temperature change.

Nature is difficult to predict

The planet's climate is closely connected to other elements in nature, including geothermal changes, solar interaction, topography, relationship with the biosphere, human involvement, and many other variables about which we have varying levels of understanding. It goes without saying, a climate model that stresses one variable over another-or simply ignores a variable altogether-will produce an incorrect climate change prediction.

This is not to denigrate the value or skill of climate scientists who are working with these extraordinarily difficult and diverse climate models. Indeed, the data collected and analyzed by climate scientists is very valuable. But to accurately predict climate change, it takes much more than data on long-term averages and seasonal variations. Some aspects of climate change are more likely to happen in small regional fluctuations. Other aspects are manifest in how the climate varies over long periods of time. There are simply too many variables in nature that we don't fully understand to accurately predict climate change with the knowledge, skill, understanding, and technology we possess today.


Scientific evidence suggests that the earth's climate over the past 1,000 years has been characterized with periods of significantly warmer-as well as significantly cooler-temperatures than the present. The "hockey stick" picture of dramatic temperature rise in the past 100 years, after 900 years of relatively constant temperature, is flawed and open to methodological and scientific questions. Mr. Chairman, as Congress examines climate change, I urge the Subcommittee to use caution when using the "hockey stick" picture of global temperature rise. Public policy-especially public policy with such wide-ranging consequences as this-should be based on science, not spin. Thank you for the opportunity to submit my views for the record.

H. Sterling Burnett, Ph.D., is a senior fellow with the National Center for Policy Analysis.