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Penn State University Earth System Science Center

Penn State University is a government-funded, public university of Pennsylvania in the U.S. It is home to the Earth System Science Center (ESSC), which was founded in 1986 to describe, model, and understand the Earth’s climate system. The ESSC is one of seven institutions that fall under the larger realm of the Earth and Environmental Systems Institute (ESSI) of Penn State University. The ESSI has tasked itself with conducting interdisciplinary, collaborative research about problems of climate, ecology, and the environment.


The ESSC carries out research into several different aspects of what are now termed Earth Systems. The idea is to look at the Earth as an interrelated set of systems rather than to treat each system as its own, separate field with little to no relation to other systems. The approach of the ESSC involves:

  • The use of climate system models to develop long-term, high-resolution experiments that attempt to resolve our understanding of internal and forced changes on the climate system. This includes the impact of humans on the climate.
  • The combination of modeling and empirical analysis to understand the ocean-atmosphere relationship and to construct models of past climates.
  • Development of specific processes and tools for studying climate problems, particularly over disparate timescales.

Current projects under the ESSC include the following topics:

  • Understanding ozone depletion and its climate effects
  • Understanding of glaciers in the Antarctic and how they are changing
  • Development of proxy data sets for understanding past climate conditions
  • Resolving the role of tree rings in understand climate response to volcanic eruptions
  • Ecological impacts of climate change
  • Potential solutions for mitigating the effects of climate change, including reducing the rate of warming and controlling sea-level rise.

Controversy and the Hockey Stick Graph

The ESSC was caught up in the Climategate controversy (see article on the University of East Anglia Climatic Research Unit) when correspondence between Dr. Michael Mann of Penn State University and several people at University of East Anglia were among the hacked emails. The correspondence appeared to be indicative of a conspiracy to suppress data that contradicted the consensus that the planet was warming due to human activity. Though the Climategate controversy was eventually determined to have been taken out of context and all parties were exonerated, Dr. Mann was implicated in yet another scandal, this time revolving around his statistical methods.

This second scandal is at least partially tied to the Climategate controversy in that the emails between faculty at the University of East Anglia refers to a “trick” used to “hide the decline.” This “trick” refers to a statistical method employed by Dr. Mann in an early paper he published in the journal nature. There is a known discrepancy in climate data in which tree ring proxies for current time periods indicate a declining temperature even while instrument data indicates an increase in global temperature. “Hiding the decline” refers to substituting instrument data for proxy data when the divergence between the two occurs. The assumption being that the instrument data is more accurate.

The problem with this approach is that many believe not enough attention is given to the discrepancy. By grafting instrument data to the end of proxy data, the graph produced looks like a hockey stick with a long, relatively stable period of temperatures followed by a more recent, dramatic rise that appears highly indicative of human-caused climate change. For many skeptics, the divergence problem suggests that tree ring data may be a poor proxy for temperature and that temperatures may therefore have been much higher in the past than the tree rings would indicate. If this is the case, then the current warming trend may be completely an artifact of using two different data sets.

There is a second problem with Dr. Mann’s graph and it stems from methods used to smooth a warm period in history known as the Medieval Warm Period (MWP). Many claim that temperatures at the time (around 800-1400 CE) were as high or higher than they are now, indicating that something other than human greenhouse gas production may be to blame for the current warming and that the cycle may, in fact, be natural, Mann’s hockey stick graph appears to have erased the MWP, thereby giving the false impression that temperatures have not been as high as they were in most of human history.

Unlike the Climategate controversy, investigations into the hockey stick graph found some systematic problems. The Proceedings of National Academy of Sciences in the United States published a response from two scientists, McIntyre and McKitrick, who have pointed out several problems with Mann’s use of the data. One of the problems seemed to include the use of data with axes reverse, making warming periods look like cooling and vice versa. Another problem found that even random data, when input into Mann’s model, comes out looking like a hockey stick more than 90% of the time. In general, random data input into any model should return random results. The fact that the hockey stick is returned so often suggests that Dr. Mann’s statistical methodology is flawed and that the hockey stick is in error.

The hockey stick is hotly contested due to its prominent use by the U.N. International Panel on Climate Change (IPCC). In fact, the prominent use of the hockey stick created by Dr. Mann in the IPCC Third Assessment Report is what promoted McKitrick and McIntryre to investigate it in the first place. The U.S. National Academy of Science (NAS) issued a 2006 report that, while affirming Dr. Mann’s work in general, has also acknowledged statistical shortcomings in his original article. In essence, the NAS is indicating that they can find no evidence that Mann did something intentionally wrong, but that the graph may be limited by the quality of the data and methods of analysis used to create it. Recent analysis of tree ring and ice-core data also indicates that both types of data may be more indicative of precipitation during a given time period than of temperature. Data based on growth of wine grapes in currently unsuitable northern climates seems to confirm that the MWP did, in fact exist, and that temperatures were likely warmer than they are currently. Ongoing research is attempting to resolve these problems, which are by no means trivial, but the hockey stick itself, at least as constructed by Dr. Mann, has been demonstrated to be false.