Study Finds Substantial Uncertainty With Natural Proxy Temperature Reconstruction

Since 1998, climate scientists have attempted to reconstruct global annual temperature over the last millennium using natural proxies such as tree rings and ice cores. However, a new study finds substantial uncertainty in these reconstructions.

In their research, professors Blakeley McShane of the Kellogg School of Management at Northwestern University and Abraham Wyner of the Wharton School at the University of Pennsylvania find that natural proxies are weak predictors of global annual temperatures over the last one thousand years.

Proxy-based temperature reconstructions such as the renowned "hockey stick" graph have arguably become the most iconic illustration of global warming. The hockey stick is a temperature reconstruction which features a long flat "handle" indicating temperatures were relatively consistent for 900 years followed by a sharp upward "blade" indicating that temperatures dramatically increased over the past 100 years.

"We conclude unequivocally that the evidence for a 'long-handled' hockey stick is lacking in the data," the authors wrote. The handle of the hockey stick "is best understood to be a feature of regression and less a reflection of our knowledge of the truth. ...The fundamental problem is that there is a limited amount of proxy data which dates back to 1000 AD; what is available is only weakly predictive of global annual temperature."  While they do not deny the existence of climate change, they believe the case as it pertains to proxy-based temperature reconstructions has been overstated.

In their study, the statisticians assessed the reliability of temperature reconstructions and their statistical significance against various models.  Additionally, they compared existing reconstructions of Northern Hemisphere average annual land temperature over the last millennium to their own reconstruction. While their reconstruction produced temperature estimates over the last 500 to 1,000 years that largely matched those produced by climate scientists' models, McShane and Wyner found substantially larger uncertainty intervals as compared with the other models.

"Natural climate variability is not well understood and is probably quite large," the authors wrote.  "It is not clear that the proxies currently used to predict temperature are even predictive of it at the annual or decadal scale over several centuries.  Nonetheless, paleoclimatoligical reconstructions constitute only one source of evidence in the Anthropogenic Global Warming (AGW) debate."

The study, "A Statistical Analysis of Multiple Temperature Proxies: Are Reconstructions of Surface Temperatures Over the Last 1000 Years Reliable?" was published in the March 2011 issue of The Annals of Applied Statistics.