- 29 Jun 2019 18:13
#15014977
Major doubts have been raised about the reliability of the historical temperature record by two US statisticians in a paper that has yet to be published but has already provoked a strong response from leading US climate scientists.
The paper re-opens the controversy over the so-called hockey stick graph, made famous by Al Gore in “An Inconvenient Truth”, and may re-energise climate change sceptics in the wake of “climategate” and on the eve of the United Nations Cancun climate conference.
The hockey-stick graph, constructed mainly from tree ring “proxy” data by US climate scientist Michael Mann, shows historic temperatures remaining fairly steady and then rising sharply since the industrial revolution. But statisticians Blakeley McShane of Northwestern University in the US and Abraham Wyner of the University of Pennsylvania in the US write in a new paper that “we conclude unequivocally that the evidence for a ”long-handled” hockey stick (where the shaft of the hockey stick extends to the year 1000 AD) is lacking in the data”.
CLIMATE SCIENTISTS “GREATLY UNDERESTIMATED THE UNCERTAINTY”
Temperature records reconstructed from tree rings, ice cores and other so called proxies that give indications of ancient temperatures “do not predict temperature significantly better than random series generated independently of temperature”, McShane and Wyner state in their paper [published] in the peer-reviewed journal Annals of Applied Statistics. The two statisticians conclude that climate scientists “have greatly underestimated the uncertainty of proxy based reconstructions and hence have been overconfident in their models”. They comment that the long flat handle of the hockey stick is best understood to be a feature of the data processing 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 weakly predictive of global annual temperature,” McShane and Wyner comment in their paper which is called “A Statistical Analysis Of Multiple Temperature Proxies: Are Reconstructions Of Surface Temperatures Over The Last 1000 Years Reliable?”.
RECONSTRUCTING PAST TEMPERATURES IS DIFFICULT
It is a complicated process to reconstruct a record of ancient temperatures. There were no thermometer measurements until around 150 years ago. This means that scientists must look at so called proxies such as tree rings, ice cores, corals, plant remains, or sediment cores from lakes, among many others. These all exhibit features that are affected by the temperature and can help scientists to infer what temperatures were in the past.
Scientists must extract the data from the proxy and then convert it into a record of surface temperatures – this enables them to “backcast” or ”reconstruct” historical temperature records. This involves significant statistical analysis and it is this part of the process that McShane and Wyner have criticised. “Our paper is an effort to apply some modern statistical methods to these problems,” they state. “While our results agree with the climate scientists findings in some respects, our methods of estimating model uncertainty and accuracy are in sharp disagreement.“
Blake McShane is a faculty member in the Marketing Department of the Kellogg School of Management at Northwestern University. He is a statistical methodologist, and has developed models for problems in a variety of fields.
Abraham Wyner is Associate Professor, Department of Statistics at The Wharton School, University of Pennsylvania.
abstract:
We find that the proxies do not predict temperature significantly better than random series generated independently of temperature. Furthermore, various model specifications that perform similarly at predicting temperature produce extremely different historical backcasts. Finally, the proxies seem unable to forecast the high levels of and sharp run-up in temperature in the 1990s either in-sample or from contiguous holdout blocks, thus casting doubt on their ability to predict such phenomena if in fact they occurred several hundred years ago.
Conclusion.
Research on multi-proxy temperature reconstructions of the earth’s temperature is now entering its second decade. While the literature is large, there has been very little collaboration with universitylevel, professional statisticians (Wegman et al., 2006; Wegman, 2006). Our paper is an effort to apply some modern statistical methods to these problems. While our results agree with the climate scientists findings in some
respects, our methods of estimating model uncertainty and accuracy are in sharp disagreement.
On the one hand, we conclude unequivocally that the evidence for a ”long-handled” hockey stick (where the shaft of the hockey stick extends to the year 1000 AD) is lacking in the data. The fundamental problem is that there is a limited amount of proxy data which dates back to 1000 AD; what is available is weakly predictive of global annual temperature. Our backcasting methods, which track quite closely the methods applied most recently in Mann (2008) to the same data, are unable to catch the sharp run up in temperatures recorded in the 1990s, even in-sample.
As can be seen in Figure 15, our estimate of the run up in temperature in the 1990s has
a much smaller slope than the actual temperature series. Furthermore, the lower frame of Figure 18 clearly reveals that the proxy model is not at all able to track the high gradient segment. Consequently, the long flat handle of the hockey stick is best understood to be a feature of regression and less a reflection of our knowledge of the truth. Nevertheless, the temperatures of the last few decades have been relatively warm compared to many of the thousand year temperature curves sampled from the posterior distribution of our model.
Our main contribution is our efforts to seriously grapple with the uncertainty involved in paleoclimatological reconstructions. Regression of high dimensional time series is always a complex problem with many traps. In our case, the particular challenges include (i) a short sequence of training data, (ii) more predictors than observations, (iii) a very weak signal, and (iv) response and predictor variables which are both strongly autocorrelated.
The final point is particularly troublesome: since the data is not easily modeled by a simple autoregressive process it follows that the number of truly independent observations (i.e., the effective sample size) may be just too small for accurate reconstruction.
Climate scientists have greatly underestimated the uncertainty of proxy based reconstructions and hence have been overconfident in their models. We have shown that time dependence in the temperature series is sufficiently strong to permit complex sequences of random numbers to forecast out-of-sample reasonably well fairly frequently (see, for example, Figure 9). Furthermore, even proxy based models with approximately the same amount of reconstructive skill (Figures 11,12, and 13), produce strikingly dissimilar historical backcasts: some of these look like hockey sticks but most do not (Figure 14).
Natural climate variability is not well understood and is probably quite large. It is not clear that the proxies currently used to predict temperature are even predictive of it at the scale of several decades let alone over many centuries. Nonetheless, paleoclimatoligical reconstructions constitute only one source of evidence in the AGW debate. Our work stands entirely on the shoulders of those environmental scientists who labored untold years to assemble the vast network of natural proxies. Although we assume the reliability of their data for our purposes here, there still remains a considerable number of outstanding questions that can only be answered with a free and open inquiry and a great deal of replication.
http://www.e-publications.org/ims/submi ... m=63ebfddf
The paper re-opens the controversy over the so-called hockey stick graph, made famous by Al Gore in “An Inconvenient Truth”, and may re-energise climate change sceptics in the wake of “climategate” and on the eve of the United Nations Cancun climate conference.
The hockey-stick graph, constructed mainly from tree ring “proxy” data by US climate scientist Michael Mann, shows historic temperatures remaining fairly steady and then rising sharply since the industrial revolution. But statisticians Blakeley McShane of Northwestern University in the US and Abraham Wyner of the University of Pennsylvania in the US write in a new paper that “we conclude unequivocally that the evidence for a ”long-handled” hockey stick (where the shaft of the hockey stick extends to the year 1000 AD) is lacking in the data”.
CLIMATE SCIENTISTS “GREATLY UNDERESTIMATED THE UNCERTAINTY”
Temperature records reconstructed from tree rings, ice cores and other so called proxies that give indications of ancient temperatures “do not predict temperature significantly better than random series generated independently of temperature”, McShane and Wyner state in their paper [published] in the peer-reviewed journal Annals of Applied Statistics. The two statisticians conclude that climate scientists “have greatly underestimated the uncertainty of proxy based reconstructions and hence have been overconfident in their models”. They comment that the long flat handle of the hockey stick is best understood to be a feature of the data processing 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 weakly predictive of global annual temperature,” McShane and Wyner comment in their paper which is called “A Statistical Analysis Of Multiple Temperature Proxies: Are Reconstructions Of Surface Temperatures Over The Last 1000 Years Reliable?”.
RECONSTRUCTING PAST TEMPERATURES IS DIFFICULT
It is a complicated process to reconstruct a record of ancient temperatures. There were no thermometer measurements until around 150 years ago. This means that scientists must look at so called proxies such as tree rings, ice cores, corals, plant remains, or sediment cores from lakes, among many others. These all exhibit features that are affected by the temperature and can help scientists to infer what temperatures were in the past.
Scientists must extract the data from the proxy and then convert it into a record of surface temperatures – this enables them to “backcast” or ”reconstruct” historical temperature records. This involves significant statistical analysis and it is this part of the process that McShane and Wyner have criticised. “Our paper is an effort to apply some modern statistical methods to these problems,” they state. “While our results agree with the climate scientists findings in some respects, our methods of estimating model uncertainty and accuracy are in sharp disagreement.“
Blake McShane is a faculty member in the Marketing Department of the Kellogg School of Management at Northwestern University. He is a statistical methodologist, and has developed models for problems in a variety of fields.
Abraham Wyner is Associate Professor, Department of Statistics at The Wharton School, University of Pennsylvania.
abstract:
We find that the proxies do not predict temperature significantly better than random series generated independently of temperature. Furthermore, various model specifications that perform similarly at predicting temperature produce extremely different historical backcasts. Finally, the proxies seem unable to forecast the high levels of and sharp run-up in temperature in the 1990s either in-sample or from contiguous holdout blocks, thus casting doubt on their ability to predict such phenomena if in fact they occurred several hundred years ago.
Conclusion.
Research on multi-proxy temperature reconstructions of the earth’s temperature is now entering its second decade. While the literature is large, there has been very little collaboration with universitylevel, professional statisticians (Wegman et al., 2006; Wegman, 2006). Our paper is an effort to apply some modern statistical methods to these problems. While our results agree with the climate scientists findings in some
respects, our methods of estimating model uncertainty and accuracy are in sharp disagreement.
On the one hand, we conclude unequivocally that the evidence for a ”long-handled” hockey stick (where the shaft of the hockey stick extends to the year 1000 AD) is lacking in the data. The fundamental problem is that there is a limited amount of proxy data which dates back to 1000 AD; what is available is weakly predictive of global annual temperature. Our backcasting methods, which track quite closely the methods applied most recently in Mann (2008) to the same data, are unable to catch the sharp run up in temperatures recorded in the 1990s, even in-sample.
As can be seen in Figure 15, our estimate of the run up in temperature in the 1990s has
a much smaller slope than the actual temperature series. Furthermore, the lower frame of Figure 18 clearly reveals that the proxy model is not at all able to track the high gradient segment. Consequently, the long flat handle of the hockey stick is best understood to be a feature of regression and less a reflection of our knowledge of the truth. Nevertheless, the temperatures of the last few decades have been relatively warm compared to many of the thousand year temperature curves sampled from the posterior distribution of our model.
Our main contribution is our efforts to seriously grapple with the uncertainty involved in paleoclimatological reconstructions. Regression of high dimensional time series is always a complex problem with many traps. In our case, the particular challenges include (i) a short sequence of training data, (ii) more predictors than observations, (iii) a very weak signal, and (iv) response and predictor variables which are both strongly autocorrelated.
The final point is particularly troublesome: since the data is not easily modeled by a simple autoregressive process it follows that the number of truly independent observations (i.e., the effective sample size) may be just too small for accurate reconstruction.
Climate scientists have greatly underestimated the uncertainty of proxy based reconstructions and hence have been overconfident in their models. We have shown that time dependence in the temperature series is sufficiently strong to permit complex sequences of random numbers to forecast out-of-sample reasonably well fairly frequently (see, for example, Figure 9). Furthermore, even proxy based models with approximately the same amount of reconstructive skill (Figures 11,12, and 13), produce strikingly dissimilar historical backcasts: some of these look like hockey sticks but most do not (Figure 14).
Natural climate variability is not well understood and is probably quite large. It is not clear that the proxies currently used to predict temperature are even predictive of it at the scale of several decades let alone over many centuries. Nonetheless, paleoclimatoligical reconstructions constitute only one source of evidence in the AGW debate. Our work stands entirely on the shoulders of those environmental scientists who labored untold years to assemble the vast network of natural proxies. Although we assume the reliability of their data for our purposes here, there still remains a considerable number of outstanding questions that can only be answered with a free and open inquiry and a great deal of replication.
http://www.e-publications.org/ims/submi ... m=63ebfddf
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