RESPONSE FOR THE RECORD OF DR. GERALD R. NORTH, DEPARTMENT OF ATMOSPHERIC SCIENCES, TEXAS A&M UNIVERSITY The Honorable Ed Whitfield 1. As you chaired the National Research Council panel that recently issued the report on millennial temperature reconstructions: a. Where in the report did the panel describe "plausible" as suggesting roughly a 2/3rds probability of being correct? In the report we shunned the use of numerical probability assessments in favor of descriptive statements (e.g., "high confidence") and statements that describe our relative confidence in different conclusions (e.g. "less confidence"). I may have mistakenly mentioned the "two to one odds" figure in the oral press release of the report, and it may also have appeared in some press accounts, but it does not appear in the report, and I avoided using it in my sworn testimony. b. In the report, did the panel attach probability estimates to the term "plausible"? No. The committee avoided numerical probability estimates because many of the uncertainties associated with reconstructing surface temperatures are not purely statistical in nature, but rather arise from physical factors associated with each proxy that are simply unquantifiable at this time. In our view it is not possible to quantify all of the inherent uncertainties associated with reconstructing surface temperatures from proxy data, which in turn precludes assigning numerical probabilities to statements regarding the unique nature of recent warmth. c. Why did the panel choose to use the term "plausible," as opposed for example to terms such as "likely," to describe confidence in millennial temperature reconstructions? In the IPCC reports, the term "likely" is used to indicate an estimated probability of between 66% and 90%, i.e. greater than two-thirds odds but less than nine-in-ten chances. We avoided numerical estimates such as these because we did not want to imply that we had performed a rigorous probability assessment. Instead, we tried to express our collective confidence in different conclusions using descriptive language. 2. When considering the panel's findings that it is "plausible" that recent decades were the warmest in a millennium, is that correct to interpret that to mean the panel's consensus view was that plausible means roughly a 2/3rds probability of being correct, as was suggested in news reports following the press conference releasing the report? Our working definition of "plausible" was that the assertion is reasonable, or in other words there is not a convincing argument to refute the assertion. We used this term to describe our assessment of the statement that "the last few decades of the 20th century were warmer than any comparable period over the last millennium" because none of the available evidence to date contradicts this assertion. In our view it is not currently possible to perform a quantitative evaluation of recent warmth relative to the past 1,000 years that includes all of the inherent uncertainties associated with reconstructing surface temperatures from proxy data. This precludes stronger statements of confidence, but it does not mean that the assertion is false. In fact, all of the large-scale surface temperature reconstructions that we examined support the assertion that global-mean temperatures during the last few decades of the 20th century were unprecedented over at least the past 1,000 years, and a larger fraction of geographically diverse proxy records experienced exceptional warmth during the late 20th century than during any other extended period from 900 A.D. onward. 3. Did the panel perform its own, in-depth technical analysis of the methods and procedures-- such as checking the underlying data sets or attempting to replicate the findings used in the various temperature reconstruction articles and presentations it considered in formulating its report? Our committee relied on the published, refereed scientific literature to reach its conclusions. We did not attempt to replicate the work of any previous author, with the lone exception of a simple computer program (reproduced in Appendix B of our report) that was used to illustrate an interesting artifact of the principal components methodology first noted by McIntyre and McKitrick. When evaluating the results of different studies, we placed higher confidence in those results that were reproduced in several different studies--for instance a number of independent lines of evidence indicate that the late 20th century warmth was unprecedented in at least the last 400 years, giving us high confidence in this conclusion. Less confidence can be placed in conclusions regarding large-scale surface temperatures prior to about 1600 A.D. because there are simply fewer independent lines of evidence to consider, although the evidence that does exist indicates that the late 20th century warmth is unprecedented for at least the last 1,000 years. 4. The NRC panel made specific reference to ice borehole studies in Greenland by DahlJensen, which suggest warmer temperatures in that region during the Medieval Warm Period than today. Please explain the value of regional temperature measurements such as this for understanding the potential effects of recent warming trends? There are two main reasons for using large-scale averages rather than individual regional measurements to evaluate global environmental changes: 1) Random measurement errors and climate fluctuations tend to cancel out when spatial averages are performed, allowing researchers to obtain a more reliable estimate than is possible for a local or a regional average; 2) The greenhouse effect operates at the global scale, hence large-scale averages are the best way to evaluate the response of the climate to increasing greenhouse gas concentrations. Current climate models also are better at computing large-scale averages than regional-scale values. Of course in order to detect large-scale climate anomalies, either in the modern temperature record or in proxy-based temperature reconstructions, it helps to have a large network of high quality measurements for geographically-diverse regions. The main reason that we have high confidence in the temperature increase over the past 100 years and in the statement that temperatures are warmer now than at any other time over the last 400 years is because we have a sufficiently large number of well-characterized local measurements to calculate a reliable large-scale average. Several proxies (including historical and archeological evidence as well as quantitative temperature estimates from ice cores and boreholes) indicate that the area around Greenland was warmer between about 1000 and 1200 A.D. than it is today. There is also evidence for warm temperatures during medieval times from other regions of the world. However, studies suggest that these warm anomalies appear to have occurred at different times at different places rather than being globally synchronous, and also appear to have been offset by cold anomalies in other regions. The few large-scale surface temperature reconstructions that extend back far enough to rigorously compare large-scale medieval temperatures to modern warmth suggest that the medieval period was, at most, comparable in warmth to the first half of the 20th century. However, as noted above in response to question (4), it is difficult to quantify the full uncertainty associated with estimates of surface temperature prior to about 1600 A.D. The Honorable Bart Stupak 1. In the study performed by a special committee of the National Academy of Sciences (NAS) on surface temperature reconstructions over the past 2,000 years, it was stated that, for the time prior to 1600 A.D., scientists are less certain about the actual average northern hemispheric surface temperatures. The Medieval Warm Period (MWP) occurred prior to 1600. How certain are climatologists that there was a globally or even hemispherically MWP that was warmer than the past several decades? Indeed, the paucity of proxy data for periods prior to about 1600 A.D., especially in the tropics and the Southern Hemisphere, limits our confidence in statements regarding the global mean temperature of the past few decades compared to medieval times. Several proxies indicate that the area around Greenland was warmer between about 1000 and 1200 A.D. than it is today. There is also evidence for warm temperatures during medieval times from other regions of the world. However, studies suggest that these warm anomalies appear to have occurred at different times at different places rather than being hemispherically or globally synchronous, and also appear to have been offset by cold anomalies in other regions. Although it is difficult to quantify the full uncertainty associated with estimates of surface temperature prior to about 1600 A.D., all of the large-scale surface temperature reconstructions that we examined support the assertion that global-mean temperatures during the last few decades of the 20th century were unprecedented over at least the past 1,000 years, and a larger fraction of geographically diverse proxy records experienced exceptional warmth during the late 20th century than during any other extended period from 900 A.D., onward. Hence we find it plausible (or in other words, no evidence exists to refute the claim) that "the last few decades of the 20th century were warmer than any comparable period over the last millennium." This statement can be more strongly applied to the Northern Hemisphere than to the globe because there is very little proxy data from the Southern Hemisphere before about 1600 A.D. 2. The 1990 Intergovernmental Panel on Climate Change (IPCC) Report contains a "schematic diagram" that shows temperature changes for 900 A.D. through 1975, but does not give specific temperatures. The text of the report notes, "it is still not clear whether all the fluctuations indicated were truly global." Am I correct in my understanding that this schematic diagram is not a graph of specific data points consisting of global temperature for particular years or time periods? Am I also correct that the scientific consensus at the time was that there was significant uncertainty about whether the diagram accurately portrayed the global temperature profile over the last 1,000 years? Yes, the schematic diagram that appeared in the 1990 IPCC Report was simply a qualitative depiction of how scientists thought that large-scale temperatures may have evolved from 900 A.D. to about 1975. There was very little proxy data available at that time, and the data that did exist tended to be concentrated in just a few geographical |