April 1, 12:17 a.m. | Update below |
There's been vigorous discussion here and elsewhere (e.g., Climate Audit and Skeptical Science) of the methods and findings in "A Reconstruction of Regional and Global Temperature for the Past 11,300 Years," a recent Science paper by Shaun A. Marcott, Jeremy D. Shakun, Peter U. Clark and Alan C. Mix.
One author, Jeremy Shakun (currently at Harvard) weighed in via Skype for Dot Earth. When more questions came in, the group of authors wrote that they would respond more completely to questions about the work and now they have done so, on the RealClimate blog. Here's a short excerpt and link to the rest:
What do paleotemperature reconstructions show about the temperature of the last 100 years?
Our global paleotemperature reconstruction includes a so-called "uptick" in temperatures during the 20th-century. However, in the paper we make the point that this particular feature is of shorter duration than the inherent smoothing in our statistical averaging procedure, and that it is based on only a few available paleo-reconstructions of the type we used. Thus, the 20th century portion of our paleotemperature stack is not statistically robust, cannot be considered representative of global temperature changes, and therefore is not the basis of any of our conclusions. Our primary conclusions are based on a comparison of the longer term paleotemperature changes from our reconstruction with the well-documented temperature changes that have occurred over the last century, as documented by the instrumental record. Although not part of our study, high-resolution paleoclimate data from the past ~130 years have been compiled from various geological archives, and confirm the general features of warming trend over this time interval (Anderson, D.M. et al., 2013, Geophysical Research Letters, v. 40, p. 189-193).
Is the rate of global temperature rise over the last 100 years faster than at any time during the past 11,300 years?
Our study did not directly address this question because the paleotemperature records used in our study have a temporal resolution of ~120 years on average, which precludes us from examining variations in rates of change occurring within a century. Other factors also contribute to smoothing the proxy temperature signals contained in many of the records we used, such as organisms burrowing through deep-sea mud, and chronological uncertainties in the proxy records that tend to smooth the signals when compositing them into a globally averaged reconstruction. We showed that no temperature variability is preserved in our reconstruction at cycles shorter than 300 years, 50% is preserved at 1000-year time scales, and nearly all is preserved at 2000-year periods and longer. Our Monte-Carlo analysis accounts for these sources of uncertainty to yield a robust (albeit smoothed) global record. Any small "upticks" or "downticks" in temperature that last less than several hundred years in our compilation of paleoclimate data are probably not robust, as stated in the paper.
There's much more at RealClimate, although there's also room for more questions — one being how the authors square the caveats they express here with some of the more definitive statements they made about their findings in news accounts.
[April 1, 12:17 a.m. | Insert | Roger A. Pielke, Jr., of the University of Colorado has compared the new output from the authors with the paper and related news releases and coverage (including mine) and finds some very big differences.]
The managers of RealClimate offered their own reaction to the paper and the authors amplification on their methods and conclusions:
Our view is that the results of the paper will stand the test of time, particularly regarding the small global temperature variations in the Holocene. If anything, early Holocene warmth might be overestimated in this study.
For convenience, here's Shakun's discussion of the paper when it was published in Science:
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