Learn how plants respond to higher atmospheric CO2 concentrations

How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Mann and Company Still Malign the Medieval Warm Period
Volume 13, Number 3: 20 January 2010

In the 27 November 2009 issue of Science, Michael Mann and eight coauthors describe how they used a global climate proxy network consisting of data derived from ice core, coral, sediment and various other records to reconstruct a Northern Hemispheric surface air temperature history covering the past 1500 years for the purpose of determining the characteristics of the Little Ice Age and Medieval Warm Period. And, once again, they have used Mann's "Nature Trick" of "ClimateGate" fame, truncating the reconstructed temperature history near its end and replacing it with modern-day instrumental data, so that the last part of the record cannot be validly compared with the earlier portion, since to do so would be akin to comparing apples and oranges, which cannot produce credible quantitative results.

This subterfuge is totally unwarranted. And in its current application, it's not just from 1981 or 1961 onwards that the ruse is applied; it's applied all the way from 1850 to 1995, which is the period of overlap between the proxy and instrumental records that was used to calibrate the proxy data. Therefore, since the proxy data were available all the way up to 1995, the reconstructed near-surface air temperature history should also have been plotted all the way up to 1995, in order to be able to make valid quantitative comparisons between the degree of warmth of the Current and Medieval Warm Periods.

So why wasn't this clearly superior method of data analysis employed? It may plausibly be concluded from the recently-revealed University of East Anglia emails that the inferior method was used in order to "hide the decline" in the reconstructed temperature history that was evident in the latter decades of some of the proxy data. And why was this done? It may likewise be plausibly concluded that it was done to "get rid of the Medieval Warm Period." And why was that a goal of Mann and his colleagues? Plausibly, it was to rid the world of the annoying fact that many prior studies had shown the Medieval Warm Period to have been slightly warmer than the Current Warm Period. And why was that a problem? It was a problem because knowledge of the existence of higher temperatures during the Medieval Warm Period makes it much more difficult for most rational people to believe that the planet's current level of warmth is due to its high atmospheric CO2 concentration, especially when it is realized that during the hotter Medieval Warm Period there was 25% less CO2 in the air than there is currently.

Interestingly, and even with the greatly biased "apples and oranges" comparison utilized by Mann et al., the nine researchers were forced to acknowledge that the warmth over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America during the Medieval Warm Period was "comparable to or exceeds that of the past one-to-two decades in some regions."

So what happens if "apples and apples" are compared? This is the approach we highlight in our Medieval Warm Period Project, where each week we present the results of a study of the Medieval Warm Period that was conducted in a different part of the world. What we denote as Level 1 Studies document the quantitative difference between the peak temperatures of the Medieval Warm Period (MWP) and the Current Warm Period (CWP). Level 2 Studies, which are qualitative in nature, only reveal which period was the warmer of the two; while Level 3 Studies merely allow the time interval of the Medieval Warm Period to be determined.

All three study types generally place the core of the Medieval Warm Period within the same time frame as that suggested by Mann et al. (2009), as may be seen by visiting our Interactive Map and Time Domain Plot. In the case of the qualitative temperature studies, the peak temperature of the MWP is usually seen to be greater than the peak temperature of the CWP, as shown graphically in our Level 2 Study Results. And in the case of the quantitative temperature studies, the peak temperature of the MWP averages about 0.75C more than the peak temperature of the CWP, as shown graphically in our Level 1 Study Results.

So does the "Nature Trick" work for Mann and Company? It sure does, as it allows them to continue to underestimate the true level of warmth of the Medieval Warm Period. And this ruse allows the Intergovernmental Panel on Climate Change and the United Nations to continue to contend that earth's current temperatures are the greatest the planet has experienced over the past millennium or more, when the vast majority of real-world data clearly proclaim otherwise. It is truly a sad day for science, when political considerations appear to take precedence over the search for truth.

Sherwood, Keith and Craig Idso

Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G. and Ni, F. 2009. Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326: 1256-1260.