February 11, 2008

A 2,000-Year Global Temperature Record

Over the past decade, considerable debate existed regarding the temperature history of the Earth on the time scale of millennia. If you followed our discussion on the subject, you know that one camp would like you believe that the highly-publicized warming of the planet over the past century is absolutely unprecedented over the past few thousand years. This group seems to fixate on the “hockey stick” representation of the temperature history of the past 1,000 years, and they hold on to the stick in spite of evidence to the contrary. Many others have argued based on proxy evidence throughout the world that the past few thousand years include a very warm period 1,000 years ago and a cold period 500 years ago; in their eyes, the warming of the past century is not at all unusual. These folks even go on to suggest that the Earth today may not be yet as warm as conditions 1,000 years ago, despite the 100 ppm increase in atmospheric carbon dioxide over the past century.

Several articles have appeared in Energy and Environment recently with results of considerable interest to us at World Climate Report. The first piece is by Dr. Craig Loehle who received his Ph.D. in mathematical ecology in 1982 from Colorado State University. He has published over one hundred papers in applied mathematics and ecology on topics that include statistical models, optimization, simulation, artificial intelligence, fractals, and wavelets. Among other accomplishments, he is the developer of “Global Optimization” which is a Mathematica application package that has been on the market since 1998. Dr. Loehle’s affiliation listed in the two articles is the National Council for Air and Stream Improvement, Inc. In the second paper, Loehle is joined by J. Huston McCulloch of the economics department at Ohio State University. There is no evidence that their work is funded by anyone with any stake in the greenhouse – global warming debate.

In the first article, Loehle notes that many long-term reconstructions of climate are based on tree rings, but “There are reasons to believe that tree ring data may not capture long-term climate changes (100+ years) because tree size, root/shoot ratio, genetic adaptation to climate, and forest density can all shift in response to prolonged climate changes, among other reasons.” Furthermore, Loehle notes “Most seriously, typical reconstructions assume that tree ring width responds linearly to temperature, but trees can respond in an inverse parabolic manner to temperature, with ring width rising with temperature to some optimal level, and then decreasing with further temperature increases.” Other problems include tree responses to precipitation changes, variations in atmospheric pollution levels, diseases, pest outbreaks, and the obvious problem of enrichment that comes along with ever higher levels of atmospheric carbon dioxide. Trees are not simple thermometers!

Loehle gathered as many non-tree ring reconstructions as possible for places throughout the world (Figure 1). There are dozens of very interesting ways to peer into the climatic past of a location, and Loehle included borehore temperature measurements, pollen remains, Mg/Ca ratios, oxygen isotope data from deep cores or from stalagmites, diatoms deposited on lake bottoms, reconstructed sea surface temperatures, and so on. Basically, he grabbed everything available, so long as it did not rely on trees.

Figure 1. Map of study sites used in the Loehle, 2007 and Loehle and McCulloch, 2008 studies

Loehle averaged the data for the 18 sites and produced the plot below, with each point representing 30-year centered average temperature (Figure 2). Loehle notes “The data show the Medieval Warm Period (MWP) and Little Ice Age (LIA) quite clearly.” The plot also shows that 20th century warming is apparently dwarfed by events in the past. If you are curious about the apparent cooling near present, Loehle explains “The series ends with a downtick because the last set of points are averages that include the cool decades of the 1960s and 1970s.” Finally, we learn “It is clear that the 1995-year reconstruction shown here does not match the famous hockey stick shape.” Believe us—we noticed that fact immediately!

Figure 2. Mean of temperature data for 18 data sets. The last point represents the period from 1966-1995. (from Loehle, 2007)

Following publication of the first article, several errors came to light regarding how temperatures were reported from the various locations. At times, they were reported relative to mean global temperature in 1950, at other times, the base year was 2000. Errors regarding smoothing came to light, other issues were raised, and Loehle and co-author J. Huston McCulloch decided to re-do all the calculations, including improvements in terms of confidence intervals.

The improved plot below shows little change from the graph above, although for data reasons, the last point now represents the 29-year average temperature centered on 1935. (Figure 3). The two statistical wizards note “The corrected data continue to show the Medieval Warm Period (MWP) and Little Ice Age (LIA) quite clearly. The confidence intervals” “indicate that the MWP was significantly warmer than the bimillennial average during most of approximately 820 – 1040 AD, at the 5% level (2-tailed). Likewise, the LIA was significantly cooler than the bimillennial average during most of approximately 1440-1740 AD.”

Figure 3. Corrected reconstruction with 95% confidence intervals (from Loehle and McCulloch, 2008)

We suspect you have been living your life unaware of the articles by Loehle and McCulloch. The reason is obvious – they found evidence that temperature variations over the past 2,000 years indicate that the earth’s average temperature bounces around naturally to a larger degree than other paleo-reconstructions indicate, and further, that temperatures about 1,000 years ago were not that dissimilar to today’s temperatures. This suggests that the earth’s ecosystems are more resilient (and adaptive) than some pessimists give them credit for—not a favorite topic in the mainstream press.


Loehle, C. 2007. A 2000-year global temperature reconstruction based on non-tree ring proxies. Energy and Environment, 18, 1049-1058.

Loehle, C. and J.H. McCulloch. 2008. Correction to: A 2000-year global temperature reconstruction based on non-tree ring proxies. Energy and Environment, 19, 93-100.

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