Can Medieval Heat Cool Warming Worries?

A flurry of recent scientific papers has tried to measure the warmth of the "Medieval Warm Period" (MWP) of about 1,000 years ago. Scientists have long debated whether it was cooler or warmer than today, and whether the warmth was global or regional. The point for nonscientists: If recent warming has precedents, some might find it less alarming.

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John S. Dykes

If recent warmth has precedents, some might find it less alarming.

Until the late 1990s, researchers generally agreed that the MWP was warmer than today and that the "Little Ice Age" of 1500-1800 was colder. Then in 2001 the Intergovernmental Panel on Climate Change adopted the "hockey stick" graph devised by Michael Mann at the University of Virginia and colleagues.

Using temperature indicators such as tree rings and lake sediments, the graph rewrote history by showing little warmth in the 11th century and little cold in the 17th, but a sharp spike in late-20th-century temperatures. That graph helped to persuade many people (such as me) that recent temperature rises were unprecedented in scale and speed in at least 1,400 years.

But critics of the graph pointed out that it used a statistical technique that overemphasized hockey-stick shaped data from unreliable indicators, such as tree rings in bristlecone pine trees and Scandinavian lake sediments influenced by 20th-century land-use changes. Four recent studies have now rehabilitated the MWP as a period of unusual warmth, though they disagree on whether it was as warm or warmer than today.

Jan Esper of the University of Mainz and his colleagues looked at pine wood densities from Sweden and Finland and found "evidence for substantial warmth during Roman and medieval times, larger in extent and longer in duration than 20th-century warmth." Bo Christiansen of the Danish Meteorological Institute and Fredrik Ljungqvist of Stockholm University looked at 32 indicators across the Northern Hemisphere and found the level of warmth during the peak of the MWP "in the second half of the 10th century equaling or slightly exceeding the mid-20th century warming."

Thomas Melvin of the University of East Anglia and colleagues reanalyzed one of the tree samples from Sweden used in the "hockey stick" and concluded: "We can infer the existence of generally warm summers in the 10th and 11th centuries, similar to the level of those in the 20th century."

A fourth study of creatures called diatoms in Chinese lake sediments found that the period "between ca. A.D. 1150 and 1200 was the warmest interval of the past 1,000 years."

Taken together, these studies cast doubt on the IPCC's conclusion in 2007 that "the evidence is not sufficient to support a conclusion that [Northern] hemispheric mean temperatures were as warm, or the extent of warm regions as expansive, as those in the 20th century as a whole, during any period in medieval times."

But was the medieval warm period confined to the Northern Hemisphere?

I consulted a database of papers collated by the climate-skeptic website CO2Science.org, run by the Center for the Study of Carbon Dioxide and Global Change, a nonprofit research center in Tempe, Ariz. The database contains numerous published studies of isotopes and other indicators in caves, lake sediments and other samples from Chile, New Zealand, South Africa and Antarctica that find the MWP warmer than today. Two Antarctic studies, for instance, concluded that current warming "is not yet as extreme in nature as the MWP" and that "the present state of reduced ice on the western Antarctic Peninsula is not unprecedented." A far smaller number of studies, such as one from Lake Tanganyika, found the MWP cooler than today.

It remains possible that today's warming is different from that of the Middle Ages. For example, while summers might have been warmer then, winters might be warmer today (if today's warming is caused by carbon dioxide, that should be true). And of course, it is the future, not the past, that scientists expect to be dangerous. Nonetheless, the evidence increasingly vindicates the scientists who first discovered the Medieval Warm Period.