Medieval Warm Epoch

Introduction

The Medieval Warm Epoch (also called the Medieval Warm Period or Little Optimum) was a period of unusually warm climate that occurred from about AD 800 to 1200. The idea of a warm medieval climate period, at least in Europe, was first discussed in the early twentieth century, but the phrase “Medieval Warm Epoch” (MWE) was not coined until the 1960s.

The extent of the MWE is in doubt. In recent years, scientific studies have shown that in the last 2,000 years, the warmest period in at least the northern hemisphere probably did occur from about AD 950 to 1100. Although some skeptics of global warming have claimed that the MWE shows that modern global warming is not anthropogenic (human-caused) or unprecedented, recent studies have concluded that the evidence does not support the conclusion that the MWE was as warm as the twentieth century. Present-day warming is unprecedented over at least the last 2,000 years.

Historical Background and Scientific Foundations

The first hints that Europe may have been significantly warmer during the Middle Ages were discovered in the early twentieth century. Scandinavian researchers found signs that larger areas of Iceland and Greenland had been cultivated in the tenth century than a few centuries later or currently in the early twenty-first century. The phrase “Medieval Warm Epoch” was coined by climatologist H. H. Lamb in 1965 to describe the period of warming in Western Europe and the Northern Atlantic that seemed indicated by several lines of evidence, mostly paleoclimatic and anecdotal (historical accounts dating from the period).

Lamb thought that the MWE had occurred from 1000 to 1200, but distinguished between an early phase (950–1200) in European Russia and Greenland and a later, overlapping one (1150–1300) in the rest of Europe. The MWE, Lamb suggested, was characterized by milder winters and warmer, drier summers, with a temperature 1.8–3.6°F (1–2°C) above the average for 1900 to 1939. The MWE was not necessarily global; Lamb's evidence was almost entirely European.

WORDS TO KNOW

ICE BOREHOLE THERMOMETRY: Measurement of temperature at different depths of a vertical hole drilled in a large mass of ice, such as the Antarctic ice cap. By mathematical processing (deconvolution) of this spatially organized record of temperatures, a time record of temperature over the last several centuries can be inferred.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE(IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

PALEOCLIMATIC: Relating to Earth's climate prior to the beginning of instrumental weather records in the late 1800s. “Paleo” means old or ancient, from the Greek palaiois for old.

In the 1990s and later, researchers found evidence for climate changes outside of Europe during the MWE. For example, study of sand dunes that built up on the Great Plains of North America has shown that a prolonged drought occurred in North America during the MWE. Normally, prairie grasses keep large dunes in the Nebraska Sand Hills region stationary, anchoring their surfaces with their roots. However, during prolonged drought, the grasses die and the dunes are slowly shifted by the wind. The orientation and internal layering of these large dunes shows that during AD 1000 to 1200, spring-summer southerly air flow was replaced by dry southwesterly air flow, causing drought and allowing the dunes to move. When the drought ended, the dunes were eventually stabilized by grasses again and have remained so ever since.

Other parts of the world also experienced unusual precipitation conditions during the MWE. For this reason, some scientists have argued that the MWE would be better named the Medieval Climatic Anomaly, because warmth may not have been its only important characteristic.

There is evidence that climate in other parts of the world was warmer during the MWE. For example, analysis of measurements of heat levels at different depths below the ground, a method called borehole thermometry, confirms the occurrence of the MWE in Greenland and may show that temperatures in many parts of the world were warmer from about 500 to 1,000 years ago than today. Ice borehole thermometry from Antarctica shows that temperatures may have been several degrees Centigrade lower in Antarctica during the MWE—an expected result if the northern hemisphere was warmer at that time, because temperatures at the north and south poles tend to tip in opposite directions like the ends of a see-saw.

Impacts and Issues

Climate does not, generally, change the same way in all areas of the world at the same time. While some parts warm, others may cool, and as some become drier, others may become wetter. A number of scientific studies have shown that this pattern of patchy (heterogeneous) change probably characterized the MWE worldwide. The gist of the evidence, as understood at the time of the Intergovernmental Panel on Climate Change's 2007 Fourth Assessment Report (which used no data collected after the end of 2005), was that over the last 2,000 years, the MWE was probably the warmest period prior to the twentieth century. However, the available evidence did not support the claim that the MWE, whether in Europe or globally, was warmer than the twentieth and early twenty-first century, particularly the period since 1980, during which time global warming has accelerated markedly.

See Also Climate Change Skeptics; Paleoclimatology.

BIBLIOGRAPHY

Books

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Bradley, Raymond S. “Climate in Medieval Times.” Science 302, no. 5644 (October 17, 2003): 404–405.

Broecker, Wallace S. “Was the Medieval Warm Period Global?” Science 291, no. 5508 (February 23, 2001): 1497–1499.

Sridhar, Venkataramana, et al. “Large Wind Shift on the Great Plains during the Medieval Warm Period.” Science 313, no. 5785 (July 21, 2006): 345–347.

Larry Gilman