Medieval Climate Not So Hot

There's not enough evidence to conclude that the Medieval Warm Period was global, or that regional warm spells between 500 and 1500 A.D. occurred simultaneously, leading paleoclimatologists report in the Oct. 17 issue of Science.

"The balance of evidence does not point to a High Medieval period (1100 to 1200 A.D.) that was as warm or warmer than the late 20th century," the team wrote. They pointed out that temperatures during High Medieval time in the Northern Hemisphere were almost the same as they were from 1901 to 1970, a time period which was cooler than the last three decades of the 20th century by about one-third of a degree Celsius (six-tenths of a degree Fahrenheit).

The authors of the article, "Climate in Medieval Time," are Raymond S. Bradley of the University of Massachusetts - Amherst, Malcolm K. Hughes of the University of Arizona Laboratory of Tree-Ring Research, and Henry F. Diaz of the National Oceanic and Atmospheric Administration Office of Atmospheric Research.

The scientists concluded that average temperatures in the Northern Hemisphere "were not exceptional" because some regions cooled whereas other regions warmed. The Northern Hemisphere is the region of the world for which scientists have the best natural records of past climate.

To reconstruct climate, scientists use natural archives, including tree rings, ice cores and laminated sediments. These natural records of temperature, precipitation and other environmental history have been carefully calibrated with instrumental observations, giving scientists quantitative information on past conditions at particular locations.

Scientists have well-calibrated, detailed data from such proxy records covering the past 1,500 years of temperature for only a few locations around the globe, the researchers noted. Most of these records are from temperate zones in the Northern Hemisphere. Few come from the tropics, and only a handful come from the Southern Hemisphere.

A common argument for the existence of a global Medieval Warm Period is that the solar radiation hitting Earth's atmosphere, or solar irradiance, was as high in medieval time as in the 20th century. Therefore, the argument goes, 20th century global warming has been largely driven by the sun, not by higher concentrations of heat-trapping greenhouse gases in the atmosphere.

However, recent modeling studies show that increased solar irradiance does not warm Earth's surface at all locations, the research team wrote. Rather, ozone absorbs ultraviolet radiation, warming the stratosphere and altering atmospheric circulation patterns. If such changes happened in the 12th century, they could well have altered large-scale atmospheric circulation patterns linked to the Arctic Oscillation, thereby warming some regions but not others.

Frequent volcanic eruptions from 1100 to 1260 A.D. may have added to regional warming. Volcanic eruptions may have resulted in mild winters in northern and western Europe during High Medieval time, just as erupting volcanoes have led to very warm winters in northern Europe and northwestern Russia in the 20th century, they added.

As first articulated in 1965, the concept of a "Medieval Warm Period" pertained only to western Europe during the High Medieval time, the authors noted.

But scientists have since stretched the term to include any climatic anomaly that occurred after the fall of Rome in 476 A.D. to the dawn of the Renaissance, around 1500 A.D., even if the climate aberration had nothing to do with temperature.

"The ill-defined evidence for a range of climate anomalies occurring over a wide time interval has created the notion that the MWE (Medieval Warm Epoch) was a definitive global phenomenon," the team wrote.

The proxy records reveal other important components of medieval climate that need to be better understood. Bradley, Hughes and Diaz said there is evidence for unusually intense and prolonged droughts in several parts of the world, including the western United States, from 900 to 1300 A.D.

Understanding the underlying mechanisms of persistent droughts and wet periods in medieval time has to be a priority, they added: "With more than 10 times as many people on Earth as in High Medieval time, (a recurrence) could be catastrophic."