Medieval Warm Period (MWP)

Definition

The Medieval Warm Period (MWP) is a term used to describe a period of several centuries that preceded the Little Ice Age (LIA). The MWP (also the Medieval Warm Epoch or the Little Climatic Optimum) was proposed in 1965 by Hubert Horace Lamb, a British meteorologist and groundbreaking climate historian, who believed it lasted from roughly 900 to 1300 C.E. During the MWP, Lamb believed, the North Atlantic and northern and western Europe experienced warmer conditions on average. He presented evidence drawn primarily from historical documentary data such as the expansion of agriculture to higher-altitude fields in mountainous regions, shifts in the cultivation of certain crops (such as wine production in the British Isles), changes in tree lines, and reports of weather and weather-related events (such as floods and droughts) in historical writings. This period also coincided with the Viking settlement of Greenland and excursions to Labrador, as well as a population boom in Europe, which grew from roughly 35 million to 80 million people between 1000 and 1347 C.E. All of these events seemed consistent with a milder climate and improved agricultural production.

In the decades following Lamb’s assertion, temperature proxies were examined to better define the MWP, the transition to the LIA, and the extent to which they were regional or global phenomena. For example, measurements of oxygen isotope ratios in marine sediments from a Sargasso Sea core indicated an ocean temperature around 1100 that was about 1° Celsius warmer than present levels. This was followed by a nearly 2° Celsius decrease between 1100 and 1700.

Dendroclimatology, or the study of climate through tree-ring growth, conducted in the Sierra Nevada and the Great Basin of the western United States suggests periods of increased warmth and later periods of severe drought during the MWP. Similar tree-ring studies in northern Sweden and the Polar Ural Mountains provide evidence of increased temperatures from 971 to 1100 and from 1110 to 1350, respectively. Ice core and borehole studies in Greenland indicate a warm period peaking around 1000, followed by a 3° Celsius decrease into the LIA. Studies of glaciers and their moraines suggest that the MWP generally coincided with glacial retreat, and the shift to the LIA with glacial advance. Other types of proxy studies include examining lake sediments, speleotherm (stalactite/stalagmite) growth, and coral growth. Many, but not all, proxy studies of sufficient length do indicate a temperature peak during the MWP and a decrease into the LIA, but with considerable variability in the details.

Significance for Climate Change

The MWP is a central issue in the debate over climate change, as it is the most recent period of climatic warmth assumed to be free of factors. Likewise, the transition to the LIA is the most recent significant temperature shift before the warming of the twentieth century. The initial estimates by Lamb suggest MWP temperatures were about 0.5° Celsius warmer than those of the late twentieth century. This peak subsequently decreased, as more Northern Hemisphere data sets were combined. While many proxy data sets showed a temperature peak in the MWP, the heterogeneity of the peak locations, magnitudes, and durations meant that averaged temperature changes were reduced from those seen at individual sites.

Even with multidecadal filtering to extract long-term trends during the MWP from year-to-year fluctuations, interludes of cold have appeared in periods of relative warmth. The 2001 report of the Intergovernmental Panel on Climate Change (IPCC) largely dismissed the existence of a distinct, global MWP and LIA, epitomized by the famous hockey stick graph that showed a small, nearly linear decrease in average temperature from 1000 to the late nineteenth century with no distinct MWP to LIA transition. After some controversy, the next IPCC report in 2007 revived the MWP and LIA, using eight proxy-based temperature reconstructions to define a mild Northern Hemisphere temperature peak between 950 and 1100 that was 0.1°–0.2° Celsius cooler than the mean global temperature between 1961 and 1990 and 0.3°–0.4° Celsius warmer than the coolest LIA period.

Attempts to measure the MWP illustrate the challenges of using temperature rather than instrumental temperature readings. Proxies are influenced by effects other than temperature—for example, growth patterns in tree-ring studies can reflect precipitation, diseases, and atmospheric carbon dioxide concentration. Studies that combine multiple proxy data sets must accurately calibrate their results against a common temperature standard, but different researchers use different approaches. Only a limited number of proxy sites extend back to the MWP, with few in the Southern Hemisphere and unbalanced coverage in the Northern. Overall, there is considerable potential error in each reconstruction, such that current views of the MWP are tentative and will likely change with future data.

Interest in the cause of the MWP has been muted by the debate over its nature. If the event was only regional, then changes in regional meteorological features such as the North Atlantic Oscillation might provide a sufficient explanation. If the MWP was more global, solar activity may account for it. Sunspot records do not exist for the MWP, but concentrations of the cosmogenic carbon 14 and beryllium 10 in and suggest that there was stronger than normal solar radiation around 1000 and again between 1100 and 1250. These isotope concentrations are also consistent with evidence of weaker radiation during much of the LIA.

Bibliography

Fagan, Brian. The Great Warming: Climatic Change and the Rise and Fall of Civilizations. New York: Bloomsbury Press, 2008.

"Global Temperature Has Risen, Not Fallen, Since Medieval Period." Reuters, 25 July 2024, www.reuters.com/fact-check/global-temperature-has-risen-not-fallen-since-medieval-period-2024-07-25/. Accessed 21 Dec. 2024.

Hogan, C. Michael. "Medieval Warm Period." Encyclopedia of Earth. Boston U, 1 Jan. 2013. Web. 23 Mar. 2015.

Hughes, Malcolm, and Henry F. Diaz, eds. The Medieval Warm Period. Berlin: Springer-Verlag, 1994.

Kaufman, Darrell S., et al. "Continental-Scale Temperature Variability during the Past Two Millennia." Nature Geoscience 6 (2013): 339–46. PDF file.

Lamb, Hubert H. Climate, History, and the Modern World. 2d ed. London: Routledge, 1995.

Singer, S. Fred, and Dennis T. Avery. Unstoppable Global Warming: Every Fifteen Hundred Years. Rev. ed. Blue Ridge Summit, Pa.: Rowman & Littlefield, 2008.