Medieval Warm Period

• 1. A Climate in Question

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  The Medieval Warm Period, known also as the Medieval Climatic Anomaly or the Medieval Climate Optimum, left traces on the floor of the Sargasso Sea. In 1996, Lloyd D. Keigwin recovered radiocarbon-dated sediment cores from those depths. The analysis found sea surface temperatures there had been roughly 1 degree Celsius warmer about 1,000 years ago. Conventionally dated from about 950 to about 1250, this era of elevated warmth was centered on the North Atlantic. A cooler interval, the Little Ice Age, followed it.

  A single measurement from one patch of ocean cannot settle what the Medieval Warm Period actually was. Was this warming global, or did it unfold differently in each region of the planet? What forces drove temperatures higher across the North Atlantic? And why, after decades of research, does the period remain one of the most studied and debated intervals in climate history?

• 2. Naming the Warm Period

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  In 1965, Hubert Lamb published research that effectively gave the warm period its name. One of the first paleoclimatologists, he combined data from botany, historical documents, and meteorology. His records covered temperature and rainfall in England. Lamb argued that evidence from many disciplines pointed to elevated temperatures across numerous regions. That warmth had persisted for several centuries before giving way to a prolonged decline. He called the warm era the Medieval Warm Period and the cold era that followed the Little Ice Age.

  The IPCC First Assessment Report of 1990 treated those labels with caution. It acknowledged the period while noting it "may not have been global." Temperatures in the late tenth to early thirteenth centuries, it stated, appeared "exceptionally warm in western Europe, Iceland and Greenland." That was a regional picture, not a universal one.

  The IPCC Third Assessment Report of 2001 went further. It concluded that "evidence does not support globally synchronous periods of anomalous cold or warmth over this time frame." The terms Medieval Warm Period and Little Ice Age, it argued, are chiefly useful for describing Northern Hemisphere temperature trends.

  Global temperature records from ice cores, tree rings, and lake deposits added a precise number. They suggested the Earth may have been slightly cooler globally during the MWP, by about 0.03 degrees Celsius, than in the early to mid-20th century. In 2019, the Pages-2k consortium used an extended set of climate proxy records to revisit the question. They confirmed that the Medieval Climate Anomaly was not a globally synchronous event. The warmest 51-year period within the MWP did not occur at the same time across different regions.

  Three candidate drivers have been proposed: increased solar activity, decreased volcanic eruptions, and changes in ocean circulation. Modelling evidence has established that natural variability alone was not sufficient to explain the warming. An external forcing had to be involved.

  Peer-reviewed research correcting for the geographic bias toward northern study areas found global temperatures essentially stable for 7,000 years before the industrial revolution.

• 3. The Norse Experiment

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  Around 985, Vikings established the Eastern and Western Settlements near the southern tip of Greenland. In those early years, the colonists kept cattle, sheep, and goats, with seafood making up roughly a quarter of their diet. The warm climate that had made those northern seas navigable also sustained a measure of agriculture.

  Iceland had been reached first, with settlers arriving between about 865 and 930. That period was believed warm enough for both sailing and farming. Researchers have since assembled detailed isotope records from Icelandic marine cores and mollusc growth patterns. Those records span from the Roman Warm Period through the MWP and into the Little Ice Age. They show that summer temperatures stayed elevated after the initial settlement, while winter temperatures fell.

  Around 1250, the climate around Greenland's settlements became colder and stormier. The settlers' diet shifted in step with the weather. By around 1300, seal hunting provided more than three quarters of their food.

  By 1350, demand for the settlements' exports had fallen and trade with Europe had effectively stopped. The last surviving document from the Greenlandic settlements is dated to 1412. Scholars attribute the Europeans' gradual withdrawal mainly to economic forces, including the increasing availability of farmland in Scandinavian countries.

  A study from Columbia University has suggested that the warming that made Greenland accessible lasted only very briefly. Mann et al. found that warmth in southern Greenland during the MWP exceeded temperature levels recorded for the 1961-1990 period. In some parts of the region, warmth during the MWP surpassed even the temperatures of the 1990-2010 period.

• 4. Europe Amid the Warmth

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  Central European winters were almost consistently cold between 1000 and the late 19th century. Against that steady backdrop, the summer temperature record stands apart. Between 1170 and 1310, the region experienced an almost continuous 140-year sequence of warm seasons. A drastic cold setback between 1195 and 1197 interrupted the sequence but did not end it.

  Southern European glaciers shrank considerably during the MWP, with several smaller ones disappearing entirely. Larger glaciers survived and now serve as direct archives of the era's conditions. The Gorner glacier in Switzerland, the second largest in the Alps, moved in the opposite direction. It advanced during cool and wet summers between 1140 and 1151, reaching its furthest extent in 1168.

  Eastern Europe experienced a period of increased flooding during the MWP. Sedimentary records attribute this to enhanced precipitation from a positive phase of the North Atlantic Oscillation.

  In western Sardinia, a coastal region that the Romans had occupied was abandoned before the MWP began. Without human settlement, the coastal zone expanded substantially into the surrounding lagoon during the warm period. When people returned, they found a landscape reshaped by climate and had to rebuild their ports from scratch. In the Iberian Central Range, the MWP brought elevated lake productivity, soil erosion, and frequent intense runoff events.

  Christian Pfister and Heinz Wanner reconstructed seasonal temperatures for Western and Central Europe in 2021. Their dataset drew on climate indices spanning from the year 1000 to 1999, with autumn temperatures traceable from 1500 onwards.

• 5. A Wider World

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  Chesapeake Bay, now divided between Maryland and Virginia, recorded large temperature excursions during the MWP, from roughly 950 to 1250. These swings may be connected to shifts in deep-ocean circulation in the North Atlantic. In Connecticut's Hammock River marsh, salt marshes extended 15 kilometres farther west than they do today, pushed there by higher sea levels. Sediments in Piermont Marsh of the lower Hudson Valley record a dry period from 800 to 1300. Prolonged droughts struck much of the western United States during the same era, particularly in eastern California and the west of the Great Basin. Droughts may also have reached Cahokia in the Eastern United States. Alaska experienced three intervals of comparable warmth: from about 1 to 300, from 850 to 1200, and from 1800 to the present.

  In central Japan, Adhikari and Kumon's 2001 study of sediments in Lake Nakatsuna identified a warm period from 900 to 1200. Northeastern Japan recorded a warm and humid interval from 750 to 1200, flanked by cold and dry periods on both sides. Ge et al. studied 2,000 years of Chinese temperature records and found high uncertainty before the 16th century. In some Chinese regions, warming from the 10th to the 14th century may have matched the magnitude of late 20th century warming. The East Asian Summer Monsoon was at its strongest point in the past millennium during the MWP. It was highly sensitive to the El Nino-Southern Oscillation. The Mu Us Desert saw increased moisture, while sites in southern China show aridification rather than wetting during the same period.

  In Lahaul, Himachal Pradesh, vegetation records confirm a warm and humid MWP from 1,158 to 647 years before the present. Pollen from Madhya Pradesh dated to the MWP provides direct evidence of increased monsoonal precipitation. The Indian Summer Monsoon strengthened during the MWP through temperature-driven changes in the Atlantic Multi-decadal Oscillation. In the Arabian Sea, surface temperatures rose and biological productivity increased, both driven by a strengthened monsoon. The Arabian Peninsula, already extremely arid in the present day, was even drier during the MWP. A hyperarid interval there persisted until around 660 years before the present.

  Equatorial eastern Africa was drier than today between 1000 and 1270. Isotopic analysis of bones from the Canary Islands documented a 5-degree Celsius drop in air temperature during the MWP-to-LIA transition. Despite that swing, the diet of the islands' inhabitants did not change appreciably.

  A 2013 study in Science found Pacific Ocean water temperatures were 0.9 degrees Celsius warmer during the MWP than during the Little Ice Age. Compared with baseline temperatures in the decades before the study, the difference was 0.65 degrees Celsius. In the northeastern Pacific, sea surface temperatures actually ran cooler during the MWP than the LIA. The MWP arrived in the Southern Ocean roughly 150 years after its onset in the North Atlantic.

  Tropical South America shows traces of the MWP from about 1050 to 1300, reconstructed from ice cores. Within the study's 1,600-year window, peak MWP temperatures there did not reach the levels recorded in the late 20th century.