The Medieval Warm Period (MWP), also called the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warmer temperatures in the North Atlantic region. This period lasted from about 950 CE to about 1250 CE. Evidence from nature shows that the warmest times happened at different times in different places. This means the MWP wasn’t the same everywhere on Earth. Some people use the term "Medieval Climatic Anomaly" to show that other climate changes, not just temperature, were important.
After the MWP, some areas in the North Atlantic and other parts of the world became cooler. This time is sometimes called the Little Ice Age (LIA).
Possible reasons for the MWP include more sunlight from the Sun, fewer volcanic eruptions, and changes in ocean currents. Computer models suggest that natural changes alone could not explain the MWP. An outside cause, such as changes in the Sun or volcanic activity, likely played a role.
Research
The Medieval Warm Period (MWP) is generally believed to have happened between about 950 CE and 1250 CE, during the European Middle Ages. Some scientists split the MWP into two parts: MWP-I, which started around 450 CE and ended around 900 CE, and MWP-II, which lasted from about 1000 CE to 1300 CE. MWP-I is called the early Medieval Warm Period, while MWP-II is called the conventional Medieval Warm Period. In 1965, Hubert Lamb, one of the first scientists who study past climates, published research using data from botany, historical records, and meteorology. His work included information about temperature and rainfall in England around 1200 CE and 1600 CE. He proposed that the time of warmer temperatures became known as the Medieval Warm Period, and the time that followed, when temperatures dropped, became known as the Little Ice Age (LIA). However, some scientists later questioned whether the MWP was a global event. The IPCC First Assessment Report from 1990 discussed: It said that temperatures in some areas were slightly cooler than in the early and mid-20th century. The IPCC Third Assessment Report from 2001 summarized newer research: Global temperature records from ice cores, tree rings, and lake deposits showed that Earth may have been slightly cooler globally (by 0.03°C or 0.1°F) than in the early and mid-20th century. Scientists who study past climates often label the coldest time in their region-specific climate reconstructions as the "Little Ice Age" and the warmest time as the "Medieval Warm Period." Some scientists use these labels when they find significant climate events in the same timeframes. However, some "Medieval Warm Period" events in certain areas, like central Antarctica, were actually cold or wet events, not strictly warm events. This is because climate patterns in Antarctica are often the opposite of those in the North Atlantic. In 2021, Christian Pfister and Heinz Wanner used methods from historical climatology to reconstruct seasonal temperature conditions in Western and Central Europe. Their study used data from the years 1000 CE to 1999 (with autumn temperatures only recorded from 1500 CE onward).
Global climate during the Medieval Warm Period
The Medieval Warm Period (MWP) has caused a lot of debate about whether it was a global event or only happened in certain areas. In 2019, the Pages-2k group used a larger set of indirect evidence to show that the Medieval Climate Anomaly was not the same everywhere at the same time. The warmest 51-year period during the MWP occurred in different regions at different times. Researchers suggest focusing on regional climate changes instead of assuming the same pattern happened globally during the preindustrial Common Era. Also, peer-reviewed studies that account for more research in northern areas show that global temperatures stayed mostly the same for about 7,000 years before the industrial revolution began. These studies found no global Medieval Warm Period or Little Ice Age.
North Atlantic
Lloyd D. Keigwin’s 1996 study of radiocarbon-dated box core data from marine sediments in the Sargasso Sea found that the sea surface temperature was about 1 °C (1.8 °F) cooler around 400 years ago during the Little Ice Age (LIA) and 1700 years ago. It was also about 1 °C (1.8 °F) warmer 1000 years ago during the Medieval Warm Period (MWP).
Using sediment samples from Puerto Rico, the Gulf Coast, and the Atlantic Coast from Florida to New England, Mann et al. found consistent evidence of a peak in North Atlantic tropical cyclone activity during the MWP. This was followed by a period of reduced cyclone activity.
Iceland was first settled between about 865 and 930, during a time considered warm enough for sailing and farming. By studying marine cores and examining mollusc growth patterns from Iceland, Patterson et al. created a stable oxygen (δO) and carbon (δC) isotope record from the Roman Warm Period to the MWP and LIA, with data at a decadal resolution. They concluded that summer temperatures remained high, but winter temperatures decreased after Iceland was first settled.
The Mann et al. study found that temperatures in southern Greenland and parts of North America during the MWP (950–1250) were warmer than 1961–1990 levels. In some regions, temperatures exceeded those of the 1990–2010 period. Much of the Northern Hemisphere experienced significant cooling during the LIA (1400–1700), but Labrador and parts of the United States were as warm as during the 1961–1990 period. Greenlandic winter oxygen isotope data from the MWP show a strong link to the North Atlantic Oscillation (NAO).
The Norse colonization of the Americas is linked to warmer periods. The common theory is that Norse people used ice-free seas to colonize Greenland and other northern areas. However, a study from Columbia University suggests that Greenland was not colonized during a long warm period, but the warming lasted only briefly. Around 1000 CE, the climate was warm enough for Vikings to travel to Newfoundland and establish a short-lived settlement there.
Around 985, Vikings founded the Eastern and Western Settlements near the southern tip of Greenland. In the early years, they raised cattle, sheep, and goats, with about a quarter of their diet coming from seafood. When the climate became colder and stormier around 1250, their diet shifted more toward ocean sources. By 1300, seal hunting provided more than three-quarters of their food.
By 1350, demand for their exports declined, and trade with Europe ended. The last document from the settlements dates to 1412. Over the following decades, the remaining Europeans gradually left, likely due to economic reasons, such as more available farmland in Scandinavia.
Winters in Central Europe were consistently cold from 1000 AD to the late 19th century. Summer temperatures showed a 140-year-long warm period between 1170 and 1310, but there was a sharp cold period between 1195 and 1197. During the MWP, glaciers in southern Europe retreated significantly. While smaller glaciers completely disappeared, larger ones remained and now help scientists understand the region’s climate history. The Gorner Glacier in Switzerland advanced during cool, wet summers between 1140 and 1151, reaching its farthest point in 1168. Sedimentary records also show increased flooding in eastern Europe during the MWP, linked to higher rainfall from a positive phase of the NAO.
Other climate impacts, such as changes in landscapes, are less obvious. Before the MWP, a coastal area in western Sardinia was abandoned by the Romans. During the MWP, the coastline expanded into the lagoon due to higher sea levels. When humans returned, they had to rebuild ports because the land had changed. In the Iberian Central Range, lakes produced more life, soil eroded, and heavy runoff events occurred frequently.
Other regions
In Chesapeake Bay (now in Maryland and Virginia, United States), scientists discovered large temperature changes during the Medieval Warm Period (MWP) (about 950–1250) and the Little Ice Age (LIA) (about 1400–1700, with cold periods lasting into the early 20th century). These changes may be linked to shifts in the strength of ocean currents in the North Atlantic. Sediments in Piermont Marsh of the lower Hudson Valley show that the MWP was dry from 800 to 1300. In the Hammock River marsh in Connecticut, salt marshes stretched 15 kilometers (9.3 miles) farther west than they do today because sea levels were higher.
Prolonged droughts affected parts of what is now the Western United States, especially eastern California and the western Great Basin. Alaska had three periods of warmth: 1–300, 850–1200, and since 1800. Understanding the MWP in North America has helped scientists date when Native American groups lived in certain areas, especially in dry regions of the Western United States. Drier conditions were more common in the southeastern United States during the MWP than during the LIA, but the difference may not be significant. Droughts during the MWP may have affected Native American settlements in the Eastern United States, such as Cahokia. Recent archaeological research shows that some early patterns, like violence and health problems, are more complex and vary by region. Other patterns, such as disrupted settlements, reduced long-distance trade, and population movements, are supported by more evidence.
In equatorial eastern Africa, the climate has alternated between being drier and wetter than today. The MWP (1000–1270) was drier. Off the coast of Africa, analysis of bones from the Canary Islands’ inhabitants during the MWP to LIA transition shows a 5-degree Celsius (9-degree Fahrenheit) drop in air temperature. The diet of people in the region did not change much, suggesting they adapted well to climate changes.
The MWP in the Southern Ocean began about 150 years after it started in the North Atlantic. A sediment core from the eastern Bransfield Basin in the Antarctic Peninsula records events from both the LIA and the MWP. Some parts of Antarctica were unusually cold, while others were unusually warm between 1000 and 1200.
Corals in the tropical Pacific suggest that cool and dry conditions may have lasted early in the millennium, matching a La Niña-like pattern in the El Niño-Southern Oscillation.
A 2013 study from three U.S. universities published in Science found that Pacific Ocean water was 0.9 degrees Celsius (1.6 degrees Fahrenheit) warmer during the MWP than during the LIA and 0.65 degrees Celsius (1.2 degrees Fahrenheit) warmer than the decades before the study. However, in the northeastern Pacific, sea surface temperatures were actually cooler during the MWP than during the LIA.
The MWP is recorded in a 1500-year sediment core from a lake in Chile and in the Eastern Cordillera of Ecuador.
A reconstruction using ice cores shows the MWP in tropical South America from about 1050 to 1300, followed by the LIA in the 15th century. Peak temperatures during the MWP did not reach the levels of the late 20th century, which were the highest in the region over a 1600-year study period.
A study by Ge et al. found that temperatures in China over the past 2000 years are uncertain before the 16th century but show consistency after that. Two cold periods (1620s–1710s and 1800s–1860s) and 20th-century warming are clearly marked. Warming from the 10th to 14th centuries in some regions may be similar in magnitude to recent 20th-century warming, which was unprecedented in the past 500 years. A warming period in China during the MWP is supported by data, but the changes varied across the country. Significant temperature changes from the MWP to the LIA were found in northeast and central-east China but not in northwest China or the Tibetan Plateau. During the MWP, the East Asian Summer Monsoon (EASM) was the strongest in the past millennium and was highly influenced by the El Niño-Southern Oscillation (ENSO). The Mu Us Desert had more moisture during the MWP. Peat cores in southeast China suggest that changes in the EASM and ENSO caused more rainfall in the region during the MWP. However, some southern Chinese sites show drier conditions during the MWP, showing that the MWP’s effects were uneven. Modeling suggests the EASM was weaker in early summer but stronger in late summer during the MWP.
In far eastern Russia, continental areas had severe floods during the MWP, while nearby islands had less rain, reducing peatland. Pollen data from the region show more warm-weather plants and fewer coniferous forests.
Adhikari and Kumon (2001) studied sediments in Lake Nakatsuna, central Japan, and found a warm period from 900 to 1200 that matches the MWP, along with three cool phases, two of which may relate to the LIA. Other research in northeastern Japan shows one warm and humid interval (750–1200) and two cold and dry intervals (1–750 and 1200 to now).
The Indian Summer Monsoon (ISM) was stronger during the MWP, bringing more rain to India due to changes in the Atlantic Multi-decadal Oscillation (AMO). Vegetation records in Lahaul, Himachal Pradesh, confirm a warm and humid MWP from 1158 to 647 BP. Pollen from Madhya Pradesh dated to the MWP shows increased monsoonal rainfall. Multi-source data from Pookode Lake in Kerala also reflect the MWP’s warmth.
Sea surface temperatures in the Arabian Sea rose during the MWP due to a strong monsoon. The Arabian Sea had higher biological productivity during this time. The Arabian Peninsula, already extremely dry today, was even drier during the MWP. Prolonged drought was common in the Arabian