Period of warm climate in North Atlantic region lasting from about 950 CE to about 1250
Global average temperatures show that the Medieval Warm Period was not a global phenomenon.[1]
TheMedieval Warm Period (MWP), also known as theMedieval Climate Optimum or theMedieval Climatic Anomaly, was a time of warmclimate in theNorth Atlantic region that lasted from about 950 CE to about 1250 CE.[2]Climate proxy records show peak warmth occurred at different times for different regions, which indicate that the MWP was not a globally uniform event.[3] Some refer to the MWP as theMedieval Climatic Anomaly to emphasize that climatic effects other than temperature were also important.[4][5]
The MWP was followed by a regionally cooler period in the North Atlantic and elsewhere, which is sometimes called theLittle Ice Age (LIA).
Possible causes of the MWP include increased solar activity, decreased volcanic activity, and changes in ocean circulation.[6] Modelling evidence has shown that natural variability is insufficient on its own to explain the MWP and that an external forcing had to be one of the causes.[7]
The MWP is generally thought to have occurred from about 950 CE to about 1250 CE, during the EuropeanMiddle Ages.[2] Some researchers divide the MWP into two phases: MWP-I, which began around 450 CE and ended around 900 CE, and MWP-II, which lasted from about 1000 CE to about 1300 CE; MWP-I is called the early Medieval Warm Period while MWP-II is called the conventional Medieval Warm Period.[8] In 1965,Hubert Lamb, one of the firstpaleoclimatologists, published research based on data frombotany, historical document research, and meteorology, combined with records indicating prevailing temperature and rainfall in England around 1200 CE and around 1600 CE. He proposed,[9]
evidence has been accumulating in many fields of investigation pointing to a notably warm climate in many parts of the world, that lasted a few centuries around 1000–1200 CE, and was followed by a decline of temperature levels till between around 1500–1700 CE the coldest phase since the last ice age occurred.
The era of warmer temperatures became known as the Medieval Warm Period and the subsequent cold period theLittle Ice Age (LIA). However, the view that the MWP was a global event was challenged by other researchers. TheIPCC First Assessment Report of 1990 discussed the:[10]
Medieval Warm Period around 1000 CE (which may not have been global) and the Little Ice Age which ended only in the middle to late nineteenth century.
evidence does not support globally synchronous periods of anomalous cold or warmth over this time frame, and the conventional terms of 'Little Ice Age' and 'Medieval Warm Period' are chiefly documented in describing northern hemisphere trends in hemispheric or global mean temperature changes in past centuries.
Globaltemperature records taken from ice cores,tree rings, and lake deposits have shown that the Earth may have been slightly cooler globally (by 0.03 °C or 0.1 °F) than in the early and the mid-20th century.[12][13]
Palaeoclimatologists developing region-specific climate reconstructions of past centuries conventionally label their coldest interval as "LIA" and their warmest interval as the "MWP".[12][14] Others follow the convention, and when a significant climate event is found in the "LIA" or "MWP" timeframes, they associate their events to the period. Some "MWP" events are thus wet events or cold events, rather than strictly warm events, particularly in centralAntarctica, where climate patterns that are opposite to those of the North Atlantic have been noticed.
Using methods of historical climatology,Christian Pfister andHeinz Wanner reconstructed the seasonal temperature conditions for Western and Central Europe in 2021 on the basis of indices from CE 1000 to 1999 (the autumns only from 1500 onwards).[15][16]
The nature and extent of the MWP has been marked by long-standing controversy over whether it was a global or regional event.[17][18] In 2019, by using an extended proxy data set,[19] the Pages-2k consortium 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 in different regions. They argue for a regional instead of global framing ofclimate variability in the preindustrialCommon Era to aid in understanding.[20]
Greenland ice sheet temperatures interpreted with 18O isotope from 6 ice cores (Vinther, B., et al., 2009).[citation needed] The data set ranges from 9690 BCE to 1970 CE and has a resolution of around 20 years. That means that each data point represents the average temperature of the surrounding 20 years.
Lloyd D. Keigwin's 1996 study ofradiocarbon-datedbox core data from marine sediments in theSargasso Sea found that itssea surface temperature was approximately 1 °C (1.8 °F) cooler approximately 400 years ago, during theLIA, and 1700 years ago, and was approximately 1 °C (1.8 °F) warmer 1000 years ago, during the MWP.[21]
Iceland was first settled between about 865 and 930, during a time believed to be warm enough for sailing and farming.[23][24] By retrieval and isotope analysis of marine cores and from examination of mollusc growth patterns fromIceland, Pattersonet al. reconstructed a stable oxygen (δ18 O) and carbon (δ13 C) isotope record at a decadal resolution from theRoman Warm Period to the MWP and theLIA.[25] Pattersonet al. conclude that the summer temperature stayed high but winter temperature decreased after the initial settlement of Iceland.[25]
The last written records of theNorse Greenlanders are from an Icelandic marriage in 1408 but were recorded later in Iceland, atHvalsey Church, which is now the best-preserved of the Norse ruins.
The Mannet al. study found warmth exceeding 1961–1990 levels in southernGreenland and parts of North America during the MWP, which the study defines as from 950 to 1250, with warmth in some regions exceeding temperatures of the 1990–2010 period.[22] Much of the Northern Hemisphere showed a significant cooling during theLIA, which the study defines as from 1400 to 1700, butLabrador and isolated parts of theUnited States appeared to be approximately as warm as during the 1961–1990 period.[2] Greenlandic winter oxygen isotope data from the MWP display a strong correlation with theNorth Atlantic Oscillation (NAO).[26]
1690 copy of the 1570Skálholt map, based on documentary information about earlier Norse sites in America.
TheNorse colonization of the Americas has been associated with warmer periods.[27] The common theory is thatNorsemen took advantage of ice-free seas to colonize areas in Greenland and other outlying lands of the far north.[28] However, a study fromColumbia University suggests that Greenland was not colonized in warmer weather, but the warming effect in fact lasted for only very briefly.[29] Around 1000 CE the climate was sufficiently warm for the Vikings to journey toNewfoundland and to establish a short-lived outpost there.[30]
Around 985, Vikings founded theEastern andWestern Settlements, both near the southern tip of Greenland. In the colony's early stages, they kept cattle, sheep, and goats, with around a quarter of their diet from seafood. After the climate became colder and stormier around 1250, their diet steadily shifted towards ocean sources. By around 1300,seal hunting provided over three quarters of their food.
By 1350, there was reduced demand for their exports, and trade with Europe fell away. The last document from the settlements dates from 1412, and over the following decades, the remaining Europeans left in what seems to have been a gradual withdrawal, which was caused mainly by economic factors such as increased availability of farms in Scandinavian countries.[31]
Substantial glacial retreat in southern Europe was experienced during the MWP. While several smaller glaciers experienced complete deglaciation, larger glaciers in the region survived and now provide insight into the region's climate history.[32] In addition to warming induced glacial melt, sedimentary records reveal a period of increased flooding, coinciding with the MWP, in eastern Europe that is attributed to enhanced precipitation from a positive phase NAO.[33] Other impacts ofclimate change can be less apparent such as a changing landscape. Preceding the MWP, a coastal region in westernSardinia was abandoned by the Romans. The coastal area was able to substantially expand into the lagoon without the influence of human populations and ahigh stand during the MWP. When human populations returned to the region, they encountered a land altered by climate change and had to reestablish ports.[34] In the Iberian Central Range, there was elevated lake productivity and soil erosion, along with frequent intense runoff events.[35]
InChesapeake Bay (now inMaryland andVirginia,United States), researchers found large temperature excursions (changes from the mean temperature of that time) during the MWP (about 950–1250) and theLittle Ice Age (about 1400–1700, with cold periods persisting into the early 20th century), which are possibly related to changes in the strength of North Atlanticthermohaline circulation.[36] Sediments inPiermont Marsh of the lowerHudson Valley show a dry MWP from 800 to 1300.[37] In the Hammock River marsh inConnecticut, salt marshes extended 15 kilometres (9.3 mi) farther westward than they do in the present due to higher sea levels.[38]
Prolonged droughts affected many parts of what is now theWestern United States, especially easternCalifornia and the west ofGreat Basin.[12][39]Alaska experienced three intervals of comparable warmth: 1–300, 850–1200, and since 1800.[40] Knowledge of the MWP in North America has been useful in dating occupancy periods of certain Native American habitation sites, especially in arid parts of the Western United States.[41][42] Aridity was more prevalent in the southeastern United States during the MWP than the following LIA, but only slightly; this difference may be statistically insignificant.[43] Droughts in the MWP may have impacted Native American settlements also in theEastern United States, such as atCahokia.[44][45] Review of more recent archaeological research shows that as the search for signs of unusual cultural changes has broadened, some of the early patterns (such as violence and health problems) have been found to be more complicated and regionally varied than had been previously thought. Other patterns, such as settlement disruption, deterioration of long-distance trade, and population movements, have been further corroborated.[46]
The climate in equatorial easternAfrica has alternated between being drier than today and relatively wet. The climate was drier during the MWP (1000–1270).[47] Off the coast of Africa,Isotopic analysis of bones from theCanary Islands' inhabitants during the MWP to LIA transition reveal the region experienced a 5 °C (9.0 °F) decrease in air temperature. Over this period, the diet of inhabitants did not appreciably change, which suggests they were remarkably resilient toclimate change.[48]
The onset of the MWP in theSouthern Ocean lagged the MWP's onset in the North Atlantic by approximately 150 years.[49] Asediment core from the eastern Bransfield Basin, in theAntarctic Peninsula, preserves climatic events from both theLIA and the MWP. The authors noted, "The late Holocene records clearly identify Neoglacial events of theLIA and Medieval Warm Period (MWP)."[50] Some Antarctic regions were atypically cold, but others were atypically warm between 1000 and 1200.[51]
Corals in the tropicalPacific Ocean suggest that relatively cool and dry conditions may have persisted early in the millennium, which is consistent with aLa Niña-like configuration of theEl Niño-Southern Oscillation patterns.[52]
In 2013, a study from three US universities was published inScience magazine and showed that the water temperature in the Pacific Ocean was 0.9 °C (1.6 °F) warmer during the MWP than during theLIA and 0.65 °C (1.2 °F) warmer than the decades before the study.[53]
A reconstruction, based on ice cores, found that the MWP could be distinguished in tropical South America from about 1050 to 1300 and was followed in the 15th century by theLIA. Peak temperatures did not rise as to the level of the late 20th century, which were unprecedented in the area during the study period of 1600 years.[56]
Geet al. studied temperatures inChina for the past 2000 years and found high uncertainty prior to the 16th century but good consistency over the last 500 years highlighted by the two cold periods, 1620s–1710s and 1800s–1860s, and the 20th-century warming. They also found that the warming from the 10th to the 14th centuries in some regions might be comparable in magnitude to the warming of the last few decades of the 20th century, which was unprecedented within the past 500 years.[57] Generally, a warming period was identified in China, coinciding with the MWP, using multi-proxy data for temperature. However, the warming was inconsistent across China. Significant temperature change, from the MWP toLIA, was found for northeast and central-east China but not for northwest China and theTibetan Plateau.[58] During the MWP, theEast Asian Summer Monsoon (EASM) was the strongest it has been in the past millennium[59] and was highly sensitive to theEl Niño Southern Oscillation (ENSO).[60] TheMu Us Desert witness increased moisture in the MWP.[61]Peat cores from peatland in southeast China suggest changes in the EASM and ENSO are responsible for increased precipitation in the region during the MWP.[62] However, other sites in southern China show aridification and not humidification during the MWP, showing that the MWP's influence was highly spatially heterogeneous.[63] Modelling evidence suggests that EASM strength during the MWP was low in early summer but very high during late summer.[64]
In far easternRussia, continental regions experienced severe floods during the MWP while nearby islands experienced less precipitation leading to a decrease in peatland. Pollen data from this region indicates an expansion of warm climate vegetation with an increasing number ofbroadleaf and decreasing number ofconiferous forests.[65]
Adhikari and Kumon (2001), investigating sediments inLake Nakatsuna, in centralJapan, found a warm period from 900 to 1200 that corresponded to the MWP and three cool phases, two of which could be related to theLIA.[66] Other research in northeastern Japan showed that there was one warm and humid interval, from 750 to 1200, and two cold and dry intervals, from 1 to 750 and from 1200 to now.[67]
Sea surface temperatures in the Arabian Sea increased during the MWP, owing to a strong monsoon.[73] During the MWP, theArabian Sea exhibited heightened biological productivity.[74] TheArabian Peninsula, already extremely arid in the present day, was even drier during the MWP. Prolonged drought was a mainstay of the Arabian climate until around 660 BP, when this hyperarid interval was terminated.[75]
There is an extreme scarcity of data fromAustralia for both the MWP and theLIA. However, evidence from wave-built shingle terraces for a permanently-fullLake Eyre[76] during the 9th and the 10th centuries is consistent with aLa Niña-like configuration, but the data are insufficient to show how lake levels varied from year to year or what climatic conditions elsewhere in Australia were like.
Temperatures derived from an18O/16O profile through a stalagmite found in aNew Zealand cave (40°40′S172°26′E / 40.67°S 172.43°E /-40.67; 172.43) suggested the Medieval Warm Period to have occurred between [... about 1050–1400 CE] and to have been 0.75 °C [1.4 °F] warmer than the Current Warm Period.
More evidence inNew Zealand is from an 1100-year tree-ring record.[78]
^Hawkins, Ed (January 30, 2020)."2019 years".Climate Lab Book.Archived from the original on February 2, 2020.The data show that the modern period is very different to what occurred in the past. The often quoted Medieval Warm Period and Little Ice Age are real phenomena, but small compared to the recent changes.
^Fletcher, M-S.; Moreno, P.I. (July 16, 2012). "Vegetation, climate and fire regime changes in the Andean region of southern Chile (38°S) covaried with centennial-scale climate anomalies in the tropical Pacific over the last 1500 years".Quaternary Science Reviews.46:46–56.Bibcode:2012QSRv...46...46F.doi:10.1016/j.quascirev.2012.04.016.hdl:10533/131338.
^Allen, Robert J. (1985). The Australasian Summer Monsoon, Teleconnections, and Flooding in the Lake Eyre Basin (Report). Royal Geographical Society of Australasia, S.A. Branch. p. 43.ISBN978-0-909112-09-7.
