Summary

The Medieval Climate Anomaly (MCA), especially with regard to temperatures also the Medieval Warm Period (MWP) or the Medieval Climate Optimum, was an interval of comparatively warm climate and other climate deviations, such as extensive continental droughts. A MWP can be determined only vaguely regionally and temporally; according to most reconstructions, it probably began after 900 and ended before 1400. The warmest period in the northern hemisphere was therefore between 950 and 1250.

It is highly probable that there were some regions during the medieval warm period that were about as warm then as they were toward the middle, and in some cases the end, of the last century. However, the warm periods of the Middle Ages were uneven in time and region, unlike the simultaneous warming that has been occurring worldwide since the 20th century. In the 21st century, the Earth has continued to warm. Mean temperatures over the past thirty years are now probably higher than those of any period of the same length in the Middle Ages. Also, the rate of global warming is greater than at any time in at least 2000 years, probably without a comparable example in recent Earth history.

Notes and initial systematic work for the North Atlantic region.

As early as the 18th century, anecdotal evidence was used to discuss whether temporarily higher temperatures might have prevailed in various regions of the North Atlantic region during the Middle Ages. The Danish missionary Hans Poulsen Egede, who in 1721 searched in vain for inhabited medieval Viking settlements in Greenland, of which nothing had been heard for 200 years, considered climate as a possible cause of their disappearance:

In 1824, the Swedish diplomat Fredrik von Ehrenheim explained the end of Viking settlements as a cooling from a high point in the 11th century to a low point in the 15th century. Bernhard Studer, 1847, François Arago, 1858, and others interpreted the end of Greenlandic settlements in the 15th century as evidence of a cooling of a previously warmer region, while Conrad Maurer rejected this view and saw the reason in the advance of Inuit. Poul Nørlund, who investigated Grænlendingar graves at Herjólfsnes, in southwestern Greenland, found abundant plant roots in shrouds under the permafrost and concluded that summer temperatures had temporarily thawed the ground and had therefore been higher there than around 1921. Changes in tree boundaries were interpreted partly as an indication of climate change, partly as caused by human intervention. Eduard Brückner pointed out in 1895 that earlier viticulture in areas such as northern Germany, where none took place around 1900, had been influenced not only by climatic but also by economic conditions: "It was more advantageous, because of the expensive freight, to accept bad harvests than to import wine from the south.

The systematic investigation of a possible medieval climate anomaly - especially in the European region - was initially primarily the field of historical climatology. For medieval Europe, long before the beginning of instrumental measurements, conclusions about climatic conditions and their consequences could be drawn from historical documents and archaeological finds, even before paleoclimatology increasingly provided high-quality reconstructions from natural climate archives since the 1990s. Thus, there are reasonably complete historical accounts of summer and winter weather for the period beginning around 1300. It was the pioneering work in this field, for example by the English climatologist Hubert Lamb or the French historian Emmanuel Le Roy Ladurie, that provided the first comprehensive overviews of higher temperatures and social contexts for the North Atlantic region and here especially Europe.

The term Medieval Warm Period was primarily coined by Lamb in the 1960s and later adopted by other fields of research. Lamb used this term to describe a warming of the climate, which he estimated regionally at up to 1 to 2 °C and whose peak he assumed to have occurred between the years 1000 and 1300. Lamb found indications of such a warming mainly around the North Atlantic, while there were indications of relatively low temperatures for the North Pacific at about the same time. As a cause he assumed displacements of the Arctic polar vortex.

Occasionally, a medieval warm period has also been defined in terms of the extent of glaciers. In this view, the MWP was characterized by a widespread glacier retreat between ca. 900 and 1300, which was assumed at that time.

A global warming anomaly?

Scott Stine published paleoclimatological analyses in 1994 showing that there had been several centuries of extreme drought in the Sierra Nevada of California and Patagonia from about 900 to 1350. Stine suggested that hydrologic anomalies may have been even more significant than temperature anomalies during the Middle Ages. To include such hydrologic anomalies, he proposed the more general term medieval climate anomaly for the time interval.

At about the same time, Hughes and Diaz (1994) concluded in a review paper that clear evidence for a uniform hemispheric or global thermal anomaly did not yet exist. At that time, high-resolution proxy data that could have provided large-scale information on temperature trends prior to 1500 were sparse. Such proxy data only became increasingly available for other regions from the mid-1990s onwards, so that by 2011 numerous reconstructions were possible for the middle and high latitudes, while the tropics and the southern hemisphere are still only covered by relatively few data series. The Intergovernmental Panel on Climate Change also concluded in its 2001 summary of the state of research that there was no clear evidence of globally simultaneous periods of unusual cold or warmth at that time.

Questions about the causes, uniqueness, and potential consequences of current global warming focused some attention on a possible medieval climate anomaly as a point of comparison. The social indications and consequences of a medieval warming anomaly in the North Atlantic region have been taken up in popular science accounts. With the advent of the media and political controversy over global warming, climate change deniers argued, primarily on the basis of Lamb's account of the Medieval Warm Period, that the temperatures of recent decades were still within the natural range of climate variability and therefore could not be considered evidence that the observed warming was a result of increased greenhouse gas concentrations. The existence and extent of a supraregional medieval warm period were also controversially discussed in the scientific community at the beginning of the 21st century.

Term and research since 2010

Further reconstructions, for example from the Pages 2k project, with increasingly better regional coverage now allow a clearer classification of at least northern hemispheric temperatures. In 2013, the Intergovernmental Panel on Climate Change's Fifth Assessment Report concluded that there were regionally and temporally inconsistent medieval climate anomalies that may have been as warm in some regions as they were during the 20th century. In the last thirty years, however, averaged temperatures were probably higher than in all equally long periods during the Middle Ages.

The use of the term "medieval warm period" or "medieval climate anomaly" is inconsistent. The American climatologist Raymond S. Bradley saw a kind of confirmation bias at work here. Many works resort to the term even when "their" studied climate anomaly lies well outside the time window of 950 to 1250 and includes periods throughout the medieval epoch between 500 and 1500. Such episodes, referred to as the medieval warm period, then occasionally include periods that are already counted in other works as belonging to the Little Ice Age that followed later or to preceding episodes of the early Middle Ages that are often characterized as more changeable or cooler (→ Pessimum of the Migration Period).

In 2005, Rudolf Brázdil et al. warned against the use of the term Medieval Warm Period in comparisons of climatic conditions with historical and social developments. The term is not very helpful because it obscures complexity and leads to hasty conclusions. The term medieval optimum can also be easily misinterpreted because it is a value-free convention in the systematics of climate variability and not a positive value judgment. The term medieval climate anomaly for the various climate deviations is now the most common in science.

Temperatures

Overall, the evaluations show a long-term slight cooling trend globally over the last 5000 years until the 19th century, which was interrupted regionally by warmer intervals in the Middle Ages. A globally simultaneous, clearly definable medieval warm period is not recognizable. Only in the last about two hundred years the slight cooling trend of the late Holocene was terminated by a now globally synchronous, unusually strong warming. The averaged northern hemispheric air temperatures of the last three decades are probably higher than those of the same length of time during the Middle Ages. In the southern hemisphere, too, the warmest decade of at least the last 1000 years probably occurred in the late 20th and early 21st centuries.

During the Medieval Climate Anomaly, much of the mid- and high-latitudes of the northern hemisphere was warmer than during the subsequent Little Ice Age. This is indicated by a large majority of the paleoclimatological evidence. Some regions, considered over periods of 100 years, may even have been as warm as during the past 20th century. For the southern hemisphere, the data are sparser. An analysis of 511 time series from tree rings, pollen, corals, lake and sea sediments, glacial ice, speleothems, and historical documents shows a warmer interval in Europe, North America, Asia, and the Arctic for the period 830-1100. South America and Australasia had a warmer interval later, from 1160 to 1370.

Parts of the tropics may have been comparatively cool; a data series from shallow waters of East Antarctica shows no clear signal of a medieval warm period. In southern South America, a reconstruction suggests that there were summer temperatures for several decades in the 13th and early 14th centuries that may have approached those in the late 20th century. Data series from Africa paint a mixed picture. Overall, there was a stronger warming signal in some areas of South Africa around the year 1000, while for Namibia, Ethiopia, and Tanzania, a more pronounced warming is not evident until later, beginning in 1100. A synthesis of 111 time series confirmed a warmer interval between 1200 and 1350 for the entire Southern Hemisphere, based on average temperatures between 1000 and 1200, the subsequent cooling trend, and the current global warming.

Sea surface temperatures were comparatively high in the North Atlantic region. However, a synthesis of 57 reconstructions of sea surface temperatures over the last two thousand years found no global medieval climate anomaly.

Hydrosphere

In addition to regional temperature anomalies, widespread hydrologic anomalies occurred.

Southern Europe was dry in the period 1000-1200 compared to the average conditions of the 20th century, southern Scandinavia and northern Central Europe were much drier. Northwestern Europe, the Balkans, and the western Levant had rather humid conditions. There is evidence that, compared to the Little Ice Age period, fewer droughts occurred in the area of influence of the East Asian monsoon.

Parts of North America experienced severe and prolonged megadroughts.

In Africa, historical sources for the Sahel indicate wetter conditions, but south of the Sahel it appears to have been relatively dry. In the western Congo Basin, available data show no clear signal. In the east, from Ethiopia to Malawi, it was dry; the Nile showed a very sharp increase in years of low water from 900 onward, with clustered years of high water from about 1150 onward. In southern Africa, most reconstructions indicate rather wet conditions overall.

Sea level has fluctuated by about ± 8 cm over the last two thousand years. It rose until about the year 700, and fell somewhat from 1000 to 1400, accompanied by a global cooling of about 0.2 °C over this period. It was not until the 19th century that sea level began to rise again, at a much faster rate than during the Middle Ages.

Cryosphere

According to reconstructions, the Arctic sea ice extent was lower before 1200 than during the Little Ice Age. However, the minimum before the beginning of industrialization fell in the period around the year 640, well before the mostly assumed core period of a medieval warm period.

Viewed over millennia, most glaciers showed a development that was consistent with long-term, gradual changes in the Earth's axis (in large parts of the Northern Hemisphere, this corresponded to a slow advance. Over the period of individual centuries or decades, reliable statements about past, simultaneous glacier changes can only be made for individual regions. From about 900, for example, the advance of glaciers in Alaska came to a temporary halt, and some glaciers in the Western Alps also showed less activity from about 760 until the 12th century. However, a uniform glacier retreat during the period of a medieval climatic anomaly is not evident. During the period of study between 1050 and 1150, glaciers grew in many high mountain regions of the world, such as the Alpine areas, Canada, Patagonia, Alaska, and others, or no difference to the Little Ice Age can be detected, for example, for the Baffin Bay area or southeast Greenland. Only in the last decades a global almost simultaneous glacier retreat can be observed, which is very unusual for the Holocene period and is proceeding rapidly.

Changes in ocean-atmospheric circulation systems probably played an important role in the non-uniform occurrence of medieval climate anomalies. Human influences through atmospheric perturbations or land use were of little importance - on a global scale. The absence of significant changes in the primary climate drivers of greenhouse gas concentration, solar activity, and volcanic activity during the period 725 to 1025 led Bradley, Wanner, and Diaz (2016) to speak of a medieval quiescent period during which the climate may have been in a near-equilibrium state.

Internal variability

The regionally and temporally inconsistent occurrence of climate anomalies suggests a substantial role of internal variability of the climate system, i.e., changes in atmospheric circulation or ocean currents.

Some work supports the thesis that changes in ocean-atmosphere circulation systems, such as a more frequent or intense occurrence of La Niña-like events, played a role. This hypothesis is consistent with reconstructions of a relatively cool tropical Pacific Ocean. Warmer sea surface temperatures in the North Atlantic, consistent with positive phases of the North Atlantic Oscillation (NAO), could explain the relatively warm climate in northern and western Europe and droughts in parts of the world. However, positive phases of the NAO are usually accompanied by colder climates in Greenland. Recent studies based on significantly more data series suggest that significantly more frequent positive phases of the NAO did not occur until about 1150-1400.

The wobbly ocean conveyor hypothesis points to periodic fluctuations (about 1000-2000 years) of the North Atlantic Current as the cause. By evaporation of 0.25 × 106 m³

Volcanism

From the 8th to the 11th century, there were unusually few strong volcanic eruptions. If gases and ash reach the stratosphere during volcanic eruptions, this can lead to the formation of aerosols, reduced solar irradiance and associated cooling. Eruptions in the tropics can have a global effect, whereas for eruptions at higher latitudes, the ejected particles are distributed less widely and the effect is more regional. Between 682 and 1108, no major eruptions are evident in the tropics and only one at higher latitudes, around 939 in Iceland, which may have had only a limited effect on global temperatures. It was not until major eruptions in 1108, 1171, 1230, and 1257 (the eruption of Samala) near the equator that the period of low volcanic activity ended. The lack of volcanic influence on climate may have contributed to relatively high temperatures in the period up to the 12th century.

Solar activity

The radiation intensity of the sun seems to have fluctuated only a little between ca. 725 and 1025, it corresponded approximately to the long-term average. After a minimum of solar activity in the 11th century, the Oortminimum, it rose again to the previous level. The solar activity from about 1150 to 1300 has sometimes been referred to as the Medieval Maximum. A period of below-average solar activity over a longer period of time is observed with the late 13th century, beginning with the Wolf Minimum. Although the direct influence of the sun through the intensity of its radiation was probably relatively small during the past millennium, it may have had greater regional significance indirectly, for example, through its influence on the ozone layer.

Since medieval climatic anomalies have been studied, the question of their influence on societies has also arisen. Many works identified temporal parallels between climatic anomalies and societal developments and attempted to infer causal relationships, often through the influence of climate on agricultural yields, which were particularly important for most medieval societies.

At times, medieval climatic conditions have been evaluated as "climatic favors" in view of a European High Middle Ages considered to be a period of prosperity. The Canadian environmental historian Richard Hoffmann warns against an overly simplistic portrayal of medieval civilization as one brought about by harsh environmental conditions in late antiquity, flourishing during favorable climatic conditions and collapsing with the onset of the Little Ice Age. This smacks of environmental determinism. A Eurocentric perspective can lead to a distorted evaluation of the medieval climate. there was a pronounced, even extreme drought in North America. The droughts of the period were accompanied by agricultural crises, famines, conflicts, and social crises. Detailed analyses of how climate fluctuations might have led to social developments, in distinction from and in interaction with other factors, are still difficult and rare - despite increasingly high-resolution rainfall and temperature reconstructions.

Europe

During the period in which the medieval warm period was located, there was a veritable population explosion in Europe. This is attributed, among other things, to a favorable climatic development. Also as a result of the warmer climate in Europe, there was an expansion of the agricultural economy; grain cultivation was now possible in much more northerly areas as well as at higher altitudes. Cereal farming has been documented as far north as Norway and in the mountains of Scotland, which was discontinued during the subsequent Little Ice Age and the associated cooling of the climate. The storage pests grain weevils and grain flat weevils, as well as the human flea, were found much more frequently in western and northern Europe between the 9th and 15th centuries, and warmer and wetter weather may have contributed to their occurrence.

However, climatic conditions were not the sole reasons for the rapid increase in population and the associated expansion with its land development. Wilhelm Abel cites agricultural progress both in the use of technical devices, such as the horse collar for draft horses, and in land use as well as the diversification of grain. This interaction made it possible to provide food for a rapidly growing population. Thus, it is believed that the population in Europe almost tripled between 1100 and 1400. As a result, there was an interaction between population growth and the acquisition of new arable land. The population began to expand, with huge areas of forest being converted into arable land (e.g., in the course of the German Ostsiedlung). Numerous cities emerged as new centers of trade and commerce, sharing labor with the agrarian areas.

For southeastern Europe and Asia Minor, the development of the agricultural Byzantine Empire, a review paper cautiously concludes that climate, among many other factors, may have played a role. From the 9th to the 10th centuries, mild, humid weather favored agriculture and population growth. Climatic conditions continued into the 11th century, meanwhile Byzantium came under pressure from the Seljuks in Anatolia and was unable to expand its agriculture there. Although in the 12th century the climate became more changeable, partly warmer, with dry periods in the autumn and winter half-years, which were particularly important for agriculture there, Byzantium experienced a new expansion and social and economic flourishing in the Comnenian period, which researchers interpreted as a sign of the society's resilience. Cooler summers and drier winters at the beginning of the 13th century, as well as the eruption of the Samala in 1257 with subsequent cool years, may have contributed to the instability and end of the Late Byzantine Empire.

Africa

From 930-1070 and 1180-1350, as a result of reduced rainfall in East Africa, there was a sharp increase in years in which the Nile carried significantly less water. From about 1150 onwards, years with high water also became more frequent. Years with low water led to famines from Egypt to the area around Lake Victoria. According to the Arab historian al-Maqrīzī (1364-1442), an extreme famine with cannibalism occurred in the years 962-967. The Arab scholar Abd al-Latif al-Baghdadi reported low water around 1200 and a subsequent famine of the years 1200-1202, which he witnessed and which, according to him, killed over 100,000 people in Cairo alone.

Along the course of the Limpopo River, the first more complex societies and urban centers in southern Africa developed from small chiefdoms around 1000. A humid climate in this semi-arid region may have promoted this development. Around 1220, the elite of this society moved its political center to nearby Mapungubwe. Changing worldviews may be revealed in this move: The leadership of the state probably also derived its legitimacy from its spiritual role of summoning scarce rain from the hill. The South African archaeologist Thomas Huffman argued that the absence of rain weakened the power of the leaders, contributed to the fragmentation of the state, and thus also to the fact that the state around Mapungubwe fell behind Great Zimbabwe, which had developed into another major regional power from the 11th century onward.

America

For the southwestern United States today, several papers have examined a possible link between exceptional warmth, droughts, and the development of Native American tribes and cultures. For example, parallels have been identified between the decline of the Chacoan, Fremont, Lovelock cultures and three pronounced medieval droughts.

The Chacoan culture on the four-county corner of what is now the U.S. states of Utah, Colorado, New Mexico, and Arizona was heavily dependent on corn cultivation. Sufficient rainfall had allowed for a sedentary lifestyle, cultural development, and rapid population growth in 700-900 and 1050-1130. Pueblos with large multistory buildings were constructed, including those of the Chaco Canyon culture and the Cliff palace in what is now Mesa Verde National Park, which depended on irrigated agriculture. However, after medieval droughts in the mid-12th century and late 13th century, nearly all settlements were abandoned. Archaeological evidence was found of a sharp increase in cannibalism in the mid-12th century.

In Central America, droughts from the 8th to the 11th centuries were probably one of the factors contributing to the demise of Maya centers in the central lowlands. Declining rainfall in the central Andes led to the end of the pre-Incan Tiwanaku and Wari cultures in present-day Bolivia and Peru from about 1000 onward; despite sophisticated irrigation systems, it was probably no longer possible to feed the population in the barren altiplano. The decisive factor for the demise of the Tiwanaku state was probably that the raised beds of the Tiwanakans were affected by the receding shoreline of Lake Titicaca and the resulting lowering of the groundwater level. Archaeological findings point to a parallel decline of the authorities.

Greenland

The extent to which climatic changes contributed to the end of the medieval Scandinavian Greenland settlement (west settlement ca. 1350, east settlement in the 15th century) is still not clear. Recent work on medieval climate change in the West and South Greenland area paints a complex picture. Overall, they point to a period of cold climate for the period between ca. 1140 and 1220 in the area of the west settlement and walrus hunting grounds. Area-wise and intermittently, there may have been cold periods even before that, i.e., during the core period of the medieval climate anomaly. In Baffin Bay and Disko Bay, glacier advances caused by lower summer temperatures occurred as early as 1000 to 1250, possibly even approaching the later extent beginning in 1400. Analyses of lake sediments paint a partly contradictory picture: An examination of sediments from a lake near Kangerlussuaq indicates increasing temperatures between 900 and 1150, then - well before the end of the settlements - a rapid cooling, followed by a warming that reached the level of 900 again as early as 1300 and continued until the 17th century.  An analysis of lake sediments from twitching mosquitoes near Narsaq to the south indicates relatively high temperatures between 900 and 1400, with a more variable climate towards the end of this period.

It had long been assumed that the Vikings had stubbornly clung to their traditional agriculture and had contributed significantly to their decline through inflexibility, also in the face of climate fluctuations, and environmental destruction. Recent excavations since the mid-2000s, however, indicate that from about 1300 onward, the sea as a source of food outweighed the previously more important agriculture and animal husbandry. Researchers interpret this as an adaptation to lower winter temperatures.

Trade probably played a crucial role for the Greenland settlement. The settlers had to import important goods such as iron. The export of the coveted walrus ivory, which they captured on regular hunting expeditions to Disko Bay, was an important economic factor. More frequent and intense storms, falling temperatures, and especially increased ice drift along the west coast - not only regional cooling but also increased ice drift from the Greenland Sea and Denmark Strait may have been causative factors - may have significantly affected hunting and trade relations. But also increasing competition from Russia (walrus) and Africa (elephants), which entered the European market and led to falling ivory prices, and a decreased demand for ivory in the wake of late medieval crises may have destroyed the economic basis of the settlement. A confrontation with the Inuit is also still considered a possible factor.

Medieval warm periods are sometimes cited by climate deniers as supposed evidence that it is by no means certain that current warming is caused by human-emitted greenhouse gases. Since greenhouse gas concentrations were no higher in the Middle Ages than before or after, only other causes could have been responsible for warm periods then. They argue that these causes alone could explain the 20th century warming. They leave out the fact that medieval warm periods were probably only regional phenomena. They also leave out of this argument the well known scientific justifications that the factors that were effective at that time cannot explain today's warming.

In doing so, they commit a logical fallacy by thinking that some factor that was solely responsible for a change in the past must be so today. Just as the occurrence of natural forest fires in the past does not rule out the possibility that forest fires could also be caused by arson, natural medieval warm periods are not evidence against anthropogenic warming. In climate research, in addition to the change in greenhouse gas concentrations - currently caused by humans - the study of other factors effective in climate history also occupies a large space. Of all the known factors that can cause global warming, only the concentration of greenhouse gases has changed so much in the 20th century that it can essentially explain the observed warming.

Occasionally, also with Eurocentric recourse to a medieval warm period, it is claimed that warm periods are generally favorable periods. When the discussion of the medieval warm period began in the mid-1960s, it was a period of global cooling that extended into the mid-1970s. Warming to the level of the Medieval Warm Period at that time might indeed have been beneficial in some regions. However, much evidence suggests that by the end of the 20th century, Europe was also already warmer than during the Medieval Warm Period. Climate historians point out that crisis-like consequences of past climate fluctuations, such as those of medieval climate anomalies, may rather serve as parables for the dangers of global warming, or that it is the rates of change and variability of the last millennia that call for climate protection.

However, if massive reductions in greenhouse gas emissions were not made, the average global temperatures expected at the end of the 21st century would very likely be higher than they have been globally for the last several hundred thousand years, and possibly even higher than they have ever been since Homo sapiens existed. The rapid global warming observed at the end of the last ice age was a warming of about one degree Celsius per 1000 years. Even if the 2-degree target were achieved (which is considered unlikely), the global warming expected by the end of the 21st century would still be an order of magnitude faster.

The discussion about the extent and consequences of the current and the probably expected man-made global warming thus refers, both in terms of speed and extent of warming, to a historically unique process for which empirical values are largely lacking and for which - as evidenced by a large number of climate proxies - no equivalent is known even from a geological and paleoclimatological point of view.

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