The Medieval Warming (900–1300 AD)

Extract from Plimer:

The Dark Ages ended as quickly as they began as the world became warm again. The Medieval Warming from 900 to 1280 AD was followed by two decades of very changeable weather as the Medieval Warming changed to the ensuing Little Ice Age. In the Medieval Warming, it was far warmer than the present and warming was widespread.239 The Medieval Warming was not all beer and skittles, because there was a cold period from 1040 to 1080 AD when the Sun was very inactive (Oort Minimum).

The Medieval Warming is just one of the many warm periods that Earth has enjoyed. The much warmer and longer Holocene Climate Optimum lasted between 7000 BC and 3000 BC. (The Late 20th Century Warming was somewhat cooler than the Medieval and Roman Warmings.) Although it was warm, there were the inevitable intermittent periods of bad weather. However, on balance summers were longer and warmer, crops were plentiful and there were few serious famines. Kings and landowners prospered and peasants rarely went hungry. The amount of land devoted to agriculture increased and fields crept up to higher altitudes where farming had previously not taken place. In the Northern Hemisphere, crops that enjoy warmth were farmed further and further north.

Europe was warm, rainfall was higher, the climate was stable and agricultural productivity was very high. There was excess food, excess labour and excess wealth. There was prosperity and there were funds to fight the Crusades. Cultivation was higher in the mountains than it had ever been, and tree ring studies in California suggest that North America was also enjoying the warm times.240 Excess food in Europe led to a 50% increase in population. Although grain could not be effectively stored from rats and other pests, the regular reliable harvests created stability and certainty.

In Europe, cities grew, transport networks were established and excess labour was employed in construction of the great monasteries, cathedrals and universities.241 The great architectural structures took generations to build, showing that there was sustained generational wealth. Universities were established to train young men for the priesthood. The modern secular universities owe their origin to the Medieval Warming. Any visitor to Europe can see the results of the boom in cathedral building, a direct result of the great prosperity that warm times brought. In Europe, new cities were built and the population increased from 30 million to 80 million. At the same time, the thousands of temples at Angkor Wat in southeast Asia were built. In China, these warmer conditions led to a doubling of the population in 100 years. The Medieval Warming was the zenith of Muslim imperialism, culture and science. Enough food was available to feed more people and travellers could venture great distances.

Economies boomed. In colder times during the Dark Ages, the economy was organised around self-sufficient estates which grew their own food, flax and wool, wove their own clothing and had little or no trade with other estates or internationally. The Medieval Warming was a time of excess when luxury goods such as spices from Oriental caravans, sugar from Cyprus and glass from Venice could be acquired through trade. Overseas and coastal trade was easier because of the decline in the frequency and intensity of high winds and fierce storms. More sunshine meant that, even if there was heavy rain, roads dried quickly, allowing more reliable traffic between farms and towns. Mountain passes were open for longer, allowing a longer season of trade. The warm climate allowed the emergence of European trade fairs. In 1000 AD there were more than 70 mints in market towns coining German silver for purchase of wool and fish. In return, the English purchased wine, furs, cloth and slaves.242

Agriculture thrived high in the Alps of Europe. At the Grosser Aletsch Glacier, a larchwood aqueduct was constructed to supply water to an alpine village around 1200 AD. It was destroyed by glacier advance in 1240 and had to be totally rerouted in 1370 during the Little Ice Age after further glacial advance.243 It was finally destroyed in the peak of the Little Ice Age. The numerous glacial retreats and advances in the Alps are a proxy for air temperature.244,245 By measuring glacial advances and retreats in New Zealand, a correlation of 1500-year cycles can be made.246 In the North Atlantic, ice packs retreated northward (especially in summer) and the incidence of severe storms decreased.

The Vikings, who were already great mariners, sailed north and west and established settlements in Greenland, Iceland and North America, such as L’Anse aux Meadows. The ice-free North Atlantic meant that the Vikings could travel and they called Newfoundland “Vinland” because of the vineyards there. Cattle, sheep and barley were grown in Greenland, tree roots could penetrate soil that was once tundra, fishing for cod and seals took place on ice-free seas, burials could be undertaken in soils that were not frozen, villages were established and the Pope sent a bishop to Greenland to care for his Norse flock. The Vikings also traded as far south as Persia and the Arab states.247

The Doomsday Book of England shows where grapes were grown, in places where no grapes could now be cultivated for wine production. England, now a cool damp place, was warmer and drier in the Medieval Warming. England thrived and its population grew from 1.4 million to 5.5 million. France’s population tripled to 18 million.

Vineyards in Germany were up to 780 metres above sea level whereas today the maximum altitude is 560 metres above sea level. Temperature usually decreases by 0.6 to 0.7ºC per 100 metres of altitude gained, so the average mean temperature must have been 1.0 to 1.4ºC warmer than now.248 Settlements, land clearing and farming in valleys and slopes spread 100 to 200 metres higher in altitude in Norway, again suggesting that summer temperatures were 1ºC higher than now.249 Tree lines moved upslope in the Medieval Warming and the stumps and roots are still preserved above the current tree line in many alpine areas. Stumps and logs of Larix sibirica 30 metres above the current tree line in the Polar Urals have been dated and show that at 1000 AD the tree line was higher than now.250 This tree line receded around 1350 AD, indicating the effects of the following Little Ice Age.

Copper, gold and emeralds were mined at high altitude in the Alps of Europe during both the Roman Warming and the Medieval Warming.251 These mines were covered by ice and abandoned during the Dark Ages and were again covered by ice during the Little Ice Age. Some of these areas have just been exposed by glacier retreat in the Late 20th Century Warming.252

Sediments from Lake Neufchatel in Switzerland show an abrupt temperature decrease of 1.5ºC at the end of the Medieval Warming. These sediments also show that mean annual temperatures in the Medieval Warming were higher than at present.253 In the Baltic Sea, the Medieval Warming allowed tropical and sub-tropical marine plankton to survive. Despite the Late 20th Century Warming, they have not returned because the Baltic Sea is still colder than it was in the Medieval Warming. At about 1200 AD, the warm water micro-organisms were replaced by cold water organisms. The faunal replacement reflects the onset of the Little Ice Age.254

Boreholes give accurate temperature histories for about 1000 years into the past because rock conducts past surface temperatures downward only slowly. In the Northern Hemisphere, borehole data shows the Medieval Warming and a cooling of about 2ºC from the Medieval Warming to the Little Ice Age.255 A study of 6000 boreholes on all continents has shown that temperature in the Medieval Warming was warmer than today and that the temperature fell 0.2 to 0.7ºC during the Little Ice Age.256

Advance of tree lines, retreat of glaciers, reduced lake-catchment erosion (i.e. fewer storms) and temperature proxies show that between 700 and 1200 AD, Scandinavia was warm.257 Other data from Scandinavia shows a cool period from 500 AD to 700 AD (i.e. the Dark Ages) with 660 AD a very cold year. An overall warm period from 720 AD to 1360 AD (Medieval Warming) had exceptionally warm times in the 10th, 11th, 12th Centuries. There was also a warm period in the early 15th Century. After 1430 it was cold (i.e. Little Ice Age).

The Medieval Warming also affected the eastern Mediterranean.258 Lake Van in eastern Turkey had a high water level259 as did lakes in the Sahara Desert.260 The Dead Sea261 and the Sea of Galilee262 were also full and rainfall in the Nile headwaters was excessive.263 The Nile has two headwaters. The Blue Nile comes from Ethiopia. This mainly contributes silt. The White Nile rises in Lake Victoria and contributes most of the water. The Nile River has a flooding and sedimentation record that integrates two widely separated areas. A 1300-year tree ring record from Pakistan shows that the warmest decades occurred between 800 AD and 1000 AD in the Medieval Warming and the coldest between 1500 and 1700 in the Little Ice Age.264

The spread of villages in southern Africa occurred during the warm wet periods of the Early Iron Age (650–300 BC). Another warm wet period extended from 900–1290 AD, the Medieval Warming. The appearance of many villages and the rise of Great Zimbabwe coincided with the beginning of the dry Little Ice Age, while a warm pulse in the 15th and 16th Centuries created the conditions for mixed farming in the highveld. Another warm and wet period at the end of the 18th Century contributed to the spread of maize, increased populations and more military action.265

In East Africa, an 1100-year rainfall record using sediments, fossil diatoms and numbers of species of midges shows alternating dry and wet conditions. Off the coast of West Africa, sea surface temperature dropped. Onshore, the landmass was drier for centuries during cool periods and there were massive rainfalls that created lakes in the Sahara Desert in warm periods. However, other work shows that in greenhouse times, such as the Medieval Warming, East Africa was drier. In the Little Ice Age a colder wetter period was frequently interrupted by droughts.266 Over the past millennium, equatorial East Africa alternated between dry and wet conditions. In the Medieval Warming, East Africa was drier than today whereas in the Little Ice Age, East Africa was far wetter than now. However, the Little Ice Age had three prolonged dry periods. Lake Naivasha in Kenya had a higher rainfall than now. The writings of Arab travellers in North Africa indicate that the rainfall was higher than in the Little Ice Age and today. In South Africa, a stalagmite from a cave in the Makapansgat Valley showed a warm period from 1000 to 1300 AD.267

China flourished in the Medieval Warming. Palace records, official histories, year books, gazettes and diaries record the arrival and departure of migratory birds; the distribution of plants, bamboo groves and fruit orchards; the patterns of elephant migrations; the flowering times of plants; and major floods and droughts.268,269,270 The growing seasons were longer and more reliable and citrus orchards moved north, only to move south once the Little Ice Age commenced.271 China enjoyed the Holocene Climate Optimum (8000 BC to 3000 BC), the Roman Warming and the Medieval Warming and, on the basis of pollen studies, China was at least 2 to 3ºC warmer than now.272

Such warming periods created great wealth in China. Wealth had been rising from 200 BC to its peak at 1100 AD with the greatest increases in the Han period (206 BC to 220 AD) and the Northern Sung Dynasty (961 AD to 1127 AD).273 These two warm periods in China coincided with the Roman Warming and the Medieval Warming elsewhere. China’s temperature history has been reconstructed for the last 2000 years from ice cores, lake sediments, peat bogs, tree rings and historic documents.274 The 2nd and 3rd Centuries AD, towards the end of the Roman Warming, were the warmest. It was also warm from 800 AD to 1400 AD during the Medieval Warming, cold in the Little Ice Age from 1400 to 1920 and then warm again after 1920 during the Late 20th Century Warming. Cave stalagmites from China show a strong warming from 700 to 1000, corresponding to the Medieval Warming, and a cooling from 1500 to 1800 when the air temperature was 1.2ºC cooler that at present.275

Similar good times were enjoyed by the Japanese as official records on weather, floods, droughts, heavy snows, long rains and mild winters show.276 It was warm from the 10th Century to the 14th Century, as in Europe. Official records allowed a detailed analysis which showed that relatively hot conditions continued until the 8th Century, then cool conditions appeared for a short time in the late 9th Century. Warm conditions existed from the 10th Century to the early 15th Century and late in the 15th Century cooling commenced. It became very cold at the beginning of the 17th Century. The carbon chemistry of Japanese cedar records the Dark Ages, the Medieval Warming and the Little Ice Age, again showing that these climate changes were widespread.277

North America also thrived in the Medieval Warming. Increased rainfall cut channels in the Great Plains278 and Alaska warmed quickly.279 Vegetation studies in northern Quebec show a cold period (760 to 860 AD) coinciding with the Dark Ages, a warming from 860 to 1000 AD reflecting the Medieval Warming and severe cold from 1025 to 1400 reflecting the Little Ice Age.280 A similar study in southern Ontario showed forest changes at the end of the Medieval Warming. Warmth-loving beech trees were replaced by the cold-tolerant oak and then later by the cold-loving pine. The change from the Medieval Warming to the Little Ice Age resulted in deforestation and a loss of 30% of the mass of the forests. The Ontario forests have still not recovered from the Little Ice Age and have not returned to the diversity and productivity of the Medieval Warming.281 In the United States, the Medieval Warming is detected by studies of moisture records of lodgepole pines at Lake Tenaya in the high Sierra Nevada Mountains.282 Lake levels have changed with the release of water from melting snow. A similar pattern was recognised at Mono Lake and the Walker River in California.283

The Anasazi Indians’ agriculture and culture spread in the early part of the Medieval Warming as rains were more consistent. Tree rings from Sand Canyon show low rainfall in 1125 to 1180 and 1270 to 1274 and a 24-year drought late in the 13th Century. These led to food scarcities, internal conflict, the building of fortress-like cliff dwellings and the eventual sacking of the fortresses in the Little Ice Age.284 By 1400, the maize crop failures had driven the Anasazi from their cliff dwellings and to extinction. In the Sierra Nevadas of California, living and dead trees provide a 3000-year record of tree line changes.285 Dense forests grew above the current tree line in the Roman Warming and from 400 AD to 1000 AD. The tree line moved rapidly down slope between 1000 and 1400 and continued to move down slope, albeit more slowly, from 1500 to 1900. The current tree line has not changed since 1900. In southern Ontario, pollen studies show that beech trees in the Medieval Warming were replaced by oaks in the Little Ice Age. Beech trees enjoy warm conditions whereas oaks are tolerant of cold conditions.286 In southern Alberta (Canada), lake sediments show the increased runoff during the Medieval Warming and the decreased runoff reflecting the drier conditions of the Little Ice Age.287

The Southern Hemisphere also experienced the Dark Ages, Medieval Warming and Little Ice Age. In Argentina, the carbon chemistry of prehistoric villages shows that villagers clustered in the lower valleys during the Dark Ages. Villages moved upslope to altitudes as high as 4300 metres in the Central Peruvian Andes during the Medieval Warming to capitalise on the stable warmer climate. In 1320, villagers moved back down slope as the colder unstable Little Ice Age commenced.288 The compilation of flood reports, sailors’ handbooks and folk records show that central Argentina had more rain during the Medieval Warming than now and that temperatures were up to 2.5ºC higher than now.289

Vegetation in South America also felt the effects of climate change, especially the Little Ice Age. Pollen from lake sediments in Peru provides a 4000-year record of climate. The Roman Warming could be seen and rainfall declined during the Dark Ages. In the Medieval Warming, increased pollen indicated warmer temperatures, more plants and greater plant diversity, followed by a pollen decline in the Little Ice Age.290 Elsewhere in South America, lake sediments from a high volcanic plateau showed that climate and rainfall changed quickly, with the Little Ice Age being a prominent feature.291

In the South Pacific, during the Roman Warming there was island-hopping migration by Polynesians. Easter Island was settled about 400 AD. Great blocks of stone were carved there between 1000 and 1350 during the time of plenty in the Medieval Warming. In 1350 famine set in and the wet tropical island became a dry cool desert island during the Little Ice Age. By 1600, Easter Islanders had resorted to cannibalism and the population greatly declined.292 The Tokelau, Society, Austral, Marshall and Marquesas Islands and Tonga and Fiji were all settled in the Roman Warming. New Zealand was first settled in the Medieval Warming293 when South Pacific island populations thrived.294

An analysis of the physical evidence from 112 studies of the Medieval Warming in Greenland, Europe, Russia, USA, China, Japan, Africa, Chile, Argentina, Peru, Australia and Antarctica295 showed that the Medieval Warming was recorded. The Medieval Warming can also be measured in sea floor sediments in the North Atlantic, the South Atlantic near Antarctica, the central and southern Indian Ocean and the Central and Western Pacific Ocean.

There were no CO2 emitting industries in the Medieval Warming. This natural warming event was greater than the Late 20th Century Warming, which we are told is due to human emissions of CO2.

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