A medieval illustration of a farm worker, part of an illuminated manuscript. Limbourg brothers (fl. 1402–1416) and Barthélemy d'Eyck (fl. 1444–1469) A heatwave lasting four months, followed by water shortages and crop failures. This is England in the 13th century, not the 21st.What is often called the Medieval warm period saw elevated temperatures in many parts of the world between the 10th and 13th centuries. These warmer temperatures were caused in part by an increase in solar irradiance. This is the amount of solar energy reaching Earth, which effects global temperature patterns and influences weather systems. There were also very few volcanic eruptions in this period, which can produce a temporary global cooling effect. Water shortages and poor harvestsIn England, the 13th century saw extreme droughts, devastating city fires and outbreaks of disease. The dry summer of 1212 contributed to the devastation of the Great Fire of Southwark in London, which killed thousands of people.Contemporary medieval chroniclers recorded a severe drought in 1252. The English Benedictine monk Matthew Paris described four months of “intolerable heat and drought” between March and July, unrelieved by “any fall of rain or dew”.The Tewkesbury Annals, a contemporary chronicle written by the monks of Tewkesbury Abbey, agreed that “drought prevailed for four months, causing the grass to disappear”. Rubbing the grass between one’s hands was said to turn it into dust. The Chronicle of the Princes of Wales (Brut y Tywysogion), a medieval record of Welsh history, similarly noted that “the heat of the sun was so great that all the earth became so dry therefrom, that no fruit grew on the trees or [crops in] the fields, and neither fish of the sea nor of the river was obtained”.Paris went on to describe the lack of available pasture, which led to cattle and livestock wasting away. The warmer weather produced an “abundance of fleas, flies and gnats”, and led to a surge in diseases, with reports of malaria – or the “sweats” and “ague” – in low-lying marshlands. Read more: Six tips from the middle ages on how to beat the summer heat Manorial accounts confirm the difficulties facing farmers, with one Bishopric of Winchester manor recording that the ground was so hard they could not plough for six weeks. Illustration of agricultural workers in a wheat field in the medieval period. Public Domain The good and the badBut it wasn’t just England that was experiencing unusual weather in this period.Most of the northern hemisphere plus South America, China and Australasia recorded temperatures 0.3-1.0°C higher than those recorded for 1960-1990 – a period that includes the hot summer of 1976. Conditions in the North Atlantic were so favourable that Norse people were able to cross oceans as the ice melted. They established settlements in Iceland and Greenland, and travelled as far as Vinland (Newfoundland).South-east England was warm enough to support vineyards – 45 were recorded in the hands of Normans or great abbeys in the Domesday Book of 1086. Yet significantly cooler conditions prevailed in the eastern Pacific Ocean. Warm water was pushed towards Asia, creating wetter conditions in Australasia, but limited rainfall in what we now refer to as the American west. And there were changes in the North Atlantic Oscillation (a weather system over the north Atlantic), which brought warmer and wetter conditions to parts of northern Europe. There are many sources of evidence for reconstructing past climates. These range from ice cores (long cylindrical samples drilled from glaciers which reveal hundreds of thousands of years of climate history) to tree rings showing the annual growing conditions – and from documentary evidence to the archaeological record. Although some of the precise chronologies and contours of this evidence are still debated, the overall trends in reconstructed climates are becoming increasingly clear. Tree rings in the Royal Ontario Museum, Royal Ontario Museum, Public Domain., CC BY-NC Not all regions experienced warmer temperatures during this period, however. Many scholars prefer the label Medieval Climate Anomaly instead. This better reflects the fact that climatic changes varied across the globe, and involved more than just rising temperatures. Although the warmer temperatures produced occasional drought conditions, the overall economic effects of the Medieval Climate Anomaly were largely positive for northern Europe and parts of Asia. It led to large-scale and widespread agricultural expansion and population growth.Between 600-900, the population of Eurasia only marginally grew from 167 million to 178 million. Between 900-1200, however, it nearly doubled to 324 million.It was during the Medieval Climate Anomaly that a highly commercialised and monetised economy developed in large parts of medieval Europe, in what historians have described as its commercial revolution. A Yale University-owned map (believed to be a copy) of the area known as Vinland by the Vikings – now Newfoundland in Canada. Public Domain Yet, the period had far more serious negative consequences elsewhere. The warm storms led to drought conditions in the southern US and parts of South and Central America. This initiated the outbreak of exceptionally large wildfires in the Rocky Mountains. Severe droughts, exacerbated by extensive deforestation and ecological degradation, were a contributing factor in the decline of the classic Maya civilisation (250-900).The Medieval Climate Anomaly period was brought to an end by climatic instability in the second half of the 13th century – including a volcanic eruption in 1257 that led to global dust clouds and crop failures. These resulted in mass starvation in medieval London.The early 14th century saw a shift in weather conditions, with exceptionally heavy rain and cool temperatures preventing crops from maturing. This culminated in the Great Famine of 1315-17, which killed around 10% of the European population. Other significant historic events of the period were also linked to unusual weather and atmospheric conditions. For instance, the Black Death of 1347-53, which killed between a third and two-thirds of the European population, was brought into Europe on grain shipments imported to alleviate famine caused by another volcanic eruption, around 1345. Around 45 vineyards were recorded in England during this medieval period, because of the warmer weather. Public domain Any parallels with today?The Medieval Climate Anomaly was a largely regional episode of natural warmth. Today’s climate change is a rapid worldwide warming trend driven primarily by human greenhouse gas emissions.Moreover, temperatures are rising today at unprecedented rates. Records continue to tumble, as weather that was once considered extreme is becoming normal. There are, however, some salutary warnings from the past. As climates change, so too will the risk of diseases spreading and the possibility of weather extremes. With increasing temperatures, it is possible, for example, that malaria may once again become endemic in the UK.As the medieval period has demonstrated, fire spread can be another acute risk in long periods of dry weather. But unlike the medieval period, we now have the technology to help us prepare, as well as national fire services and more ways to share information quickly. That gives us an advantage that medieval communities just did not have.Alex Brown is a member of the Institute of Medieval and Early Modern Studies at Durham University and is currently the Principal Investigator of a Leverhulme Trust funded project on 'Modelling the Black Death'.