Environmental History Resources

535—555 CE: Dendrochronological (tree-ring) evidence in oak trees salvaged from Irish peat bogs indicates a sequence of colder than average summers at this time. A similar effect is seen in Scandinavian pine trees, and a study of European oak tree data as a whole shows that the event starts in 536 and lasts until 545. The decrease in rate of growth in these years corresponds to a global temperature decrease of up to ~3 °C. In fact, 536-39 is noted as one of the coldest two or three years globally in the last 2000 years.

Tree ring growth in Europe, 523 - 552CE

The consequences were experienced worldwide. In Britain, the period 535—555 saw the worst weather that century. In Mesopotamia there were heavy falls of snow and in Arabia there was flooding follow by famine. In China, in 536, there was drought and famine and yellow dust rained down like snow. In Korea, AD 535 and 536 were the worst years of that century in climatic terms with massive storms and flooding, followed by drought.
In the Americas, the pattern was similar. In all cases observers describe a dust veil hanging in the atmosphere obscuring the sun which led to lowering to the surface temperature.

What caused it?
There is much speculation about this and explanations range from a volcanic eruption to the impact of a comet that exploded in the atmosphere. Since there is no major volcanic eruption at that time it is now thought that the explosion of a comet in the atmosphere is the most plausible explanation. However, the jury is still out. (Further reading: Rigby, Emma, Melissa Symonds and Derek Ward-Thompson, ‘A comet impact in AD536?’, Astronomy and Geophysics, 45 (February 2004), 23-26.

This catastrophic event can be regarded as the trigger that ended the classical world and the beginning of the Middle Ages. It blotted the sun out and resulted directly and indirectly, in climate chaos, famine, migration, war and massive political change on all continents.

Note: Climatic and environmental disasters are "funnels", not direct causes for historical events. They reinforce already existing historical patterns such as migration or political instability, over exploitation etc.

Harvest time in the Middle Ages -
Les Tres Riches Heures

10th – 14th century: The Medieval Warm Period (MWP) or Medieval Climate Optimum. During the High Middle Ages in Europe experienced a climate slightly warmer than today. The summer temperatures were between 1 and 1.4 degrees higher than the average temperature of the 20th century. The winters were even warmer with an average temperature in England of 6 degrees, which is about 1 to 2 degrees warmer than today. The warmer conditions were caused by the fact that the air circulation above the Atlantic changed position, as did the warm sea currents, transporting warmer water to the arctic.

In Europe the warm conditions had positive effects. Summer after summer the harvests were good and the population increased rapidly. As a result thousands of hectares were cleared of woodland and farmers expanded their fields high into the hills and on mountain slopes. It was even possible to grow successfully grapes as far north as Yorkshire.

Under these conditions, art, literature and even science were developing apace and we see the height of medieval civilisation. The most visible achievements of this period are undoubtedly the construction of the many cathedrals all over Europe. The good harvests had made Europe rich and the good weather freed people from the burden of the struggle against the elements. It created the wealth and labour force to build cathedrals. It was a golden period for European Architecture and art.

The Black Death, 1340s-1350s
The dramatic decline of the European population caused by the Black Death coincided with a decline in global temperature. Coincidence? The climate was already getting colder because the northern hemisphere was heading for the Little Ice Age. At the same time agricultural land was taken out of production in Europe because of the 25-40% decline of the European population (depending on region). This means ploughing of less ground, which releases greenhouse gasses (Methane and carbonates) and forest clearance was reversed. More trees and scrubs mean that more carbon (CO2) was taken out of the atmosphere and stored in biomass. The abandoned farmland acted as a significant carbon sink because trees store carbon taken from the CO2 in the air.

From about 1350 CO2 levels in the atmosphere appear to fall following the Black Death. However, a long term declining trend may have already started before the Black Death. We know that the first two decades of the 14th century were wetter, windier and climatically more unstable than before. The declining trend also continued after the recovery of agriculture after 1440. The reforestation that followed the Black Death and the resulting decrease of CO2 in the atmosphere pronounced a natural cooling trend that was already underway. This was the beginning of the LIA. This means the LIA was not triggered by the Black Death but possibly contributed to it, although temporarily.