Angkor Demise

25 Oct 2018

This story was sent in by William - go to ... the headline of which puts it into context - what contributed to the downfall of the city of Angkor in modern Cambodia? A new study claims that monsoon flooding weakened the water infrastructural network of canals, reservoirs, moats and ponds, and led to the collapse of the system (and abandonment of the city). Angkor was one of the world's largest cities back in the 13th century AD. It had a unique system to store and distribute water resources as well as to regulate flooding. However, irregularities in monsoonal rainfall may have led to sedimentation and erosion. The authors of the study modelled the city infrastructural network as a computer analysis and found that large floods caused by heavy monsoon rains would create instability in the network - with severe damage at upstream junctions. Extensive levels of rainfall occurred in the late 14th century across SE Asia. It is interesting to note that prolonged rainfall was also a feature in NW Europe - in the 14th century. This suggests that the jet stream at the time was positioned south of its normal position which in turn indicates that global cooling was a distinct feature of the weather - and indeed the 13th and 14th centuries are regarded as the first phase of the Little Ice Age (although that term is usually applied mainly to the 16th and 17th centuries when average temperatures were much colder than nowadays). Climatically, this is defined as a period of reduced sunshine (warmth) which is usually blamed on volcanic activity. Chinese and Korean astronomers record a variety of transient phenomena in the sky during the same period - and not all due to volcanoes. The mid 14th century coincides with the Black Death epidemic. A sudden lack of human labour may have caused the infrastructure at Angkor to collapse - due to lack of maintainence (in association with increased rainfall).

Mike Baillie, in @new Light on the Black Death' (Tempus:2006) has collated lots of evidence of earthquakes, cosmic phenomena, and volcanic activity - and a climatic event in trees that was global (various dendrochronologies) and all this led up to the Black Death which broke out in 1348. From the mid 12th century to around 1330 there was a continuous C14 enrichment spike which suggests the Sun was in a quiet mode (which allowed more cosmic radiation from outside the solar system to penetrate our atmosphere). After 1330 the enrichment ceased and the calibration curve changes direction  - to one of depletion of the radio carbon reservoir. Baillie suggests this was due to dilution by old carbon. What may do that? Baillie says volcanoes can emit lots of old carbon - and comets and meteors may do the same thing.

Baillie goes on to say that in the Americas tree rings have also shown a similar reversal - with increased precipitation levels from 1300 onwards (following a 50 year dry period in the SW). Indeed, the SW became attractive to farmers - but after 1335 the pueblo culture in semi desert zones fell into decline - with a second phase of settlement in the 1370s. We may also note swings in drought and flood in China - as well as a 6 day earthquake event. Baillie's analysis does not involve the late 14th century - but one can surmise a similar set of circumstances was in play (droughts and famine followed by floods).

HH Lamb, in 'Climate, History and the Modern World' page 191 - says in the 13th century storms increased in frequency and severity around the North Sea basin, and sea floods resulted in an appalling loss of life. This was especially true of the Dutch and German coastline. Devastating floods occurred in 1240 and 1362. Up to 60 parishes were lost in Schleswig, swallowed up by the salt sea. The island of Heligoland measured 60km across in 800AD but was reduced to 25km by 1300. Today it measures just 1.5km at its largest axis - a fraction of its former size. In England Dunwich was lost, and Ravenburgh (east of modern Hull). Storms that involved large loss of life occurred in 14521, 1446 and 1570 - and in 1634 there was again a great loss of land in Denmark and Germany. In the Netherlands devastating storm surges were common in the early 1400s and late 1600s. Lamb attributed this in part to a cooling Arctic which produced a strong thermal gradient in between 50 and 65 degrees North.

On page 195-8 he describes a cool and wet climate in early 14th century Europe  with an extraordinarily run of wet summers and wet springs, even wet autumns, between 1313 and 1317 (continuing to a degree into the 1320s). This seems to have followed a period of dry warm summers between 1284 and 1311. The wet weather caused failure of grain (rotting in the fields) and famine (and death by starvation). Great numbers of sheep and cattle also died in outrbreaks of diesease feeding on the wet weather and a sodden landscape. One of the most common of diseases in that century was St Anthony's Fire (ergotims) produced by the ergo of blight (which affected grain husks). The Black Death epidemic has tended to obscured the blighted failure in crops that preceded it - and the number of deaths causesd by the blight.

On page 200 he says the Black Death came on the back of exceptional rain and flooding. In 1332 7 million people died in China. In India, according to KS Lal, population was between 200 and 300 million people in the Medieval Warm Period. It fell dramatically after AD1200 - and even sharper during the 16th century AD. Basil Cracknell, in 'Global Warming and Coastal Change in Britain through 2000 years' (Philimore of Chichester@2005) says that climatic warming is not the only factor affecting sea levels. Oner of these is the level of storminess (and storm surges caused by wind and tide). On page 11 (of the Introduction) he says the 13th and 14th centuries experienced a peak in sea flooding events (a period characterised by storminess). He says over a million people were drowned in sea floods around the North Sea basin between 1200 and 1600 - and not only that, the coast has changed significantly.

When one consider the evidence from India and China we can only think in terms of an unstable monsoon cycle - as a result of a the polar circle widening and forcing the temperate zone to contract (with a more southern jet stream in the north and pressure on the monsoon system to shift likewise). Hence, we can only conclude that the demise of Angkor was also related to climate change (and monsoonal variability) as modelled by the authors of the study in question. However, the fact that loss of life (and therefore labour to keep the water system intact) also played a role is also a factor. Climate change alone may not be enough.