Built upon on a complex system of canals, water catchments and embankments, Angkor was once the largest city in the world, covering an area of approximately 1000km2. However, in the 15th Century it saw a massive population fall.

The multidisciplinary team of academics, researchers and students led by Professor Mikhail Prokopenko, Director of the Complex Systems Research Group, and Associate Professor Daniel Penny, Director of the Greater Angkor Project, found that the medieval city suffered external climate stress coupled with overloaded infrastructure within the canal system, which through in-depth mapping showed evidence of a vulnerability to catastrophic failures.

Professor Prokopenko believes that the study is crucial to improving infrastructure in an era of increasing frequent extreme weather events which are creating new and pressing risks to urban environments.

He said, "Complex infrastructural networks provide critical services to cities but can be vulnerable to external stressors, including climatic variability.

"The cascading failure of critical infrastructure in Angkor which resulted from climate extremes re-emphasises the importance of building resilience into modern networks."

Having worked on the Greater Angkor Project for 18 years, Professor Daniel Penny said, "For the first time, identifying a systemic vulnerability in Angkor's infrastructural network has provided a mechanistic explanation for its demise, which comes with an important lesson for our contemporary urban environments."

Both of the researchers believe that the risks of network collapse have become more acute as urban conglomerations become larger, more complex, and have more people living in them.

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"The water management infrastructure of Angkor has been developed over centuries, becoming very large, tightly interconnected, and dependent on older and ageing components. The change in the middle of the 14th Century C.E., from prolonged drought to particularly wet years, put too much stress on this complex network, making the water distribution unstable," said Professor Prokopenko.

The factors behind Angkor's demise are also comparable to the challenges faced by modern urban communities that are struggling with complex critical infrastructure and extreme weather events, such as floods and drought.

Professor Prokopenko continued, "We found that infrastructural networks in preindustrial urban environments in fact share very common topological and functional characteristics with modern complex networks."

In complex systems, catastrophic responses and cascading failures, such as an abandonment of a city, can also be caused by very small changes or even a series of accumulated changes which cause a system or network to reach a 'tipping point'.

There are several examples of tipping points faced by the modern world, such as the Amazon die-back, the 2016 South Australian power outage and the El Niño-Southern Oscillation.

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Professor Prokopenko believes the team's research emphasises the need for governments and communities to focus on building resilience into modern urban networks, particularly in the face of a changing climate.

"Not only is it possible that catastrophic, infrastructural failure may also have occurred in the past, but the results from this research are critical to our community's understanding of how climate and distributed resources affect the functioning of our cities and societies.

"If we don't build resilience into our critical infrastructure, we may face severe and lasting disruptions to our civil systems, that can be intensified by external shocks and threaten our environment and economy," he concluded.

The research was conducted as part of the Greater Angkor Project in conjunction with the University of Sydney's CRISIS initiative which models social risks and extreme events.

The Greater Angkor Project is a collaborative research program between the University of Sydney (Archaeology and Geosciences), the APSARA National Authority and the École Française d'Extrême-Orient, that has attempted to explain the demise and abandonment of Angkor for the past 16 years.

The new study was published in the Science Advances.