Pre-existing ethnic tensions increased, creating fertile grounds for violence and conflict. Many flooded into urban centres, overwhelming limited resources and services there. That crisis left large numbers of people вЂ“ especially young men вЂ“ unemployed, discontent and desperate. A severe drought in the late 2000s, likely made worse by human-induced climate change, combined with groundwater shortages to cripple agricultural production. Syria, for example, enjoyed exceptionally high fertility rates for a time, which fueled rapid population growth. Indeed, some nations are already serving as canaries in the coal mine for the issues that may eventually pull apart more affluent ones. While we are all in this together, the worldвЂ™s poorest will feel the effects of collapse first. вЂњThe climate problem will get worse and worse and worse because we wonвЂ™t be able to live up to what weвЂ™ve promised to do in the Paris Agreement and elsewhere.вЂќ вЂњThe world will not rise to the occasion of solving the climate problem during this century, simply because it is more expensive in the short term to solve the problem than it is to just keep acting as usual,вЂќ says Jorgen Randers, a professor emeritus of climate strategy at the BI Norwegian Business School, and author of 2052: A Global Forecast for the Next Forty Years. Unfortunately, some experts believe such tough decisions exceed our political and psychological capabilities. вЂњBut we cannot wait forever to make those decisions.вЂќ вЂњIf we make rational choices to reduce factors such as inequality, explosive population growth, the rate at which we deplete natural resources and the rate of pollution вЂ“ all perfectly doable things вЂ“ then we can avoid collapse and stabilise onto a sustainable trajectory,вЂќ Motesharrei said.

If the carrying capacity is overshot by too much, collapse becomes inevitable. Similarly, about half the worldвЂ™s population lives on less than $3 per day.ДЇor both scenarios, the models define a carrying capacity вЂ“ a total population level that a given environmentвЂ™s resources can sustain over the long term. For example, the top 10% of global income earners are responsible for almost as much total greenhouse gas emissions as the bottom 90% combined. The inequalities we see today both within and between countries already point to such disparities. Eventually, the working population crashes because the portion of wealth allocated to them is not enough, followed by collapse of the elites due to the absence of labour. Under this scenario, elites push society toward instability and eventual collapse by hoarding huge quantities of wealth and resources, and leaving little or none for commoners who vastly outnumber them yet support them with labour. That economic stratification may lead to collapse on its own, on the other hand, came as more of a surprise to Motesharrei and his colleagues. What are they, and which, if any, have already begun to surface? It should come as no surprise that humanity is currently on an unsustainable and uncertain path вЂ“ but just how close are we to reaching the point of no return? Putting aside species-ending events like an asteroid strike, nuclear winter or deadly pandemic, history tells us that itвЂ™s usually a plethora of factors that contribute to collapse. Regardless of how well things are going in the present moment, the situation can always change. Such collapses have occurred many times in human history, and no civilisation, no matter how seemingly great, is immune to the vulnerabilities that may lead a society to its end. Should we find no way to get the wheels back in motion, weвЂ™d eventually face total societal collapse.

Our world would become an increasingly ugly place, one defined by a scramble over limited resources and a rejection of anyone outside of our immediate group. Should that forward-propelling motion slow or cease, the pillars that define our society вЂ“ democracy, individual liberties, social tolerance and more вЂ“ would begin to teeter. The political economist Benjamin Friedman once compared modern Western society to a stable bicycle whose wheels are kept spinning by economic growth.

This story is featured in BBC FutureвЂ™s вЂњBest of 2017вЂќ collection.

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