An abrupt, significant, and wide-spread regression in quality of life, coupled with cultural and technological stagnation. Something like the Late Bronze age collapse (
https://en.wikipedia.org/wiki/Late_Bronze_Age_collapse )
It is a reasonable low-probability high-risk concern, given (unordered):
* Depleting fossil fuel reserves: most obvious risk; dominant fuel source for transportation and agriculture, dominant energy source for industries, dominant material in many consumer goods.
* Climate change: threatens some regions' agricultural and ocean productivity, pressuring human migration, spreading diseases.
* Exhausted oceans: oceanic deadzones are spreading, loss of fishing as a food source is a serious risk for parts of Asia, which will put additional pressures on agriculture and motivate economic migration.
* Over-use of antibiotics and vaccines: risk of super-bacteria/virus plagues, mostly in agriculture and 3rd world nations.
* Depletion of aquifers: imminent risk amplified by climate change. America's food production is heavily reliant on effectively non-renewable aquifers, depletion or poisoning through fracking are serious risks.
* Globalized economy/supply chains: coupling economies improves efficiency but introduces the risk of cascade failures.
No one of these problems are an existential risk by themselves... but they are all interrelated and poorly understood, which impacts our ability to effectively preempt wide-spread impact.
Humanity will recover should the worst happen, but I think it's better to avoid the set back in the first place.