Byzantium was in decline for seven centuries until the Ottomans pulled the plug. Rome was in decline for several centuries prior to its sacking.
Climate change and the accompanying plagues, droughts, famines, and calamities that accompany it might accelerate and exacerbate the state's capacity and willingness to respond to these crises, but all it might mean is that this is a new normal added to the reproleterization of American life.
I don't really have a point but it is just a thought that I (perhaps others) are going to have to accept that future, and that is a kind of new world I am unsure as to how to adapt to.
It's very unlikely pax Americana is going to end any time soon. China is 20 years behind the US in terms of military technology innovation (something they can essentially never catch up on so long as the US still funds this stuff).
China has no real incentive for pax americana to end either, their recent military investment is mostly related to specific local concerns for them like Taiwan. China is a reserve currency within the IMF, they're on the security council.
The general consensus is just more regional economic powers. Automation is increasing a return to domestic manufacturing all over the world due to cheaper shipping costs. Renewable energy means less of a need to force oil markets open.