In Persepolis Rising there is a reference to the evacuation of London, Beijing, and Denver at some unspecified point(s) in the history of The Expanse:
She looked at them and saw all the other times children had been carried away from a disaster that was approaching and that could not be stopped: London, Beijing, Denver. History, she reminded herself, was peppered with moments like this one.
Persepolis Rising, Chapter 27: Drummer
The mention of three specific cities indicates to me that Drummer is referencing event(s) that I, the reader, should be familiar with.
When did these evacuations occur and why were they needed? Were they all due to the same cause or did they occur at different points in history?
The obvious historical event in The Expanse which would cause cities to be evacuated is
the militant OPA's attack on Earth using asteroids in Nemesis Games.
However, I could find no mention of London, Beijing, or Denver being evacuated in any of the seven novels which have been released so far. The only relevant mention of any of those cities in the novels is in Babylon's Ashes, in which there is a mention that people were hoarding relief supplies in Denver.
Perhaps Drummer is referring to event(s) which were mentioned in one of the short stories (which I don't have)? Or to event(s) that happened in real life?