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Good point, but the chance of a given city having a 500-year flood in three given successive years is only (0.002)³ = 8 × 10⁻⁹. So even if we allowed for 100 U.S. cities, 10 different natural disasters and 10 different three-year periods we're still talking about a probability of less than 0.01%.

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And it's not like Houston is the only city having 100 year+ floods regularly.

yes, it's almost as if the climate is changing

Or our assumptions about climate / wheather were wrong.

...the assumption being that the risks are separate. This is clearly false - you are not rolling dice here, the probabilities of the same disaster at geographically "close" cities are linked.

That is true but it's an error in the unproblematic direction: fixing it makes the argument even stronger.

That is, if the probability for each city flooding is 0.002 but the different cities are positively correlated, then the probability of at least one city flooding is lower than it would be if they were uncorrelated. (Because the probability of no cities flooding is higher, because of positive correlation.)

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