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> Separately the HVAC systems for the DCs will have been designed for a specific climate, with margins.

Exactly right. For a Tier IV data centre that margin is ASHRAE N=20. For London City until recently that was only 34.5C (dry bulb).

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Interesting rabbit hole! Does that mean the margin is defined to contain once-in-twenty-years extremes?

That’s right. However these estimates are now being regularly exceeded. Going forward, it would probably be wiser to do what the mining industry does here in Australia - still design to a reasonable extreme temp, but also specify that the equipment must run at a much higher temperature at a derated capacity. Here that’s 50C ambient.

Even disregarding climate change, a 5% chance each year that your equipment will fail to handle the weather seems lacking for a high-availability facility. (You'll factor in many other this type of figures together, so your total risk budget would be higher still).

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