[throwaway6532]

>The problems with water and sand are not physical limits. Like climate change and pollution generally they're just unpriced externalities.

I'd say it's a little bit of column A and a little bit of column B. They're definitely are physical limits to those things, and they are definitely subject to unpriced externalities at present. I think the water shortages in South Africa are a good case study where the public reacted to avert a crisis once they were made acutely aware that a crisis was imminent if they didn't act. People just take water for granted, until they no longer can. But in saying that there is almost certainly a point at which things just can't keep up, or the alternative cannot be procured/switched to in a short enough time to avoid the disaster. I see it less as a problem of pure physical limits and more as a problem in control theory.