> All the water tanks in the world don't add up to a trillion km³. Not by many orders of magnitude.
You're missing my point: that hoarding water is at the expense of anyone else who needs water. One milliliter, one gigaliter, doesn't matter; that's still less for everyone else unless and until it is released, and during that time the mere storage of that water externalizes an opportunity cost on everyone else needing that water.
> The sun does that for free, and has done for billions of years.
Right, because the sun magically drops all precipitation into lakes and rivers, and not a drop of it into the oceans. I'm sure the sun has some sort of tracking system that realizes which molecules of water come from where and puts them right back whence they came, right?
> You're missing my point: that hoarding water is at the expense of anyone else who needs water.
Someone with a water tank in (say) Seattle, Washington is not "hoarding water" "at the expense" of someone in Addis Ababa, Ethiopia, because there is no practical way of getting the "hoarded" water from Washington State to Ethiopia. If the Seattleite doesn't "hoard" that water, it's going to run straight into Puget Sound. It's not going to somehow appear at a tap in Addis Ababa.
> Right, because the sun magically drops all precipitation into lakes and rivers, and not a drop of it into the oceans.
Now you're just strawmanning. Of course a lot of it falls on the oceans. But enough of it falls on land that the hydrologic cycle continues, as it has for billions of years.
I think fundamentally you're just wedded to the the idea that water gets "used up" in the same sense that, say, oil, gets used up.
> Someone with a water tank in (say) Seattle, Washington is not "hoarding water" "at the expense" of someone in Addis Ababa, Ethiopia,
Well it sure ain't contributing to the "hydrologic cycle" if it's in a water tank. That's a tankful less water in that cycle. And multiply that by everyone else storing water, and before you know it that's a noticeable impact globally, including both for Addis Ababa and the rest of Seattle.
This is, mind you, precisely why a lot of municipalities don't take kindly to people collecting rainfall, or to people damming up streams or rivers without doing the necessary due diligence on ecological impact assessment. There are downstream impacts to these seemingly-innocuous things.
> I think fundamentally you're just wedded to the the idea that water gets "used up" in the same sense that, say, oil, gets used up.
Nowhere have I even suggested that to be the case, and yet somehow you think I'm the one strawmanning here. If you're going to deliberately ignore my point and substitute it for one that's obviously false, then why even bother to respond?
All the water tanks in the world don't add up to a trillion km³. Not by many orders of magnitude.
> the resulting labor and energy costs of converting the latter back into the former
The sun does that for free, and has done for billions of years.