[PeterisP]

Pumped hydro is great for these circumstances and we absolutely should build it where it's feasible even though it's often considered anti-green due to ecosystem damage by the dam/reservoir.

However, as far as I understand, this is not a sufficient solution to the storage problem because you can't build them just anywhere; (a) the feasible locations are not enough to fill the enormous storage need required for a full transfer from fossils to wind/solar; and (b) in general, the type of locations that are good for pumped hydro (e.g. rivers on mountains) are not the type of locations where we have major cities (e.g. fertile plains) and mass production of renewables, so they are far from the major consumers and producers and we have a bit of a grid transit problem and international cooperation problem for that.

So while we build pumped hydro we still need to look forward to something else (e.g. mass battery storage) that can fill the gap between what we can store today and what we'll need to store to stop burning fossil fuels.