Can you give an example of an industrialized first-world nation using Wind + Solar to provide baseload power on a consistent basis? Because if you can't then they're far from proven as anything more than a partial supplement to other energy sources. The economics of Solar/Wind get much worse as you scale past ~30% of the grid because utilization factor goes down and the need for energy storage goes way up.
Right now grid-scale batteries are uneconomical and pumped hydro is only feasible in specific locations (not to mention its ecological impact) so the 'storage' backing renewable installations takes the form of big tanks of methane next to gas turbines. Even when conditions are good and it's not being burned, that methane has a tendency to leak and wipe out a fair proportion of the face-value greenhouse emission savings from swapping to renewables in the first place.
That's not correct. Denmark is currently is currently at something like 45% of electricity from wind, and going past 50% in a year or two IIRC. It's not base load, because you don't need base load - it's an obsolete model from a world of capital-intensive, constant output plants. What you need is to match the demand.
Regarding long-term storage: Exactly. Tanks or caverns filled with methane generated from a renewable source is one possible solution. Of course, you need to have a firm grip on leaks.
Renewables aren't any better at load following than current nuclear technology, and unlike nuclear energy there's no potential for the development of proposed designs that do have load-following capability. Never mind the fact that intermittent renewables themselves are the reason the 'mostly base load with some load following' model is obsolete, precisely because they can't be relied upon to generate electricity on demand.
Denmark is definitely the poster child of wind energy (and intermittent renewables in general), but it benefits enormously from utilizing the existing pumped hydro storage of Sweden and Norway, which is an option the vast majority of countries don't have. The economics would look at lot worse if they were having the build out batteries to provide that storage capacity instead of taking advantage of what's already there.
Just saying 'you need to have a firm grip on leaks' is trivializing a significant engineering problem. Existing natural gas plants have a 1-9% leak rate, and given than leaking fuel is a complete waste of money we can safely assume that doing better than that is non-trivial. Given that in the case of carbon-neutral 'synthetic' methane you have the possibility of leaks at the manufacturing plant as well as in transmission/storage, it's reasonable to expect a figure closer to the higher end of that spectrum, which makes lifetime emissions from Wind/Solar sources significantly worse than Nuclear on a per-GWh basis.
Right now grid-scale batteries are uneconomical and pumped hydro is only feasible in specific locations (not to mention its ecological impact) so the 'storage' backing renewable installations takes the form of big tanks of methane next to gas turbines. Even when conditions are good and it's not being burned, that methane has a tendency to leak and wipe out a fair proportion of the face-value greenhouse emission savings from swapping to renewables in the first place.