> If I recall, the paper was based on wind data for 1 year

They selected 2011 as representative and detailed it, however the underlying numerical series cover decades (see the 'Ninja' project https://www.renewables.ninja/ )

> how much variability ((...) will be in the future with climate change

Indeed, there is a non-negligible challenge here. It is also true for nuclear, for example when it comes to cool a reactor down: water ill be more and more scarce, heating it isn't neutral, installing a reactor on seashore exposes it to sea level rise...

> capacity factor

Is way less a criteria for production units not consuming any fuel (wind, solar...) not waste.

>. They are able to rely so much on wind because they still burn fossil fuels and fortunately the neighboring countries don't heavily rely on intermittent energy sources and can buffer Denmark's under and over supply of electricity.

Yes, and each and every clean energy source type now needs such a backup.

No industrialized nation (bar maybe very low-population density coupled with huge natural resources, mainly hydro) can produce all its electricity without either not negligible curtailment or burning fossil fuel in 'backup' powerplants.

Even hugely-nuclearized France could never, cannot (and don't even plan to) produce all its electricity thanks to nuclear: around 9% of their electricity is produced by backup 'thermal' plants: gas (methane), oil, coal, see https://www.statistiques.developpement-durable.gouv.fr/editi... ), either due to load-following or peak consumption.

> For Germany one estimate is that for it to rely only on solar and wind

AFAI no nation targets such an ordeal, they plan to reduce the need for backup units to an acceptable (and always lower) level thanks to production units geographic spread-out, mix (solar, wind...), storage (including V2G) and low-impact curtailment.

> If making more grid storage was cheap and easy, we would have built it decades ago

No, mainly because there was no perceived need (burning fossils was practical and cheap), and also as some rather recent advances (network management enabling better integration of intermittent sources, low-loss long-haul powerlines ((U)HVDC...) facilitate such projects.

Applying nuclear reactors to real-world problems (in submarines, aircraft carriers and ice breakers) date back the 1950's, and the very same reactor architecture (PWR) wasn't adopted as the main type of electricity-production unit before the 1970's.

> (renewables) ((...)) what if that doesn't pan out?

It better does, as our current (worldwide) ability to build nuclear reactors is very weak, and weakening, and as there isn't any third approach in sight.

> If you were forced to choose, would you prefer the world to suffer through catastrophic climate change rather than use nuclear power?

For the time being, and quite probably for (at best) the next 20 years, there is no such option. Nuclear produces about 2.2% of world's total final consumed energy, with approx running 450 reactors. In order for it to produce about 22% we need to build approx 4500 reactors. This isn't even an option. France is one of the leaders and doesn't hope to have more than 3 new reactors by 2040...