I like some concepts and design of Mbin, something to learn from, but I'd believe more in its growth potential if not written mainly in php.

Interesting observation and analysis, and illustrates the potential of more lemmy-mastodon interaction.
Indeed mdon like-federation seems weird but I presume it was setup this way for efficiency, to reduce the number of small communications? Although Lemmy has a backend in rust - more efficient than mdon's ruby - still I wonder whether the lemmy system of federating all upvotes would scale well if the number of users grows to that of mastodon and beyond ? Could there be some intermediate compromise solution (e.g. federate batches of 100 likes)?

Indeed to use scala-native you'd need pure-scala libraries, but the core lib re-implements most java lib, and there are now small simple external libs available for common tasks like file management, database, etc. - for example check out the lihaoyi suite.
I mainly use scala-js (to make this) which was formerly a java app - as it compiles to both js and jvm (cross-project) can gradually convert stuff you already wrote. I've tried native for stuff like pre-processing data files.

I didn't discover Lemmy through search, nor did I ever use reddit - I found it from mastodon where a few people promote lemmy posts. Then gradually realised I preferred the community-focus here, compared to the individual-focus of mdon (although combining both could be good). As mdon has many more users, improving this inter-op would help to bring people here.

Scala compiles either to native, js or jvm - obviously the IO / interface options vary between these envs, but the lang is the same. Recently Scala 3.5 incorporates a simple-to-use CLI which makes it easier to compile to native (or just run a small file as a script, or experiment with a repl), native binaries are small and fast, and there are some simple io libraries. Since you can also compile to jvm to interop with java, that might help with transition.

I now use Scala 3, and very happy with syntactic whitespace (combined with an intelligent compiler)

Trying to imagine what's the application of mats of electric seaweed - if the energy could somehow make them self propelling, and self replicating, could get interesting, big potential surface area ...?

Indeed that's strange, and the flat slope in 2060 seems inconsistent with declared net-zero policies of China and even India. Russia has no such policy, but still strange to assume continuation of current government concepts there until 2060. (you can see the regional breakdown in supplem Fig 1. )

Regarding the map - an annual average cost is not so meaningful - in higher latitudes solar is not enough in winter - especially where it’s mostly cloudy during the first half of winter. Wind helps the balance but not everywhere, always. Of course, the sophisticated models behind the article know all that, the issue is simplistic presentation. I note "we assume hydrogen is used for seasonal storage" - this may be rather optimistic - how many dark months can that cover?

Scala-js is working on it - as its compiler design may facilitate this.
I haven't yet tried (on todo list) and am not an expert, but bookmarked in passing:
recent github implementation, some history, following older discussion

Stability is indeed a strength of EU - effectively averaging over all the countries smooths over political oscillations - which is useful for tackling long-term policy problems (like climate). I'm not advocating majoritarian voting where 51% overrides 49%. However with ± 30 countries, one or two should not block the rest - the current system leads to transactional brinkmanship where the last hold-outs get some prize in return for postponed obstruction. I've seen similar (worse) problems in UN climate negotiations - also due to "consensus" principle.

I agree. The key symbiosis between coral and microalgae depends on fundamental thermodynamic equilibria of the carbonate chemistry of seawater - which are highly sensitive to temperature and atmospheric CO2, in very predictable ways. When living in coral becomes unprofitable for the algae, they leave. My instinct, from some experience with this system, is that introducing new species won't do better than nature, nobody can beat thermodynamics. We have to reduce the CO2.