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the American study from 2022 is just warmed over tilman Clark (that American study from 2016), and the Danish study also depends on tilman Clark. so we should look at their methodology.
I did.
they compare a wide range of data from lca studies, even though this violated the best guidance on lca data.
lca studies are a bit like grand juries: the person designing the study can pretty much get any result they want.
and since these studies are all disparately methodized, you cannot combine them.
it's possible the conclusions are correct, but these papers are not sufficient evidence to be believed.
I don't have full access to the danish study, so I will have to take your word for it.
I do see that Tilman D, Clark M (2014) Global diets link environmental sustainability and human health. Nature 515(7528):518–522. is referenced in the 2016 study and the 2022 study.
the danish study is actually worse in some ways. it additionally cites poore-nemecek 2018, who themselves referenced tilman-clark, but egregiously gathered even more lca meta-analyses, and created something of a meta-meta-analysis of lcas. it's bad science all the way down.
Do you have any links for someone who wants to read more about these LCA and why they're not combinable?
hilariously, you can read the references from poore-nemecek, where the meta-studies they cite, themselves explain the problems with combining lcas, but then say "we're gonna do it anyway".
understanding how lca studies are conducted should be sufficient to understand why meta-analyses are misuses of the data, and the wikipedia article about lcas does a pretty good job of explaining the issues with the methodologies