@Unixbigot I've recently been enjoying reading a bunch of papers on topics including mechanisms for extracting carbon dioxide from our atmosphere, and I've learned we already have many options—so the open questions are how to extract a lot quickly and efficiently, and what to do with the carbon we get. Turning captured carbon into usable fuel, for example, is a neat trick and well understood, but then using that fuel puts the carbon back in the atmosphere.

I probably wouldn't want to rely on advice from aliens about growing bacteria on carbon to then use as animal feed, as some researchers have demonstrated; biology strikes me as a little more sensitive to local conditions than physics or chemistry.

I might however ask aliens for suggestions on reversibly adjusting the albedo of a planet. It's been estimated that radiative sky cooling could counteract a significant amount of greenhouse effect warming if we can do it across a mere 1% of the Earth's surface, an area the size of the Sahara desert; maybe our galactic neighbors have easier alternatives.