What does methānum mean in Latin?

methānum, a noun, means “methane”.

What is the genitive of methānum?

The genitive singular is methānī. methānum is neuter, second declension.

In classical Latin, [mɛˈtʰaː.nũː]; in ecclesiastical (church) Latin, [meˈtaː.num].

noun · neutersecond declension

methānum n (genitive methānī)

Classical[mɛˈtʰaː.nũː]Ecclesiastical[meˈtaː.num]

methane
[…]sed multae inquisitiones scientificae demonstrant maiorem partem calefactionis universalis postremis decennis gasiorum ex cumulatione oriri […] (dioxydum carbonicum, methanum, oxyda nitrogeni et alia) quae emittuntur potissimum ob humanam industriam.[…] yet a number of scientific studies indicate that most global warming in recent decades is due to the great concentration of greenhouse gases (carbon dioxide, methane, nitrogen oxides and others) released mainly as a result of human activity.2015, Pope Franciscus, “Laudato si’. http://w2.vatican.va/content/francesco/la/encyclicals/documents/papa-francesco_20150524_enciclica-laudato-si.html, Litterae Encyclicae, Vatican

Internationalism. Compare English methane.