"As" is an adverb that modifies the adjective "visible". The clause "as ... can form" is an adverbial clause modifying "visible" and describing "how" the evidence is visible.

In the sentence provided, "as" is used as a preposition. It is used to indicate the reason or cause for the evidence being visible. It shows that the worldwide decline in "ship tracks" is the result of or caused by the presence of long, thin clouds created by sulphate particles in a ship's exhaust. To replace "as" in the sentence, you can use words like "due to," "because of," or "resulting from." For example, "The evidence is visible due to a worldwide decline in 'ship tracks'..." or "The evidence is visible because of a worldwide decline in 'ship tracks'..." These alternatives maintain the same meaning and help clarify the relationship between the evidence and the decline in ship tracks.