Interested in Language
Could you paraphrase or say in other words the underlined part in the following title:
"Stratospheric Water Vapor Is a Global Warming Wild Card"
It is the thing that is unknown and might change the way things turn out.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.