Which factor is most important to consider when evaluating a journal article about global warming?

Since you didn't include the options, I will answer the question as well as I can: the one factor I would be considering is whether the journal is known to be biased, for example whether it's a peer-reviewed journal (those are less biased) or whether they journal is paid by some interest group.

Otherwise I would look at whether the data is reliable, for example whether the author cites the sources for their data