I think I know why that happened.

citation for claim that it's the longest-dated climate dataset?

This comes off a bit curt, which can be read as a challenge. I don't typically ask for citations unless I'm implying the other person either doesn't have them or has bad information. It's almost always an attack on their character.

I'm just musing from the sidelines, by the way, don't take this as a lecture:

This is actually what the social function of filler words like 'please' and 'thank you' are for. They provide a base level of politeness that, much like a tone-tag, shapes the way the rest of the message is meant to be received. Sort of similarly, a question like "Is this really the longest-dated record? Do you have a citation?" shifts focus away from the person and toward validity, making the citation just a matter of course.