Technically it's a diaeresis, not an umlaut. It indicates the syllable break in the word "cooperate". It's not a common thing in written English, but the New Yorker has a certain writing style that calls for that sort of thing.
You're not joking, here's an extract from the Wikipedia entry [1]:
"Nowadays in English, the diaeresis is normally left out (cooperate), except by The New Yorker, or a hyphen is used (co-operate). It is, however, still common in loanwords such as naïve and Noël."
