Someone else recently asked me the same question. I have read enough times from numerous literature sources how Orca have killed polar bears & in every account the author state that such a predation event is very rare. I have not read any specific literature detailing such an event although I am sure some exist. We know Orca swim as far north as the Arctic & as you mentioned global warming & decreasing ice are forcing polar bears to swim longer distances often ending in an exhausted hungry polar bear drowning but also increasing the chance to marine predation by Orca. If I do find relating literature (there is a lot of obtained literature I need to review - maybe I already access) I will post here.