The extraordinary heat wave that scorched the Pacific Northwest last week would almost certainly not have occurred without global warming, an international team of climate researchers said Wednesday.

Temperatures were so extreme — including readings of 116 degrees Fahrenheit in Portland, Ore., and a Canadian record of 121 in British Columbia — that the researchers had difficulty saying just how rare the heat wave was. But they estimated that in any given year there was only a 0.1 percent chance of such an intense heat wave occurring.