Nearly impossible without man-made global warming, this year's freak Siberian heat wave is producing climate change's most flagrant footprint of extreme weather, a new flash study says.

International scientists released a study Wednesday that found the greenhouse effect multiplied the chance of the region's prolonged heat by at least 600 times, and maybe tens of thousands of times. In the study, which has not yet gone through peer review, the team looked at Siberia from January to June, including a day that hit 100 degrees (38 degrees Celsius) for a new Arctic record.

Scientists from the Unite...