Across the Arctic, snow melted earlier and more completely this year than any in recorded history. In the same way ice loss exposes dark water to the sun’s radiant heat, melting snow causes exposed ground to heat up, adding to the Arctic’s already super-sized warming.

“In the past, whatever happened in the Arctic stayed in the Arctic. But now it seems to be reaching down from time to time in the mid-latitudes,” said climatologist James Overland of the National Oceanographic and Atmospheric Administration. “When you combine the new influence of the Arctic with other effects, such as El Niño, we’re seeing the more extreme weather events.”

Over the last several weeks, public attention has been seized by the disappearance of ice in the Arctic Ocean, which in September covered a smaller area than at any other time in the climate record, a fitting exclamation point to its 50 percent decline since the late 1970s.