This past winter was an exceptionally strange one across North America. Rain deluged California, as unseasonable warmth fanned across the Midwest and Eastern seaboard. In New York, sales of salt and snow shovels plunged; in Washington, some of the famous cherry trees bloomed too early and died. When the weather gets weird, many people now think of climate change. And nearly as many people know, too, that it’s hopeless to try to figure out if it’s to blame.