An East Coast blizzard howling, global temperatures peaking, the desert Southwest flooding, drought-stricken California drying up—surely there’s a common thread tying together this “extreme” weather. There is. But it has little to do with what recent headlines have been saying about the hottest year ever. It is called business as usual.

Surface temperatures are indeed increasing slightly: They’ve been going up, in fits and starts, for more than 150 years, or since a miserably cold and pestilential period known as the Little...