Scorching heat waves have gripped the world in recent weeks from the Pacific Northwest to Northern Europe and, most tragically, Japan, where more than 100 mostly elderly people have died. The usual caveat applies: no single event can be specifically tied to climate change. Nonetheless, it offers an unsettling preview of what may be in store for the coming century.

Just how much should the world worry? Optimists often note that most countries will be richer by the end of this century, and societies are adaptable, both of which...