The soft path, as Amory Lovins defined it in the 1970s, is energy conservation and power from the sun, wind and plants — the technologies that Senator Barack Obama emphasizes in his plan to reduce greenhouse emissions. Senator John McCain is more enthusiastic about building nuclear power plants, the quintessential hard path.


As a rule, it’s not a good idea to revive anything from the 1970s. But this debate is the exception, and not just because the threat of global warming has raised the stakes. The old lessons are as good a guide as any to the future, as William Tucker argues in “Terrestrial Energy,” his history of the hard-soft debate.

The initial debate over nuclear power seemed to end not long after the partial meltdown in 1979 of the reactor at Three Mile Island. Utilities canceled orders and stopped building reactors, partly because of public fears, but perhaps mainly because of rising costs. Mr. Lovins and his allies liked to say that nuclear power, once promoted as “too cheap to meter,” had now become “too expensive to matter.”