No one could have predicted that oil prices would rise to today’s levels. Saudi Arabia’s oil minister, Ali al-Naimi, says that they are irrationally high, pointing out that world demand is lower than the available supply and that Saudi oil inventories around the world are largely untapped. The “irrational” cause, of course, is fear of a war with Iran. But it would also have been unpredictable that a 47 percent hike in oil prices since November 2010 would not cause a major slowdown in the U.S. economy. One reason it hasn’t might well be the rise of shale gas.